<p>First, it's good to note that it really wasn't much of Ida that went that far... in terms of the strength of the low pressure (and the <a href="https://earthscience.stackexchange.com/a/542/6298">resultant wind</a>), while <a href="https://www.researchgate.net/figure/Land-use-land-cover-LULC-types-for-the-four-parish-study-area-in-southwestern_fig2_257129109" rel="nofollow noreferrer">the swampy waters in southern Louisiana</a> may've helped it maintain category 4 strength for an extra few hours inland, it decayed rapidly soon after, <a href="https://www.nhc.noaa.gov/archive/2021/graphics/AL092021/AL092021_5day_cone_no_line_and_wind_19.png" rel="nofollow noreferrer">becoming a depression in Mississippi</a>, and <a href="https://en.wikipedia.org/wiki/Extratropical_cyclone" rel="nofollow noreferrer">extratropical</a> <a href="https://www.wpc.ncep.noaa.gov/tropical/tropical_advisories.php?storm=IDA&amp;adnum=26&amp;dt=2021090115&amp;status=posttrop" rel="nofollow noreferrer">near the Kentucky/West Virginia border</a>.  By the time it passed through the northeast, it was just a 998-1000 mb low pressure with sustained winds of only 30-40 mph.  As a low pressure system it was quite pedestrian in even fairly recent history:</p>
<p><em>(Lower on the list = more intense low pressure), values are mostly peaks, at landfall if applicable</em></p>
<div class="s-table-container">
<table class="s-table">
<thead>
<tr>
<th>Storm</th>
<th>When</th>
<th>Pressure</th>
<th>Sustained Winds</th>
</tr>
</thead>
<tbody>
<tr>
<td><a href="https://w1.weather.gov/data/obhistory/KNYC.html" rel="nofollow noreferrer">Typical daily pressures</a></td>
<td></td>
<td>~ 1005-1020 mb</td>
<td></td>
</tr>
<tr>
<td>Tropical Storm Fay</td>
<td>July 2020</td>
<td>1000 mb</td>
<td>45 mph</td>
</tr>
<tr>
<td><a href="https://www.wpc.ncep.noaa.gov/tropical/IDA/IDA_archive.shtml" rel="nofollow noreferrer"><strong>Remnants of Ida</strong></a></td>
<td><strong>Sept 2021</strong></td>
<td><strong>998-1000 mb</strong></td>
<td><strong>30-40 mph</strong></td>
</tr>
<tr>
<td>Tropical Storm Henri</td>
<td>Aug 2021</td>
<td>989 mb</td>
<td>60 mph</td>
</tr>
<tr>
<td>2017 Nor'easter</td>
<td>Feb 2017</td>
<td>969 mb</td>
<td>43 mph</td>
</tr>
<tr>
<td>Hurricane Irene</td>
<td>Aug 2011</td>
<td>965 mb</td>
<td>65 mph</td>
</tr>
<tr>
<td>2021 Nor'easter</td>
<td>Feb 2021</td>
<td>960 mb</td>
<td>80 mph</td>
</tr>
<tr>
<td>Hurricane Sandy</td>
<td>Oct 2012</td>
<td>946 mb</td>
<td>80 mph</td>
</tr>
<tr>
<td><em>Ida at Louisiana landfall</em></td>
<td><em>Aug 2021</em></td>
<td><em>930 mb</em></td>
<td><em>150 mph</em></td>
</tr>
</tbody>
</table>
</div>
<p>Most tornadoes and many heavy rainfall events come outside of hurricanes, and a large number come from fairly weak lows.  They're both so much about how specific localized conditions lineup, not so much the strength of the low pressure itself.</p>
<hr>
<p>Now, first to get a couple of the secondary questions out of the way:</p>
<p>In regard to rain shadows, they're just as much about wind direction and surrounding factors.
In the fairly arid intermountain western US for example, storms come in with fairly limited total moisture as is (due to the cold east Pacific temperatures), then wring out quite a bit of what they have as widespread precipitation forced by its air rising over the Cascades and Sierras... but as important is that pre-storm wind directions from the northeast/east/southeast/south have very little access to water due to the basin nature of the whole region and large distances to water.  In comparison, most precipitation comes ahead of low pressures in the northeast because this is the wind direction:</p>
<p><a href="https://i.sstatic.net/hRP24m.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/hRP24m.jpg" alt="enter image description here" /></a></p>
<p>As transitioning tropical cyclones work north from the Gulf (or regular lows work east from the Plains), they drop plenty of precipitation, and sure <a href="https://d32ogoqmya1dw8.cloudfront.net/images/eslabs/weather/us_ppt_1971-2000.v2_1368.png" rel="nofollow noreferrer">you can pick out</a> some extra moisture is deposited from rising air on the windward side of the Appalachians... but the pre-storm fetch for the east coast is over <a href="https://www.ospo.noaa.gov/data/sst/contour/namerica.cf.gif" rel="nofollow noreferrer">the warm Gulf-Stream</a> of the western Atlantic, so moisture is well preserved... as the northeast coastal areas often finds out too well in the winter months when snow piles in from <a href="https://en.wikipedia.org/wiki/Nor%27easter" rel="nofollow noreferrer">nor'easters</a>.  Moisture profile isn't about where the center of the low pressure goes, but the flow around it.  In nor'easters, a just-offshore track tends to favor more snow not because a track over the Appalachians would steal the moisture, but because of how it maximizes the onshore flow most perpendicularly to the strongest temperature gradient to create the most lift (and because a track offshore means you're in the cold sector, a path over land means you spend more time in the warm sector (= perhaps not snow).  The low profile of the Appalachians and ability of air to flow around it from ample water areas of the Gulf, Atlantic, and even Great Lakes means there really isn't any great rainshadow in the eastern US.</p>
<p>In terms of tornadoes, the overall count was <a href="https://en.wikipedia.org/wiki/List_of_tornadoes_spawned_by_tropical_cyclones#2020%E2%80%932021" rel="nofollow noreferrer">fairly regular</a> for tropical cyclones.</p>
<hr>
<p>Now for rainfall... how much was there?</p>
<p><a href="https://i.sstatic.net/xXK9d.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/xXK9d.jpg" alt="enter image description here" /></a>
<sup><sub><em><strong>(remind me in a few months to update with a better graphic that the WPC produces for tropical cyclones)</strong></em></sub></sup></p>
<p>Is that extraordinary?  Yes... and no... and yes... and no... and then very much yes!!!</p>
<p>Those are some rainfall records listed up above, so it's certainly a notable amount in spots.</p>
<p>However, certainly compared to rainfall when tropical cyclones stall further south, it's a <a href="https://en.wikipedia.org/wiki/List_of_wettest_tropical_cyclones" rel="nofollow noreferrer">drop in the bucket</a>.  In this very event, Ida, a few spots in Louisiana and Mississippi had <a href="https://twitter.com/GCarbin/status/1432302397720862723" rel="nofollow noreferrer">nearly double the rain</a> that even some of the harder hit spots in the northeast got.  Spots along the northeast Texas Gulf Coast got nearly <em>60 inches</em> (1500 mm) of rain from <a href="https://en.wikipedia.org/wiki/Hurricane_Harvey" rel="nofollow noreferrer">Hurricane Harvey</a> in 2017, and then another 40 inches (1000 mm) from weak <a href="https://en.wikipedia.org/wiki/Tropical_Storm_Imelda" rel="nofollow noreferrer">Tropical Storm Imelda</a> in 2019 (and looks like they could see another potential event next week).</p>
<p>Still, one person's mountain is another person's molehill.  The northeast doesn't have particularly great permeable soil.</p>
<p><a href="https://i.sstatic.net/o00Q2.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/o00Q2.png" alt="Enter image description here" /></a>
<sub><sup>(<a href="https://earthobservatory.nasa.gov/images/87220/soil-composition-across-the-us" rel="nofollow noreferrer">Source NASA's Earth Observatory</a>)</sup></sub></p>
<p>And that's where there's soil.  The northeast is... kind of known as a <a href="https://en.wikipedia.org/wiki/List_of_United_States_urban_areas#/media/File:USA-Urban-Areas.svg" rel="nofollow noreferrer">heavily urbanized area</a>.</p>
<p>So a comparable amount of rain in the New York metro versus most of Louisiana means more.</p>
<p>But... what's most surprising to me is that, when you compare the rainfall maps to recent events overall... it really doesn't stand out much!!!</p>
<p><a href="https://i.sstatic.net/Fs970.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Fs970.png" alt="enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/sIJqz.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/sIJqz.png" alt="enter image description here" /></a>
A swath of like 5-8 inch rainfall in all three of the (at some point) hurricanes, each with a few higher inches spots.  Not off the charts.</p>
<p>Yet, at least from what we saw on television from afar around the country, it seemed like this storm's (freshwater) flooding was comparably quite extreme.  (Though honestly the picture from the media often makes it seem like total widespread devastation when it's really not.  Many areas in southeastern Louisiana are quite reasonable, as were decent amounts of Houston in Harvey and south/central Florida in <a href="https://en.wikipedia.org/wiki/Hurricane_Irma" rel="nofollow noreferrer">Hurricane Irma</a>.  It does seem harder to get a real feel of the true scope of an event from the media anymore ☹️)  But based upon the proliferation of awful videos of raging waters and collapsing basements around NYC and satellite images <a href="https://www.npr.org/sections/pictureshow/2021/09/05/1034459845/satellite-photos-flooding-ida-new-jersey" rel="nofollow noreferrer">like these</a>, and the list of different areas involved, it seems to have been fairly notable compared to most systems.</p>
<p>So where does that leave us.  Get to the point, was it really that bad?!?</p>
<p>The answers looks be yes.  For two reasons I can see/guess at.</p>
<p>#1 is that the rainfall came two weeks after similar rainfall from Henri.  The soil was saturated, so less rainfall went into the soil.  The great Coast NOAA hurricane finder suggests only three times in past history has New York City had two tropical cyclones (or remnants) pass within 60 miles of NYC within a month... 1888, 1893, and  1985.  And the rainfall analyses from <a href="https://www.wpc.ncep.noaa.gov/tropical/rain/henri1985filledrainwhite.gif" rel="nofollow noreferrer">1985-Henri</a> and <a href="https://www.wpc.ncep.noaa.gov/tropical/rain/gloria1985filledrainwhite.gif" rel="nofollow noreferrer">Gloria</a> produced less rain and most of it further from the I-95 corridor.  So having two storms in quick succession (and additionally, <a href="https://www.weather.gov/wrh/Climate?wfo=okx" rel="nofollow noreferrer">suggestions the NYC area was also about 4&quot; above average in July</a>), both happening to line up the heavy rain, made things tough.</p>
<p>And #2 is the rainfall <strong>rate</strong>.  Here are the greatest 1 hour rainfalls in modern history at Times Square (from Iowa State's dataset):</p>
<p><a href="https://i.sstatic.net/9zza2.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/9zza2.png" alt="enter image description here" /></a></p>
<p>Making <a href="https://mesonet.agron.iastate.edu/plotting/auto/?_wait=no&amp;q=155&amp;network=NY_ASOS&amp;zstation=LGA&amp;var=max_p01i&amp;threshold=100&amp;hour=12&amp;sdate=2000%2F01%2F01&amp;edate=2000%2F12%2F31&amp;month=all&amp;dpi=100&amp;_fmt=png" rel="nofollow noreferrer">other plots</a> there's a range of results... <a href="https://en.wikipedia.org/wiki/New_Brunswick,_New_Jersey" rel="nofollow noreferrer">New Brunswick</a>/<a href="https://en.wikipedia.org/wiki/Somerville,_Massachusetts" rel="nofollow noreferrer">Somerville</a>, NJ, another badly hit area, similarly blew out their record... other spots weren't quite as bad but some still up flirting with records (as some did with Henri the week before), like Laguardia (2 of top 3 were in Ida) and Trenton (slight record)... whereas Philadelphia and Providence were nowhere on the list, showing it was a fairly narrow swath.</p>
<p><em>Why</em> such heavy rainfall rates?</p>
<p>Because it's getting well beyond a week since the event, data is a bit hard to come by on some key things (many of the best sites keep their data only a few hours to days because of the size... it's something I wish we'd focus on improving, but there's enough to piece together some things these days at least).  But I think it actually kind of relates back full circle in connection to the other topic touched on... tornadoes.  Intense tornadoes are quite rare in the northeast.  The central reason that both intense rain (like &gt;2-3&quot;) and tornadoes are scarcer up that way is that very intense deep convection is quite infrequent in the region because it's pretty rare to get instability/strong lift (hot moist air at the surface, cold air aloft, things like strong fronts [all giving the energy for rapidly rising air, allowing moisture to build faster/larger raindrop size]) and helicity (rotation of the winds with height which <a href="https://en.wikipedia.org/wiki/Supercell" rel="nofollow noreferrer">allowing the storms to maintain intensity</a>) to line up.  <a href="https://www.spc.noaa.gov/exper/soundingclimo/" rel="nofollow noreferrer">SPC's sounding analysis</a> shows that summer has some decent instability (CAPE) in the northeast, but lower helicity/deep layer shear, as the area is usually far removed from the jet stream and it's fairly rare to gets the enhancement a tropical cyclone can offer.  Whereas the ingredients were all reasonably in place for strong persisting updrafts = tornado potential and very heavy rainfall rates.</p>
<p><a href="https://i.sstatic.net/waWav.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/waWav.png" alt="Enter image description here" /></a></p>
<p>(<a href="https://twitter.com/burgwx/status/1433567519810494465" rel="nofollow noreferrer">Here's a fantastic radar track video of Ida's US life</a>... you can see the deep convection flare up brighter in the brighter yellow and reddish colors in a fairly tight band over NJ, also seen in the white deeper clouds that show up in the same area on <a href="https://www.youtube.com/watch?time_continue=180&amp;v=0HlhEB2GT88&amp;" rel="nofollow noreferrer">satellite</a>.)</p>
<p>Why'd it come together like this?  Going to be hard to answer too confidently with a quick analysis with limited data.</p>
<p>Was there extra moisture, perhaps from global warming?  Sea surface temperatures certainly are above normal in the region, as they are this year in much of the Atlantic basin.</p>
<p><a href="https://i.sstatic.net/y7lpKm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/y7lpKm.png" alt="Enter image description here" /></a></p>
<p>But a rough try at lining up NYC sounding data shows that while precipitable water level was very high at NYC, above the daily record, it wasn't miles off the charts for this time of year:</p>
<p><a href="https://i.sstatic.net/D6hXXm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/D6hXXm.png" alt="Enter image description here" /></a></p>
<p>And plotting record dewpoints shows the surface moisture was also not extraordinary whatsoever (for even just fall months, which have lower absolute humidity than summer) at Central Park (<a href="https://mesonet.agron.iastate.edu/sites/obhistory.php?station=NYC&amp;network=NY_ASOS&amp;metar=0&amp;madis=0&amp;year=2021&amp;month=9&amp;day=1&amp;sortdir=asc" rel="nofollow noreferrer">obs</a> vs <a href="https://mesonet.agron.iastate.edu/plotting/auto/?_wait=no&amp;q=155&amp;network=NY_ASOS&amp;zstation=NYC&amp;var=max_dwpf&amp;threshold=&amp;hour=12&amp;sdate=2000%2F01%2F01&amp;edate=2000%2F12%2F31&amp;month=fall&amp;dpi=100&amp;_fmt=png" rel="nofollow noreferrer">records</a>) or Somerville (<a href="https://mesonet.agron.iastate.edu/sites/obhistory.php?station=SMQ&amp;network=NJ_ASOS&amp;metar=0&amp;madis=0&amp;year=2021&amp;month=9&amp;day=1&amp;sortdir=asc" rel="nofollow noreferrer">obs</a> vs <a href="https://mesonet.agron.iastate.edu/plotting/auto/?_wait=no&amp;q=155&amp;network=NJ_ASOS&amp;zstation=SMQ&amp;var=max_dwpf&amp;threshold=&amp;hour=12&amp;sdate=2000%2F01%2F01&amp;edate=2000%2F12%2F31&amp;month=fall&amp;dpi=100&amp;_fmt=png" rel="nofollow noreferrer">records</a>).</p>
<p>Did the extra intensity from the existing rainfall / &quot;brown ocean effect&quot; over the deep south sustaining the storm a bit longer and giving it perhaps a bit larger of moisture envelope, shift the common ET-transition tornado zone caused by <a href="https://journals.ametsoc.org/view/journals/wefo/19/2/1520-0434_2004_019_0411_mdiaaf_2_0_co_2.xml?tab_body=pdf" rel="nofollow noreferrer">dry slot interaction</a> further up north than the more common mid-Atlantic/<a href="https://en.wikipedia.org/wiki/Tennessee_Valley" rel="nofollow noreferrer">Tennessee Valley</a> areas?  Maybe?  But it would take a lot more to prove.</p>
<p>Or was it just a small band happened to come together just right to produce a few supercells, as does happen on slight occasion (tornadoes are rare in the northeast, but not entirely unknown, <a href="https://worldpopulationreview.com/state-rankings/tornado-alley-states" rel="nofollow noreferrer">usually having a modest regional outbreak every few years</a> in areas like MD/PA/NJ/NY)?  Though it certainly was fairly <a href="https://twitter.com/JakeCarstens/status/1433220767261593602" rel="nofollow noreferrer">above the typical</a>.</p>
<p>Regardless, the models did absolutely remarkable with it, leading to very strong forecasts for both tornadoes and rainfall by the morning of:</p>
<p><a href="https://i.sstatic.net/MF8NFm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/MF8NFm.png" alt="Enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/s7go0m.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/s7go0m.png" alt="Enter image description here" /></a></p>
<p>both rare levels of risk, especially for the northeast (ditto success on <a href="https://twitter.com/TAG42481/status/1432441352424468485" rel="nofollow noreferrer">NHC</a> forecast skill).</p>
<p>The extreme flood damage was quite clearly due in very large part to the excessive rain <strong>rates</strong> and wet soils from recent storms.  What were the exact causes for those factors?  It's a complex task to unravel, and pinning it to one specific thing is often dubious meteorologically... just as there are a lot of factors that lead to any particular tornado outbreak or heavy rain event -- that's why they have historically been very hard to forecast until the advent of improved computer models.</p>

