Crozet Weather Almanac: December 2012
By Heidi Sonen & Roscoe Shaw
Strangely enough, Crozet had snow in December when the air was 52 degrees and rain when the temperature was just 30. Although the 52 was our personal record for warm snow, Heidi and I once woke up to pouring rain with thunder and lightning and just 14 degrees! That happened in Ohio and the next day, kids were skating through the neighborhood lawns on ice skates.
How can this happen? The precipitation type is determined mainly by what is happening above you. If rain is falling at 10,000 feet, then nothing can turn it back to snow no matter how cold at the ground. A snowflake, however, can melt into rain at any point on its descent.
As forecasters, correctly assessing precipitation type is fascinating but also tricky. Often, the heaviest snow falls close to the rain/snow line. If your forecast storm track is off by a few miles, your rain forecast can end up 10” deep or vice versa. Heidi and I boil the forecast down into a big math problem with dozens of factors that looks something like this:
POPT = Tsfc + T850 + THK85-5 +Thk10-5 + Tdsfc +Twetave + Twrmlayer
On December 18th, we enjoyed a 60 degree morning before a cold front came through. Temperatures dropped to 52 by 3 p.m. but much colder air had already arrived in the upper atmosphere. Snow fell for about a minute. I ran to my thermometer and it read 52. Amazing.
We have seen snow many times in the 40’s and even upper 40’s but never 52. A snowflake can usually only survive about a 1000 foot fall above the freezing point. In this case, the temperature was remarkably colder just above us for a brief moment.
The opposite case, freezing rain, happens quite a bit here in the winter. The typical scenario is when dense, cold Canadian air wedges up against the Appalachian Mountains. Then, warm, wet air from Gulf of Mexico moves our way. The warm moist air is lighter and gets forced aloft and rain falls into the cold air stuck at the ground. Typically, this will start as snow, then sleet, then freezing rain and finally rain as the cold air erodes and the warm air wins.
December temperatures were almost identical to November. That means November was colder than normal, but December averaged nearly five degrees warmer than usual. Snow or ice fell five days including the day before and the day after our pleasant Christmas day. The day after Christmas was a horrendously miserable day of mixed precipitation resulting in a pile of slop an inch and a half deep.
Our dry spell ended with good rain on the 20th and the messy slop on the 26th got the streams running fast again. Snow totaled two inches but that came in bits and pieces, mostly on the 26th.
- Crozet 2.58”
- CHO Airport 2.36”
- Greenwood 2.79”
- White Hall 2.98”
- Univ. of VA 2.95”
- Nellysford 2.42”
- Shipman 2.87”