<p>Great question!<br>
First, let's look at typical data: here are the 1981-2010 <a href="http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.derived.pressure.html" rel="nofollow noreferrer">Reanalysis</a> mean and standard deviation:</p>

<p><a href="https://i.sstatic.net/fW8cf.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/fW8cf.png" alt="enter image description here"></a><a href="https://i.sstatic.net/TkgyE.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/TkgyE.png" alt="enter image description here"></a></p>

<p>So looks like the mean 500 height in September for Wisconsin ranges from about 571-580 Dm, with a standard deviation of 36-42 Dm.  These are mean pressures, not lows or highs, so already, not that far off your low.  If (<a href="http://www.sjsu.edu/faculty/watkins/normalvariation.htm" rel="nofollow noreferrer">a big if</a>) meteorological statistics are normally distributed, using this <a href="http://onlinestatbook.com/2/calculators/normal_dist.html" rel="nofollow noreferrer">calculator</a>, this would suggest a 560 Dm height would be in about the 29th-40th percentile, depending upon location.  A little lower than usual, but apparently fairly typical.</p>

<p>And just for good measure, looking back at the <a href="http://weather.uwyo.edu/upperair/uamap.html" rel="nofollow noreferrer">University of Wyoming</a>, I was able to fairly quickly come upon:</p>

<p><a href="https://i.sstatic.net/ehqkf.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ehqkf.gif" alt="enter image description here"></a></p>

<p>A 554 or 555 Dm low.  If you want to look for more images (perhaps even the event you are reading about), you could also look in the Pix directory for the dates of interest at Iowa State's <a href="http://mtarchive.geol.iastate.edu/" rel="nofollow noreferrer">MTArchive</a>.</p>

<p>But it appears that 560 Dm would be a fairly typical height for a low/trough during early fall.  Some people might still call it deep (30 Dm of contours on that plot from September would be fair), but it's nothing too abnormal.</p>

CC BY-SA 3.0
2016-11-22T16:17:14.090Z
I think I remember that event, even living out in Oklahoma at the time.  Coming after September 11th, it stood out I guess.  Think I might've chased the system when it came through the Plains, though it wasn't as noteworthy as October 9th.  But the strength of the low isn't as vital as the surface conditions and phasing.
JeopardyTempest
CC BY-SA 3.0
2016-11-23T06:09:10.883Z
No problem, keep up the good questions!  And, a better place to look for info on the day than the link I did would be http://www.spc.noaa.gov/exper/archive/events/, or particularly http://www.spc.noaa.gov/exper/archive/event.php?date=20010924.  It doesn't have day-before maps, but has morning of, and perhaps would help you look into it more.
JeopardyTempest
CC BY-SA 3.0
2016-11-22T16:10:11.797Z
2016-11-22T16:10:11.797Z
JeopardyTempest
<p>I was reading the paper <a href="http://ams.confex.com/ams/pdfpapers/47601.pdf" rel="nofollow noreferrer">The Washington DC tornado of 24 september 2001: pre-storm environment and radar perspectives</a> which had this passage: </p>

<blockquote>
  <p>At 1200 UTC 24 September 2001, at 500 hPa a deep (560 [gpdm]) cut-off extratropical low (midlatitude cyclone that is cut off from the jet stream) was located over Wisconsin with a central core temperature of -20 C." (). </p>
</blockquote>

<p>Is 560 dm deep for that part of the country and that time of year? </p>

<p>I have revised the question based on suggestions fomr the comments.</p>

CC BY-SA 3.0
2016-11-16T12:09:50.947Z
I'm not sure any set criteria for deep low, even at the surface, as it's a rather subjective criteria, and also varies by season and nearby pressures.  As to why they use it as the plot variable rather than pressure on isoheight surfaces... I don't know, a good question.  It does allow direct thickness calculation... but if you use the hydrostatic approximation, you can switch to p coordinates.  Perhaps it's because soundings historically reported the special pressure levels, and so you'd have needed to record/transmit more data for isoheight?
JeopardyTempest
CC BY-SA 3.0
2016-11-16T12:12:27.700Z
As to what you ask about hydrostatic pressure changing with gravity, I'm not sure I understand your question fully?  If you're asking do variations of gravity come into it, I suppose they would... but for the troposphere, gravity is for all-intents-and-purposes constant.  And hydrostatic is already an approximation indeed... such gravity variations are likely of a smaller scale than other neglected terms.  But perhaps this is better as it's own question and with more explanation\details?
JeopardyTempest
<meteorology><barometric-pressure><troposphere>
With respect to latitude and time of year, what geopotential heights are associated with low pressure systems on the North American landmass?
spillthrill
# With respect to latitude and time of year, what geopotential heights are associated with low pressure systems on the North American landmass?