CC BY-SA 4.0
2021-09-12T01:20:35.923Z
Given your commentary on my answer I hoped you'd put something together, and you haven't disappointed.
Ash
CC BY-SA 4.0
2021-09-12T01:24:39.060Z
To all: for a broad idea of what can bring heavy rainfall, it may also be good to look at a somewhat related [question on Hurricane Harvey from 2017](https://earthscience.stackexchange.com/questions/12159/how-can-a-storm-drop-40-inches-1-metre-of-rain)... I never focused that much on tropical cyclone flooding, but seems to be a topic that I always end up in around here! 8-)
JeopardyTempest
CC BY-SA 4.0
2021-09-12T06:02:40.197Z
@gansub never took any courses on it particularly (a very basic hydrology), never paid it much attention (much more focus on the more "exciting" less straight-out-deadly severe weather and tropical cyclones themselves)... but somehow I always seem to find it a more nuanced and unique question than a basic supercell\severe weather\hurricane one (not that we get many of those!), and always find myself in a season to let myself ramble this time of year I guess! (I really didn't even get a chance to follow Ida too excessively compared to other storms... pretty straightforward, see forecast skill)
JeopardyTempest
CC BY-SA 4.0
2021-09-12T06:13:46.793Z
@gansub I can't seem to get the 36 km to show up.  Always forget about the more exotic satellites for stuff like that.  But never seem to be able to get them to work half the time!  Seems a neat website, exciting to see such a wide array of data potentially in one place (believe it or not I'm not sure I've ever been there before, maybe long ago... amazing how there's still new websites out there every day to be stumbled upon) if it'd work.  UX these days seems miserable, I'd rather have a basic interface, even basic data, that'd load quick and reliably.
JeopardyTempest
CC BY-SA 4.0
2021-09-12T06:40:47.407Z
I kind of gave a soapbox to Ash about what I envision the term brown ocean to mean (standing water rather than soil moisture) (despite not hearing it before this storm!!!)  The 2002 Chen paper solutions suggest to me that there's a giant difference between vast amounts of shallow standing water (swamps and extreme floods) versus just soil moisture.
JeopardyTempest
CC BY-SA 4.0
2021-09-12T06:44:19.873Z
I'm certainly no expert, but considering so much of the moisture budget is usually from not just core evaporation but convergence over giant regions I'd think it'd be hard to maintain inches per hour for long at all and quietly have a hunch synoptics are much more the key than realized and the models used overdo the effects.  For this storm overall rainfall, air moisture levels, and intensity didn't look too abnormal, but rate and runoff, so would think it's not a huge thing, albeit a small factor at least.
JeopardyTempest
CC BY-SA 4.0
2021-09-12T06:59:14.633Z
But you sound like you actually know the moisture topic more thoroughly... have similar effects been seen in more typical cold core cyclones (since it seemed fairly post tropical, though also can't seem to find phase space diagrams for it :-( )... any attempt to cobble together an MPI for such conditions... or any satellite products that attempt to show evaporation? I'd think those ~25C ocean waters would add a greater moisture pump, especially given spatial scale and unquenchable nature, compared to the 26-28 C max air around those days in the area.  But I certainly don't know with certainty!
JeopardyTempest
CC BY-SA 4.0
2021-09-12T07:53:15.103Z
Does NOAA really still work in these arcane units of measurement?
Vladimir F Героям слава
CC BY-SA 4.0
2021-09-12T01:08:56.343Z
2023-06-30T09:38:14.290Z
2023-06-30T09:38:14.290Z
JeopardyTempest
<p>You're asking about several different aspects so let me try and get them in order.</p>
<p>The slow drop off in rainfall is in fact because of the <a href="https://en.wikipedia.org/wiki/Brown_ocean_effect" rel="nofollow noreferrer">Brown Ocean Effect</a> just a bigger one than we've seen before. Once Ida made landfall it was crossed one hot, saturated watershed after another and fed on their heat and moisture to extend its range. Because of how unseasonably wet and hot the land was Ida was, in effect, over open, tropical, water until it hit the Appalachian watershed divide.</p>
<p>The Appalachians average only 910m (3000ft) so they don't block as much weather as the name &quot;mountains&quot; suggests they should, there is very little <a href="https://en.wikipedia.org/wiki/Rain_shadow" rel="nofollow noreferrer">rain shadow</a> effect across the range and then only around the higher peaks. They made very little difference to Ida just as they have to many other weather systems.</p>
<p>The higher precipitation at the northeastern end of Ida's track was largely down to the fact that it finally lost it's brown ocean and started to lose strength and dump rain in a way that is more typical of a land bound tropical storm. It also ran into colder air that force it to lose yet more moisture.</p>

