Storm Total Snowfall Analysis
Water Vapor Image at 6:15 a.m. (12:15 UTC) 28 December 2012
Water Vapor Image at 12:15 p.m. (18:15 UTC) 28 December 2012
Water Vapor Image at 5:45 p.m (23:45 UTC) 28 December 2012
300mb Upper Air Analysis at 6:00 p.m (00:00 UTC 29 December 2012)
Radar Loop of Snow Event (Noon until 9 p.m. 28 December 2012)
Infrared Satellite Image and Surface Map around 6:00 p.m. (00:00 UTC 29 December 2012)

NWS
St. Louis, Missouri

December 28-29 2012
Surprise Heavy Snow

Heavy snow developed across southeast Missouri and southwest Illinois during the afternoon of Friday December 28th and continued through the evening before tapering off from west to east early Saturday morning. Storm total snowfall ranged from around an inch in St. Louis to more than six inches from Bixby, Missouri to Nashville, Illinois.

Storm Total Snowfall

The official forecast from the National Weather Service the morning of the December 28th only mentioned a chance of snow across portions of southeast Missouri and southwest Illinois.  The heaviest snowfall was expected to remain south of the St. Louis Weather Forecast Office County Warning Area.  So what happened? Why did the heaviest snowfall occur nearly 100 miles to the northwest of the morning forecast?

Forecasters noticed shortly after coming on shift Friday morning that the shortwave (mid-upper level energy) appeared to be much stronger and further north than model guidance had indicated just less than 24 hours ago. This shortwave can easily be seen in water vapor imagery (below) as the dark streak ejecting from northern Mexico across Texas and into the middle to lower Misssissippi River Valley.

water vapor
water vapor

water vapor

Water Vapor at 6:15 a.m.

Water Vapor at 12:15 p.m.

Water Vapor at 5:45 p.m.

Short term model guidance (updated every hour), along with regional radar trends, gave confidence to forecasters to adjust the forecast by midday with accumulating snow now expected across southeast Missouri and southwest Illinois. There was still some question as to how much accumulation would occur and how far north the snow would fall, but a general 1 to 3 inches was put into the forecast. The midday update and issuance of winter weather advisories heightened awareness of the rapidly changing forecast.

Winter weather advisories were posted further north by the time the afternoon forecast package was issued and forecast snowfall amounts were increased further north based on the latest radar trends. The snow continued through the evening with the forecast being fine tuned to capture where the heaviest snow was falling.  A dominant snowband developed (see radar loop below) from western Iron County through southeast Washington County, northern St. Francois and Ste. Genevieve Countie in Missouri and across parts of Monroe, Randolph, St. Clair, and Washington Counties in Illinois.  This band of heavy snow was responsible for producing the heaviest snowfall rates (up to 2 inches per hour) and greatest accumulation. 

The stronger upper level disturbance caused the best dynamics to be located nearly 100 miles further north than what was expected.  In fact, a weak area of low pressure developed along an inverted trof that extended north from the main synoptic low along the Gulf Coast (right image below).  The set-up ended up being ideal for heavy snow with a coupled jet structure noted at 300mb (center image below) which led to increased vertical motion and moisture transport into the area.  In addition, the presence of weak instability noted on short term model cross section analysis (not shown) was responsible for the banded nature of the snow and locally heavy snowfall.    

radar

 300mb

300mb

St. Louis Radar Loop

 300mb 6:00 p.m (00Z)

 IR Satellite and Surface Map

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