(Asked by: username_0 on 2016-11-16)

I was reading the paper [The Washington DC tornado of 24 september 2001: pre-storm environment and radar perspectives](http://ams.confex.com/ams/pdfpapers/47601.pdf) which had this passage:

> At 1200 UTC 24 September 2001, at 500 hPa a deep (560 [gpdm]) cut-off extratropical low (midlatitude cyclone that is cut off from the jet stream) was located over Wisconsin with a central core temperature of -20 C." ().

Is 560 dm deep for that part of the country and that time of year?

I have revised the question based on suggestions fomr the comments.

**Tags:** meteorology, barometric-pressure, troposphere

> **Comments:**
> **username_1**: I'm not sure any set criteria for deep low, even at the surface, as it's a rather subjective criteria, and also varies by season and nearby pressures.  As to why they use it as the plot variable rather than pressure on isoheight surfaces... I don't know, a good question.  It does allow direct thickness calculation... but if you use the hydrostatic approximation, you can switch to p coordinates.  Perhaps it's because soundings historically reported the special pressure levels, and so you'd have needed to record/transmit more data for isoheight?  
> **username_1**: As to what you ask about hydrostatic pressure changing with gravity, I'm not sure I understand your question fully?  If you're asking do variations of gravity come into it, I suppose they would... but for the troposphere, gravity is for all-intents-and-purposes constant.  And hydrostatic is already an approximation indeed... such gravity variations are likely of a smaller scale than other neglected terms.  But perhaps this is better as it's own question and with more explanation\details?  

---

## 1 answers

### Answer by username_1 on 2016-11-22. Score: 2 | Accepted answer

Great question!  
First, let's look at typical data: here are the 1981-2010 [Reanalysis](http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.derived.pressure.html) mean and standard deviation:

[![enter image description here](https://i.sstatic.net/fW8cf.png)](https://i.sstatic.net/fW8cf.png)[![enter image description here](https://i.sstatic.net/TkgyE.png)](https://i.sstatic.net/TkgyE.png)

So looks like the mean 500 height in September for Wisconsin ranges from about 571-580 Dm, with a standard deviation of 36-42 Dm. These are mean pressures, not lows or highs, so already, not that far off your low. If ([a big if](http://www.sjsu.edu/faculty/watkins/normalvariation.htm)) meteorological statistics are normally distributed, using this [calculator](http://onlinestatbook.com/2/calculators/normal_dist.html), this would suggest a 560 Dm height would be in about the 29th-40th percentile, depending upon location. A little lower than usual, but apparently fairly typical.

And just for good measure, looking back at the [University of Wyoming](http://weather.uwyo.edu/upperair/uamap.html), I was able to fairly quickly come upon:

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

A 554 or 555 Dm low. If you want to look for more images (perhaps even the event you are reading about), you could also look in the Pix directory for the dates of interest at Iowa State's [MTArchive](http://mtarchive.geol.iastate.edu/).

But it appears that 560 Dm would be a fairly typical height for a low/trough during early fall. Some people might still call it deep (30 Dm of contours on that plot from September would be fair), but it's nothing too abnormal.

> **Comments:**
> **username_1**: I think I remember that event, even living out in Oklahoma at the time.  Coming after September 11th, it stood out I guess.  Think I might've chased the system when it came through the Plains, though it wasn't as noteworthy as October 9th.  But the strength of the low isn't as vital as the surface conditions and phasing.  
> **username_1**: No problem, keep up the good questions!  And, a better place to look for info on the day than the link I did would be http://www.spc.noaa.gov/exper/archive/events/, or particularly http://www.spc.noaa.gov/exper/archive/event.php?date=20010924.  It doesn't have day-before maps, but has morning of, and perhaps would help you look into it more.  

---