CC BY-SA 4.0
2021-09-11T09:07:46.110Z
Very very poor/misleading graphic and flawed conclusions, as https://mobile.twitter.com/nwswpc/status/1433448454898999299 and https://origin.wpc.ncep.noaa.gov/discussions/nfdscc4.html indicate clearly, other than right near landfall, Ida produced it's heaviest rainfall over the northeast. The movement speed seemed fairly common by that point (4 days to transit from the Gulf to Canada).
JeopardyTempest
CC BY-SA 4.0
2021-09-11T09:33:34.120Z
The brown ocean effect as noted in the Wiki article and papers like [Shen et al](https://journals.ametsoc.org/view/journals/atsc/59/4/1520-0469_2002_059_0789_aniols_2.0.co_2.xml) and [Evans et al](https://journals.ametsoc.org/view/journals/mwre/139/12/2011mwr3593.1.xml) is about [re]strengthening the storm itself... and is firstmost about excess surface water which even with local heavy rains in TN a few days earlier, RFC data suggest was mainly only a factor in S LA [much of the midAtlantic has been below normal in recent months)
JeopardyTempest
CC BY-SA 4.0
2021-09-11T09:51:20.803Z
... indeed the cyclone weakened rapidly further inland, downgraded to a TD over central MS, much further south than many storms make it. If it was "in effect over tropical water" until the Appalachians, it wouldn't have died like that.  So in the traditional meaning of the term, the "brown ocean effect" wasn't a real player further north, as it didn't restrengthen.  Certainly some of the moisture over the south may've made it further north, and what you say about the Appalachians and airmass meetup have some truth, but overall this answer is fairly poor and misleading :-(
JeopardyTempest
CC BY-SA 4.0
2021-09-11T17:33:02.270Z
@JeopardyTempest's data syncs with what I was seeing when I was researching this a bit, can you expand on your answer to discuss this impact in greater detail? Based on the graphic you're depicting, it seems that maybe when considering overall volume the storm's precipitation reduced by virtue of the width of the impact band narrowing but that doesn't account for the startling high rainfall peaks observed in PA, NJ, NY, etc. The observed precipitation depths were akin to at least a 100-year storm, 24-hour storm if not greater. I would expect that from a hurricane off the ocean, not overland.
Pyrotechnical
CC BY-SA 4.0
2021-09-11T19:31:00.080Z
That graphic really looks like a forecast to me, not a map of actual precipitation totals measured after the fact.  Can you cite where you got it from?
Michael Seifert
CC BY-SA 4.0
2021-09-11T19:59:39.737Z
The graphic is definitely an AccuWeather forecast and not precip totals. See @JeopardyTempest's WPC twitter link. As he's said, the brown ocean effect was not a factor after Mississippi.
Bob Smith
CC BY-SA 4.0
2021-09-12T00:57:46.803Z
@JeopardyTempest Apologies it was presented at source as a finals graphic.
Ash
CC BY-SA 4.0
2021-09-12T01:31:03.737Z
No worries, I'm sorry if I came off harsh :-) It nudged some frustrations in me, nothing personal to you... and I'm guessing your sources (like many in weather media) may be some of the cause. Got a bit of passion to see things presented with proper depth, and half the time it seems Accuweather, Weather Channel, the national news networks, etc, just kludge together a 5 minute article written by some inexperienced (at best) meteorologist, often wrong/poor. Anyways, I appreciate your answer, even flawed details give room for greater understanding. Hopefully my answer isn't too much of a book.
JeopardyTempest
CC BY-SA 4.0
2021-09-12T01:33:28.893Z
Just this week NYPost had an article suggesting Larry was going to hit the northeast, likely because they kind of merged the Ida and Larry information and the person didn't know better, or at least made a lot of typos.  Like I said, not that the bigger networks can brag, as how often there's not just some bad grammar but bad (out of date, confused, or just plain wrong) information in many of theirs.
JeopardyTempest
CC BY-SA 4.0
2021-09-12T01:38:28.550Z
I should say after digging in a lot, you're not maybe not 111% technically wrong on the brown ocean... only because it did grow from 1000 mb to 998 and 30 mph to 40. And the way the wikipedia article puts it, any existing soil moisture effects are under the scope of brown ocean (though honestly I'd think most of the slight deepening in the midatlantic were synoptic in cause rather than soilmoisture, but can't conclusively say)
JeopardyTempest
CC BY-SA 4.0
2021-09-12T01:41:07.233Z
(I'd personally never heard the term until the day of Ida's landfall... though plenty familiar with the two separate effects, warm swamps played a part in Katrina maintaining/intensifying over the FL Everglades, setting it up for trouble in Louisiana... and soil moisture helped extratropical Erin do damage in OK in 2007.  But would say they're two fairly different effects\features, and wouldn't group them under one term... I doubt soil moisture will keep a tropical cyclone going, and likening it to an ocean seems a bit excessive :-p  The Everglades\Louisiana swamps on the other hand, totally.)
JeopardyTempest
CC BY-SA 4.0
2021-09-12T01:48:07.040Z
@JeopardyTempest I'd never heard the term but instinctively knew from the track that the effect was a factor.
Ash
CC BY-SA 4.0
2021-09-11T02:47:53.770Z
2021-09-12T00:58:47.147Z
2021-09-12T00:58:47.147Z
Ash
<p>A hurricane is a system dynamic of moisture and warm air. As they go north, they get colder, and when they run out of energy, the moisture condenses in cooler climate. Precipitation is inevitable.</p>
<p>An average thunderstorm cloud contains enough water drops to fill up approximately 275 million gallons, and a hurricane can store several billion.</p>

