Major Winter Storm For The Great Lakes - December 20, 2012

General Description:

A major winter storm affected southern Wisconsin and the western Great Lakes region Wednesday evening through Thursday night, Dec 19th and 20th.

Below is a map of the actual track of the low pressure system Wednesday through Thursday.  The small 2 digit numbers next to the "L" is the pressure in millibars with the "9" missing.  So, "82" would be 982 mb, which was the lowest pressure in the storm when it was crossing the Chicago area.  The storm deepened 1 millibar per hour for 15 hours.  Storms that strengthen 1 mb per hour or more for at least 24 hours are considered "explosive cyclogenesis", a technical term for rapidly deepening storms that are relatively rare.   So, although this storm did not meet the criteria, it came rather close.  The times are in UTC or Zulu, so 18z along the CO/KS border is Noon Wed, 00z over Kansas was 6 pm Wed.

Click on image for larger version. Subtract 6 hours from UTC to arrive at local standard time. 

The image below left, is a 36 hour radar animation of the storm (about 8mb.)  It starts around 1 PM CST Wednesday afternoon, and ends around 8 PM CST Thursday evening.  For reference, see the color scale at the top of the image.  Note the high radar reflectivity returns Wednesday night into Thursday pivoting across south central and southeast Wisconsin.  This lead to a prolonged period of moderate to heavy snow across south central and portions of southeast Wisconsin.  Rain fell across far southeast Wisconsin Wednesday night into Thursday afternoon, before changing to snow late in the day.  

The image below right, is a Water Vapor animation running from mid morning to mid evening on the 20th, Thursday.  This is a large file, about 15mb.  This is from our friends over at UW-Madison's Cooperative Institute for Metoerological Satellite Studies.  Here is a tutorial about WV satellite imagery from the CIMSS folks.

Click the images to see the animation: 

   Photos:

Snow totals:

Two-day snow totals (around 7 am 12/19 through 7 am 12/21) ranged from less than an inch along the Lake Michigan shoreline from Milwaukee south to Kenosha, to 12 to 22 inches in Dane County.  Below are three maps showing the snow accumulations across:

1) South-central/Southeast Wisconsin (automated map),

2) for all of Wisconsin (automated map - caution, this middle map does not capture snow totals of 10 to 12 inches from Appleton to the Green Bay area, and

3) for all of Wisconsin - a hand-drawn version that incoporates all official and unnofficial reports of total accumulations.

Click on image for larger version: 

Below is an image of a few snowfall totals across southern Wisconsin:

The greatest 2-day snow totals (official & unofficial) include 21.9" in Cottage Grove, 21.5" in Mt. Horeb, 21.1" in Columbus 1S, 20.5" in Barneveld, 20.1" in the Town of Westford (near Beaver Dam), 20.0" at 7SW Madison, 19.9" in Middleton, 19.6" in Portage 7SW, 19.0" at the Arlington UW Farm, and 18.8 inches in Horion and DeForest. Officially, Madison's Truax Field's 2-day total was 15.2" and Milwaukee's Mitchell Field came in with 2.8."

What we Predicted:

The thumbnail images below are the snow amounts we predicted. These were posted on our web site during the days leading up to the storm.

Record Snowfalls:

Madison recorded 13.3 inches of snow on Thursday, December 20th, which broke the previous record of 4.6 inches, set in 2000.  

The tables below show how this snowfall ranked in 1, 2 and 3 Day Totals.

Impacts/Winds:  Scattered power outages were reported.  Drifting and falling snow made plowing activities difficult or almost impossible at times.  Road surfaces were either snow covered and/or icy inland away from Lake Michigan.  Hundreds of vehicle accidents were reported, and many vehicles became stuck in snow drifts.  The Wisconsin DOT reported roads in Rock County were nearly impassable Thursday.

The snow and blowing snow likely caused some road closures in rural areas. Northwest winds gusted to 40 to 50 mph during the afternoon and early evening hours of Thursday.  These strong winds, along with falling and/or blowing snow, resulted in blizzard or near blizzard conditions in open and exposed areas, with visibilities reduced to one quarter mile or less. 

Here is a table of Peak Winds during the storm:

Rain/Liguid Equvalent Totals:

Rain amounts of 1.50 to over 2.00 inches, southeast of a line from Ozaukee County to Walworth County, caused water levels in most rivers and streams to rise to near bankfull or bankfull.  However, flood stages were not reached, due to the snow and colder temperatures halting runoff.  The liquid equivalent totals across the rest of the area were high as well, with amounts of 0.75 inches to around 2.00 inches.  This illustrated the high water content in the snow that fell, which resulted in the "heavy and wet" snow.

Click on image for larger version:

Madison recorded 1.16 inches of liquid equivalent precipitation on Thursday, December 20th, which broke the previous record of 0.80 inches, set in 1882.

Milwaukee recorded 2.16 inches of rain and liquid equivalent precipitation on Thursday, December 20th, which broke the previous record of 1.48 inches, set in 1977. 
 

See the table below for the 1, 2 and 3 day rankings for the highest rain/liquid equivalent precipitation for December at Milwaukee:

National Weather Service, Milwaukee/Sullivan, WI