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:
 

Radar loop (8mb)

Water Vapor Imagery (15mb)

 


   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: 

 StormTotalSnowMKX   StormTotalSnowWI   Dec 19-20, 2012 Final Snow Map

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.

Our Forecast:

Click images for
larger version


Sunday Afternoon (12/16)

 
Monday Morning (12/17)

 
Monday Afternoon (12/17)

 

Tuesday Morning (12/18)

 

Tuesday Afternoon (12/18)

 

Wednesday Morning (12/19)

 

Wednesday Afternoon (12/19)

 

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.

Highest 1 Day Total - Madison:

Highest 2 Day Total - Madison:

Highest 3 Day Total - Madison:

Rank Value Date
1 17.3 12/3/1990
2 13.3 12/20/2012
3 13.2 12/15/1987
4 12.9 4/9/1973
5 12.7 12/10/1970
6 12.6

1/26/1996, 1/19/1995

8 12.1 12/1/1985, 1/13/1910
10 12.0 3/18/1971
Rank Value Dates
1 17.3 12/2-12/3, 1990
2 16.1 12/10-12/11, 1970
3 15.4 2/22-2/23, 1994
4 15.2 12/19-12/20, 2012
5 14.5 3/18-3/19, 1971
6 14.3 2/1-2/2, 2011
7 14.2 1/25-1/26, 1996
8 14.1 12/8-12/9, 2009
9 14.0 4/8-4/9, 1973
10 13.7 12/19-12/20, 2008
4/9-4/10, 1973
Rank Value Dates
1 19.0 2/23-2/25, 1994
2 18.7 1/31-2/2, 2011
3 18.0 1/23-1/25, 1898
4 17.3 12/1-12/3, 1990
12/2-12/4, 1990
12/3-12/5, 1990
7 17.1 12/10-12/12, 1970
8 16.6 12/18-12/20, 2012
9 16.1 12/9-12/11, 1970
10 15.7 12/7-12/9, 2009
4/9-4/11, 1973


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:

Airport

Peak Gust (MPH)

Peak Gust (Knots)

Monroe

48

42

Sheboygan

45

39

Platteville

44

38

Mineral Point

44

38

Middleton

44

38

Fond Du Lac

44

38

Milwaukee

44

38

Racine

44

38

Kenosha

43

37

Lone Rock

43

37

Madison

41

36

Waukesha

41

36

Wisconsin Dells

36

31

Janesville

36

31

Timmerman

35

30

Burlington

33

29

Juneau

32

28

Watertown

30

26

West Bend

28

24


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:

StormTotalLiquidMKX   StormTotalLiquidWI

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:

Highest 1 Day Rain or Liquid Equivalent Precipitation - Milwaukee:

Highest 2 Day Rain or Liquid Equivalent Precipitation - Milwaukee: Highest 3 Day Rain or Liquid Equivalent Precipitation - Milwaukee:
Rank Value Date
1 2.24 12/2/1982
2 2.16 12/20/2012
3 1.84 12/24/1979
4 1.76 12/24/1965
5 1.68 12/4/1973
6 1.67 12/1/1985
7 1.59 12/13/1928
8 1.57 12/15/1987
9 1.48 12/20/1977
10 1.45 12/22/2006
Rank Value Dates
1 2.41 12/2-12/3, 1982
2 2.35 12/19-12/20, 2012
3 2.24 12/1-12/2, 1982
4 2.04 12/26-12/27, 1942
5 1.93 12/24-12/25, 1965
6 1.91 12/21-12/22, 2006
7 1.89 12/4-12/5, 1973
8 1.86 12/23-12/24, 1979
9 1.85 12/24-12/25, 1979
10 1.82 12/23-12/24, 1965
Rank Value Date
1 2.54 12/18-12/20, 2012
2 2.46 12/2-12/4, 1982
3 2.41 12/1-12/3, 1982
4 2.11 12/22-12/24, 1979
5 2.07 12/1-12/3, 1873
6 2.05 12/26-12/28, 1942
7 2.04 12/25-12/27, 1942
8 2.01 12/20-12/22, 2006
9 1.99 12/4-12/6, 1973,
12/23-12/25, 1965

Storm summary write-ups from surrounding offices:

NWS Green Bay

NWS La Crosse

NWS Chicago

NWS Twin Cities

NWS Quad Cities

NWS Des Moines


National Weather Service, Milwaukee/Sullivan, WI 


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