CC BY-SA 4.0
2021-09-13T02:17:35.493Z
The missing thing with what you say is that air cooling doesn't really directly mean precipitation, because without rising motion to cause more widespread mass-condensation, you've just got fog (you also have the issue of how it cools... self-cooling is slow in humid air... and if the air is cooling by mixing with a separate source of cooler air, that also mixes the dryness of the new air in too). The big thing is that warm air rises over cool air because it's less dense... so encountering cool air provides the lift needed. Not by directly adding cool to the air, but lift\adiabatic cooling.
JeopardyTempest
CC BY-SA 4.0
2021-09-11T05:03:39.793Z
2021-09-13T11:32:57.077Z
2021-09-13T11:32:57.077Z
LazyReader
<p>The US Northeast was hit surprisingly hard by Hurricane Ida, dumping a phenomenal amount of rain on the region, causing significant flooding and tornadic activity. I'm having difficulty understanding why the hurricane was able to travel so far inland with as much strength, particularly with regards to precipitation, given several mitigating factors:</p>
<ul>
<li>Nearly 1,200 miles (1,900 km) of overland travel</li>
<li>The intervening <a href="https://en.wikipedia.org/wiki/Appalachian_Mountains" rel="noreferrer">Appalachian Mountain range</a></li>
</ul>
<p><a href="https://www.livescience.com/hurricane-ida-brown-ocean.html" rel="noreferrer">This article</a> from LiveScience suggests that a <a href="https://en.wikipedia.org/wiki/Brown_ocean_effect" rel="noreferrer">brown ocean effect</a> occurred, which allowed Ida to retain strength further inland than normal due to the marshy areas north of Louisiana, but I would think that impact would've dissipated upon hitting the mountains.</p>
<p>Additionally, per <a href="https://www.nbcnews.com/news/weather/map-here-s-how-much-rain-ida-dumped-northeast-n1278428" rel="noreferrer">several</a> <a href="https://www.northjersey.com/story/news/2021/09/02/rainfall-totals-nj-ida-tropical-storm-hurricane/5693060001/" rel="noreferrer">news</a> <a href="https://www.nbcboston.com/news/local/how-much-rain-did-you-get-from-ida-check-the-latest-totals/2482492/" rel="noreferrer">articles</a> the rainfall amounts in the Northeast were peaking between 5-10 inches (130-260 mm). But somehow, this is more than totals seen <a href="https://www.tennessean.com/story/news/local/2021/08/30/ida-warnings-and-alerts-middle-tennessee/5657600001/" rel="noreferrer">for</a> <a href="https://www.tennessean.com/story/weather/2021/09/01/rainfall-totals-hurricane-ida-nashville-middle-tennesse/5678999001/" rel="noreferrer">Tennessee</a>, which saw around 5 inches (130 mm).</p>
<p>Were there other sources that could've caused Ida's precipitation capacity to increase as it traveled more than 1,200 miles (1,900 km) overland?</p>

<tropical-cyclone>
Why did Hurricane Ida have so much precipitation when it reached the Northeast?
Pyrotechnical
# Why did Hurricane Ida have so much precipitation when it reached the Northeast?

(Asked by: username_0 on 2021-09-10)

The US Northeast was hit surprisingly hard by Hurricane Ida, dumping a phenomenal amount of rain on the region, causing significant flooding and tornadic activity. I'm having difficulty understanding why the hurricane was able to travel so far inland with as much strength, particularly with regards to precipitation, given several mitigating factors:

* Nearly 1,200 miles (1,900 km) of overland travel
* The intervening [Appalachian Mountain range](https://en.wikipedia.org/wiki/Appalachian_Mountains)

[This article](https://www.livescience.com/hurricane-ida-brown-ocean.html) from LiveScience suggests that a [brown ocean effect](https://en.wikipedia.org/wiki/Brown_ocean_effect) occurred, which allowed Ida to retain strength further inland than normal due to the marshy areas north of Louisiana, but I would think that impact would've dissipated upon hitting the mountains.

Additionally, per [several](https://www.nbcnews.com/news/weather/map-here-s-how-much-rain-ida-dumped-northeast-n1278428) [news](https://www.northjersey.com/story/news/2021/09/02/rainfall-totals-nj-ida-tropical-storm-hurricane/5693060001/) [articles](https://www.nbcboston.com/news/local/how-much-rain-did-you-get-from-ida-check-the-latest-totals/2482492/) the rainfall amounts in the Northeast were peaking between 5-10 inches (130-260 mm). But somehow, this is more than totals seen [for](https://www.tennessean.com/story/news/local/2021/08/30/ida-warnings-and-alerts-middle-tennessee/5657600001/) [Tennessee](https://www.tennessean.com/story/weather/2021/09/01/rainfall-totals-hurricane-ida-nashville-middle-tennesse/5678999001/), which saw around 5 inches (130 mm).

Were there other sources that could've caused Ida's precipitation capacity to increase as it traveled more than 1,200 miles (1,900 km) overland?

**Tags:** tropical-cyclone


---

## 3 answers

### Answer by username_1 on 2021-09-12. Score: 19

First, it's good to note that it really wasn't much of Ida that went that far... in terms of the strength of the low pressure (and the [resultant wind](https://earthscience.stackexchange.com/a/542/6298)), while [the swampy waters in southern Louisiana](https://www.researchgate.net/figure/Land-use-land-cover-LULC-types-for-the-four-parish-study-area-in-southwestern_fig2_257129109) may've helped it maintain category 4 strength for an extra few hours inland, it decayed rapidly soon after, [becoming a depression in Mississippi](https://www.nhc.noaa.gov/archive/2021/graphics/AL092021/AL092021_5day_cone_no_line_and_wind_19.png), and [extratropical](https://en.wikipedia.org/wiki/Extratropical_cyclone) [near the Kentucky/West Virginia border](https://www.wpc.ncep.noaa.gov/tropical/tropical_advisories.php?storm=IDA&adnum=26&dt=2021090115&status=posttrop). By the time it passed through the northeast, it was just a 998-1000 mb low pressure with sustained winds of only 30-40 mph. As a low pressure system it was quite pedestrian in even fairly recent history:

*(Lower on the list = more intense low pressure), values are mostly peaks, at landfall if applicable*

| Storm | When | Pressure | Sustained Winds |
| --- | --- | --- | --- |
| [Typical daily pressures](https://w1.weather.gov/data/obhistory/KNYC.html) |  | ~ 1005-1020 mb |  |
| Tropical Storm Fay | July 2020 | 1000 mb | 45 mph |
| [**Remnants of Ida**](https://www.wpc.ncep.noaa.gov/tropical/IDA/IDA_archive.shtml) | **Sept 2021** | **998-1000 mb** | **30-40 mph** |
| Tropical Storm Henri | Aug 2021 | 989 mb | 60 mph |
| 2017 Nor'easter | Feb 2017 | 969 mb | 43 mph |
| Hurricane Irene | Aug 2011 | 965 mb | 65 mph |
| 2021 Nor'easter | Feb 2021 | 960 mb | 80 mph |
| Hurricane Sandy | Oct 2012 | 946 mb | 80 mph |
| *Ida at Louisiana landfall* | *Aug 2021* | *930 mb* | *150 mph* |

Most tornadoes and many heavy rainfall events come outside of hurricanes, and a large number come from fairly weak lows. They're both so much about how specific localized conditions lineup, not so much the strength of the low pressure itself.

---

Now, first to get a couple of the secondary questions out of the way:

In regard to rain shadows, they're just as much about wind direction and surrounding factors.
In the fairly arid intermountain western US for example, storms come in with fairly limited total moisture as is (due to the cold east Pacific temperatures), then wring out quite a bit of what they have as widespread precipitation forced by its air rising over the Cascades and Sierras... but as important is that pre-storm wind directions from the northeast/east/southeast/south have very little access to water due to the basin nature of the whole region and large distances to water. In comparison, most precipitation comes ahead of low pressures in the northeast because this is the wind direction:

[![enter image description here](https://i.sstatic.net/hRP24m.jpg)](https://i.sstatic.net/hRP24m.jpg)

As transitioning tropical cyclones work north from the Gulf (or regular lows work east from the Plains), they drop plenty of precipitation, and sure [you can pick out](https://d32ogoqmya1dw8.cloudfront.net/images/eslabs/weather/us_ppt_1971-2000.v2_1368.png) some extra moisture is deposited from rising air on the windward side of the Appalachians... but the pre-storm fetch for the east coast is over [the warm Gulf-Stream](https://www.ospo.noaa.gov/data/sst/contour/namerica.cf.gif) of the western Atlantic, so moisture is well preserved... as the northeast coastal areas often finds out too well in the winter months when snow piles in from [nor'easters](https://en.wikipedia.org/wiki/Nor%27easter). Moisture profile isn't about where the center of the low pressure goes, but the flow around it. In nor'easters, a just-offshore track tends to favor more snow not because a track over the Appalachians would steal the moisture, but because of how it maximizes the onshore flow most perpendicularly to the strongest temperature gradient to create the most lift (and because a track offshore means you're in the cold sector, a path over land means you spend more time in the warm sector (= perhaps not snow). The low profile of the Appalachians and ability of air to flow around it from ample water areas of the Gulf, Atlantic, and even Great Lakes means there really isn't any great rainshadow in the eastern US.

In terms of tornadoes, the overall count was [fairly regular](https://en.wikipedia.org/wiki/List_of_tornadoes_spawned_by_tropical_cyclones#2020%E2%80%932021) for tropical cyclones.

---

Now for rainfall... how much was there?

[![enter image description here](https://i.sstatic.net/xXK9d.jpg)](https://i.sstatic.net/xXK9d.jpg)
***(remind me in a few months to update with a better graphic that the WPC produces for tropical cyclones)***

Is that extraordinary? Yes... and no... and yes... and no... and then very much yes!!!

Those are some rainfall records listed up above, so it's certainly a notable amount in spots.

However, certainly compared to rainfall when tropical cyclones stall further south, it's a [drop in the bucket](https://en.wikipedia.org/wiki/List_of_wettest_tropical_cyclones). In this very event, Ida, a few spots in Louisiana and Mississippi had [nearly double the rain](https://twitter.com/GCarbin/status/1432302397720862723) that even some of the harder hit spots in the northeast got. Spots along the northeast Texas Gulf Coast got nearly *60 inches* (1500 mm) of rain from [Hurricane Harvey](https://en.wikipedia.org/wiki/Hurricane_Harvey) in 2017, and then another 40 inches (1000 mm) from weak [Tropical Storm Imelda](https://en.wikipedia.org/wiki/Tropical_Storm_Imelda) in 2019 (and looks like they could see another potential event next week).

Still, one person's mountain is another person's molehill. The northeast doesn't have particularly great permeable soil.

[![Enter image description here](https://i.sstatic.net/o00Q2.png)](https://i.sstatic.net/o00Q2.png)
([Source NASA's Earth Observatory](https://earthobservatory.nasa.gov/images/87220/soil-composition-across-the-us))

And that's where there's soil. The northeast is... kind of known as a [heavily urbanized area](https://en.wikipedia.org/wiki/List_of_United_States_urban_areas#/media/File:USA-Urban-Areas.svg).

So a comparable amount of rain in the New York metro versus most of Louisiana means more.

But... what's most surprising to me is that, when you compare the rainfall maps to recent events overall... it really doesn't stand out much!!!

[![enter image description here](https://i.sstatic.net/Fs970.png)](https://i.sstatic.net/Fs970.png)

[![enter image description here](https://i.sstatic.net/sIJqz.png)](https://i.sstatic.net/sIJqz.png)
A swath of like 5-8 inch rainfall in all three of the (at some point) hurricanes, each with a few higher inches spots. Not off the charts.

Yet, at least from what we saw on television from afar around the country, it seemed like this storm's (freshwater) flooding was comparably quite extreme. (Though honestly the picture from the media often makes it seem like total widespread devastation when it's really not. Many areas in southeastern Louisiana are quite reasonable, as were decent amounts of Houston in Harvey and south/central Florida in [Hurricane Irma](https://en.wikipedia.org/wiki/Hurricane_Irma). It does seem harder to get a real feel of the true scope of an event from the media anymore ☹️) But based upon the proliferation of awful videos of raging waters and collapsing basements around NYC and satellite images [like these](https://www.npr.org/sections/pictureshow/2021/09/05/1034459845/satellite-photos-flooding-ida-new-jersey), and the list of different areas involved, it seems to have been fairly notable compared to most systems.

So where does that leave us. Get to the point, was it really that bad?!?

The answers looks be yes. For two reasons I can see/guess at.

#1 is that the rainfall came two weeks after similar rainfall from Henri. The soil was saturated, so less rainfall went into the soil. The great Coast NOAA hurricane finder suggests only three times in past history has New York City had two tropical cyclones (or remnants) pass within 60 miles of NYC within a month... 1888, 1893, and 1985. And the rainfall analyses from [1985-Henri](https://www.wpc.ncep.noaa.gov/tropical/rain/henri1985filledrainwhite.gif) and [Gloria](https://www.wpc.ncep.noaa.gov/tropical/rain/gloria1985filledrainwhite.gif) produced less rain and most of it further from the I-95 corridor. So having two storms in quick succession (and additionally, [suggestions the NYC area was also about 4" above average in July](https://www.weather.gov/wrh/Climate?wfo=okx)), both happening to line up the heavy rain, made things tough.

And #2 is the rainfall **rate**. Here are the greatest 1 hour rainfalls in modern history at Times Square (from Iowa State's dataset):

[![enter image description here](https://i.sstatic.net/9zza2.png)](https://i.sstatic.net/9zza2.png)

Making [other plots](https://mesonet.agron.iastate.edu/plotting/auto/?_wait=no&q=155&network=NY_ASOS&zstation=LGA&var=max_p01i&threshold=100&hour=12&sdate=2000%2F01%2F01&edate=2000%2F12%2F31&month=all&dpi=100&_fmt=png) there's a range of results... [New Brunswick](https://en.wikipedia.org/wiki/New_Brunswick,_New_Jersey)/[Somerville](https://en.wikipedia.org/wiki/Somerville,_Massachusetts), NJ, another badly hit area, similarly blew out their record... other spots weren't quite as bad but some still up flirting with records (as some did with Henri the week before), like Laguardia (2 of top 3 were in Ida) and Trenton (slight record)... whereas Philadelphia and Providence were nowhere on the list, showing it was a fairly narrow swath.

*Why* such heavy rainfall rates?

Because it's getting well beyond a week since the event, data is a bit hard to come by on some key things (many of the best sites keep their data only a few hours to days because of the size... it's something I wish we'd focus on improving, but there's enough to piece together some things these days at least). But I think it actually kind of relates back full circle in connection to the other topic touched on... tornadoes. Intense tornadoes are quite rare in the northeast. The central reason that both intense rain (like >2-3") and tornadoes are scarcer up that way is that very intense deep convection is quite infrequent in the region because it's pretty rare to get instability/strong lift (hot moist air at the surface, cold air aloft, things like strong fronts [all giving the energy for rapidly rising air, allowing moisture to build faster/larger raindrop size]) and helicity (rotation of the winds with height which [allowing the storms to maintain intensity](https://en.wikipedia.org/wiki/Supercell)) to line up. [SPC's sounding analysis](https://www.spc.noaa.gov/exper/soundingclimo/) shows that summer has some decent instability (CAPE) in the northeast, but lower helicity/deep layer shear, as the area is usually far removed from the jet stream and it's fairly rare to gets the enhancement a tropical cyclone can offer. Whereas the ingredients were all reasonably in place for strong persisting updrafts = tornado potential and very heavy rainfall rates.

[![Enter image description here](https://i.sstatic.net/waWav.png)](https://i.sstatic.net/waWav.png)

([Here's a fantastic radar track video of Ida's US life](https://twitter.com/burgwx/status/1433567519810494465)... you can see the deep convection flare up brighter in the brighter yellow and reddish colors in a fairly tight band over NJ, also seen in the white deeper clouds that show up in the same area on [satellite](https://www.youtube.com/watch?time_continue=180&v=0HlhEB2GT88&).)

Why'd it come together like this? Going to be hard to answer too confidently with a quick analysis with limited data.

Was there extra moisture, perhaps from global warming? Sea surface temperatures certainly are above normal in the region, as they are this year in much of the Atlantic basin.

[![Enter image description here](https://i.sstatic.net/y7lpKm.png)](https://i.sstatic.net/y7lpKm.png)

But a rough try at lining up NYC sounding data shows that while precipitable water level was very high at NYC, above the daily record, it wasn't miles off the charts for this time of year:

[![Enter image description here](https://i.sstatic.net/D6hXXm.png)](https://i.sstatic.net/D6hXXm.png)

And plotting record dewpoints shows the surface moisture was also not extraordinary whatsoever (for even just fall months, which have lower absolute humidity than summer) at Central Park ([obs](https://mesonet.agron.iastate.edu/sites/obhistory.php?station=NYC&network=NY_ASOS&metar=0&madis=0&year=2021&month=9&day=1&sortdir=asc) vs [records](https://mesonet.agron.iastate.edu/plotting/auto/?_wait=no&q=155&network=NY_ASOS&zstation=NYC&var=max_dwpf&threshold=&hour=12&sdate=2000%2F01%2F01&edate=2000%2F12%2F31&month=fall&dpi=100&_fmt=png)) or Somerville ([obs](https://mesonet.agron.iastate.edu/sites/obhistory.php?station=SMQ&network=NJ_ASOS&metar=0&madis=0&year=2021&month=9&day=1&sortdir=asc) vs [records](https://mesonet.agron.iastate.edu/plotting/auto/?_wait=no&q=155&network=NJ_ASOS&zstation=SMQ&var=max_dwpf&threshold=&hour=12&sdate=2000%2F01%2F01&edate=2000%2F12%2F31&month=fall&dpi=100&_fmt=png)).

Did the extra intensity from the existing rainfall / "brown ocean effect" over the deep south sustaining the storm a bit longer and giving it perhaps a bit larger of moisture envelope, shift the common ET-transition tornado zone caused by [dry slot interaction](https://journals.ametsoc.org/view/journals/wefo/19/2/1520-0434_2004_019_0411_mdiaaf_2_0_co_2.xml?tab_body=pdf) further up north than the more common mid-Atlantic/[Tennessee Valley](https://en.wikipedia.org/wiki/Tennessee_Valley) areas? Maybe? But it would take a lot more to prove.

Or was it just a small band happened to come together just right to produce a few supercells, as does happen on slight occasion (tornadoes are rare in the northeast, but not entirely unknown, [usually having a modest regional outbreak every few years](https://worldpopulationreview.com/state-rankings/tornado-alley-states) in areas like MD/PA/NJ/NY)? Though it certainly was fairly [above the typical](https://twitter.com/JakeCarstens/status/1433220767261593602).

Regardless, the models did absolutely remarkable with it, leading to very strong forecasts for both tornadoes and rainfall by the morning of:

[![Enter image description here](https://i.sstatic.net/MF8NFm.png)](https://i.sstatic.net/MF8NFm.png)

[![Enter image description here](https://i.sstatic.net/s7go0m.png)](https://i.sstatic.net/s7go0m.png)

both rare levels of risk, especially for the northeast (ditto success on [NHC](https://twitter.com/TAG42481/status/1432441352424468485) forecast skill).

The extreme flood damage was quite clearly due in very large part to the excessive rain **rates** and wet soils from recent storms. What were the exact causes for those factors? It's a complex task to unravel, and pinning it to one specific thing is often dubious meteorologically... just as there are a lot of factors that lead to any particular tornado outbreak or heavy rain event -- that's why they have historically been very hard to forecast until the advent of improved computer models.

> **Comments:**
> **username_2**: Given your commentary on my answer I hoped you'd put something together, and you haven't disappointed.  
> **username_1**: To all: for a broad idea of what can bring heavy rainfall, it may also be good to look at a somewhat related [question on Hurricane Harvey from 2017](https://earthscience.stackexchange.com/questions/12159/how-can-a-storm-drop-40-inches-1-metre-of-rain)... I never focused that much on tropical cyclone flooding, but seems to be a topic that I always end up in around here! 8-)  
> **username_1**: @gansub never took any courses on it particularly (a very basic hydrology), never paid it much attention (much more focus on the more "exciting" less straight-out-deadly severe weather and tropical cyclones themselves)... but somehow I always seem to find it a more nuanced and unique question than a basic supercell\severe weather\hurricane one (not that we get many of those!), and always find myself in a season to let myself ramble this time of year I guess! (I really didn't even get a chance to follow Ida too excessively compared to other storms... pretty straightforward, see forecast skill)  
> **username_1**: @gansub I can't seem to get the 36 km to show up.  Always forget about the more exotic satellites for stuff like that.  But never seem to be able to get them to work half the time!  Seems a neat website, exciting to see such a wide array of data potentially in one place (believe it or not I'm not sure I've ever been there before, maybe long ago... amazing how there's still new websites out there every day to be stumbled upon) if it'd work.  UX these days seems miserable, I'd rather have a basic interface, even basic data, that'd load quick and reliably.  
> **username_1**: I kind of gave a soapbox to Ash about what I envision the term brown ocean to mean (standing water rather than soil moisture) (despite not hearing it before this storm!!!)  The 2002 Chen paper solutions suggest to me that there's a giant difference between vast amounts of shallow standing water (swamps and extreme floods) versus just soil moisture.  
> **username_1**: I'm certainly no expert, but considering so much of the moisture budget is usually from not just core evaporation but convergence over giant regions I'd think it'd be hard to maintain inches per hour for long at all and quietly have a hunch synoptics are much more the key than realized and the models used overdo the effects.  For this storm overall rainfall, air moisture levels, and intensity didn't look too abnormal, but rate and runoff, so would think it's not a huge thing, albeit a small factor at least.  
> **username_1**: But you sound like you actually know the moisture topic more thoroughly... have similar effects been seen in more typical cold core cyclones (since it seemed fairly post tropical, though also can't seem to find phase space diagrams for it :-( )... any attempt to cobble together an MPI for such conditions... or any satellite products that attempt to show evaporation? I'd think those ~25C ocean waters would add a greater moisture pump, especially given spatial scale and unquenchable nature, compared to the 26-28 C max air around those days in the area.  But I certainly don't know with certainty!  
> **username_3**: Does NOAA really still work in these arcane units of measurement?  

---

### Answer by username_2 on 2021-09-11. Score: 8

You're asking about several different aspects so let me try and get them in order.

The slow drop off in rainfall is in fact because of the [Brown Ocean Effect](https://en.wikipedia.org/wiki/Brown_ocean_effect) just a bigger one than we've seen before. Once Ida made landfall it was crossed one hot, saturated watershed after another and fed on their heat and moisture to extend its range. Because of how unseasonably wet and hot the land was Ida was, in effect, over open, tropical, water until it hit the Appalachian watershed divide.

The Appalachians average only 910m (3000ft) so they don't block as much weather as the name "mountains" suggests they should, there is very little [rain shadow](https://en.wikipedia.org/wiki/Rain_shadow) effect across the range and then only around the higher peaks. They made very little difference to Ida just as they have to many other weather systems.

The higher precipitation at the northeastern end of Ida's track was largely down to the fact that it finally lost it's brown ocean and started to lose strength and dump rain in a way that is more typical of a land bound tropical storm. It also ran into colder air that force it to lose yet more moisture.

> **Comments:**
> **username_1**: Very very poor/misleading graphic and flawed conclusions, as https://mobile.twitter.com/nwswpc/status/1433448454898999299 and https://origin.wpc.ncep.noaa.gov/discussions/nfdscc4.html indicate clearly, other than right near landfall, Ida produced it's heaviest rainfall over the northeast. The movement speed seemed fairly common by that point (4 days to transit from the Gulf to Canada).  
> **username_1**: The brown ocean effect as noted in the Wiki article and papers like [Shen et al](https://journals.ametsoc.org/view/journals/atsc/59/4/1520-0469_2002_059_0789_aniols_2.0.co_2.xml) and [Evans et al](https://journals.ametsoc.org/view/journals/mwre/139/12/2011mwr3593.1.xml) is about [re]strengthening the storm itself... and is firstmost about excess surface water which even with local heavy rains in TN a few days earlier, RFC data suggest was mainly only a factor in S LA [much of the midAtlantic has been below normal in recent months)  
> **username_1**: ... indeed the cyclone weakened rapidly further inland, downgraded to a TD over central MS, much further south than many storms make it. If it was "in effect over tropical water" until the Appalachians, it wouldn't have died like that.  So in the traditional meaning of the term, the "brown ocean effect" wasn't a real player further north, as it didn't restrengthen.  Certainly some of the moisture over the south may've made it further north, and what you say about the Appalachians and airmass meetup have some truth, but overall this answer is fairly poor and misleading :-(  
> **username_0**: @username_1's data syncs with what I was seeing when I was researching this a bit, can you expand on your answer to discuss this impact in greater detail? Based on the graphic you're depicting, it seems that maybe when considering overall volume the storm's precipitation reduced by virtue of the width of the impact band narrowing but that doesn't account for the startling high rainfall peaks observed in PA, NJ, NY, etc. The observed precipitation depths were akin to at least a 100-year storm, 24-hour storm if not greater. I would expect that from a hurricane off the ocean, not overland.  
> **username_4**: That graphic really looks like a forecast to me, not a map of actual precipitation totals measured after the fact.  Can you cite where you got it from?  
> **username_5**: The graphic is definitely an AccuWeather forecast and not precip totals. See @username_1's WPC twitter link. As he's said, the brown ocean effect was not a factor after Mississippi.  
> **username_2**: @username_1 Apologies it was presented at source as a finals graphic.  
> **username_1**: No worries, I'm sorry if I came off harsh :-) It nudged some frustrations in me, nothing personal to you... and I'm guessing your sources (like many in weather media) may be some of the cause. Got a bit of passion to see things presented with proper depth, and half the time it seems Accuweather, Weather Channel, the national news networks, etc, just kludge together a 5 minute article written by some inexperienced (at best) meteorologist, often wrong/poor. Anyways, I appreciate your answer, even flawed details give room for greater understanding. Hopefully my answer isn't too much of a book.  
> **username_1**: Just this week NYPost had an article suggesting Larry was going to hit the northeast, likely because they kind of merged the Ida and Larry information and the person didn't know better, or at least made a lot of typos.  Like I said, not that the bigger networks can brag, as how often there's not just some bad grammar but bad (out of date, confused, or just plain wrong) information in many of theirs.  
> **username_1**: I should say after digging in a lot, you're not maybe not 111% technically wrong on the brown ocean... only because it did grow from 1000 mb to 998 and 30 mph to 40. And the way the wikipedia article puts it, any existing soil moisture effects are under the scope of brown ocean (though honestly I'd think most of the slight deepening in the midatlantic were synoptic in cause rather than soilmoisture, but can't conclusively say)  
> **username_1**: (I'd personally never heard the term until the day of Ida's landfall... though plenty familiar with the two separate effects, warm swamps played a part in Katrina maintaining/intensifying over the FL Everglades, setting it up for trouble in Louisiana... and soil moisture helped extratropical Erin do damage in OK in 2007.  But would say they're two fairly different effects\features, and wouldn't group them under one term... I doubt soil moisture will keep a tropical cyclone going, and likening it to an ocean seems a bit excessive :-p  The Everglades\Louisiana swamps on the other hand, totally.)  
> **username_2**: @username_1 I'd never heard the term but instinctively knew from the track that the effect was a factor.  

---

### Answer by username_6 on 2021-09-11. Score: 1

A hurricane is a system dynamic of moisture and warm air. As they go north, they get colder, and when they run out of energy, the moisture condenses in cooler climate. Precipitation is inevitable.

An average thunderstorm cloud contains enough water drops to fill up approximately 275 million gallons, and a hurricane can store several billion.

> **Comments:**
> **username_1**: The missing thing with what you say is that air cooling doesn't really directly mean precipitation, because without rising motion to cause more widespread mass-condensation, you've just got fog (you also have the issue of how it cools... self-cooling is slow in humid air... and if the air is cooling by mixing with a separate source of cooler air, that also mixes the dryness of the new air in too). The big thing is that warm air rises over cool air because it's less dense... so encountering cool air provides the lift needed. Not by directly adding cool to the air, but lift\adiabatic cooling.  

---

