A storm system moved out of the Rockies, tracked through the Central Plains, and then moved over the Great Lake States on Saturday, December 1st. Strong warming out ahead of the heart of the system led to areas of precipitation developing well to the northeast of the surface low. Precipitation was spreading across Iowa early Saturday morning, working into southeast Minnesota and northeast Iowa by mid morning, and then across the Mississippi River into western and southwest Wisconsin by late morning. Snow fell initially, leading to some quick accumulations. However, warm air just above the surface was pressing north into the region, which caused the snow to change over to sleet over northeast Iowa and southwest Wisconsin by late morning. The warm air would continue to advance northward, leading to a change over to sleet over southeast Minnesota and western Wisconsin between 2 and 4 pm. The only areas locally to stay mostly snow were north of Highway 29 in northern Wisconsin (north of Neillsville).
Snow and sleet accumulations of 4 to 6 inches were common from an Austin, MN to Decorah, IA to Mauston, WI line. In most cases, a quarter of the totals were the result of the sleet. Some locations reported as much as 1 to 2 inches of sleet, with one report coming in at over 3 inches!
By the evening, sleet was the dominant precipitation type, and the bulk of the accumulations were shifting into eastern Wisconsin by 9 pm. However, areas of freezing drizzle and light freezing rain developed behind the sleet, leading to some glazing of ice (a couple tenths of an inch in accumulation in many cases).
The warm air kept on coming though, with temperatures warming to, or above freezing across much of the area by mid to late evening. This reduced, or eliminated, the threat for freezing precipitation...and thus lessened the potential for even more ice accumulation.
Travel was very treacherous for much of the afternoon and evening, with the ice glaze adding another dangerous ingredient to the travel conditions. Some area road crews were pulled off the roads for a short time Saturday night on account of the very slippery roads.
The NWS in La Crosse issued a Winter Storm Watch around 2 pm on Thursday for Saturday and Saturday Night calling for significant winter weather to impact the area. On Friday afternoon, that Watch was upgraded to a Winter Storm Warning for Saturday and Saturday Night. The graphical Weather Story, which summarized the threats to the public on Friday afternoon, remained very consistent through the storm (graphic below).
Graphic Forecast of Storm Totals
Snow and Sleet Totals
This storm will be remembered for the wintry mix of precipitation it brought, rather than its snow totals. Starting as snow, then becoming dominated by sleet, with freezing rain and freezing drizzle mixing in at the end...this storm brought a bit of everything. So why did this happen?
Warm air was the culprit...and a lot of it. A very strong push of warm, moist air in the low levels of the atmosphere was pulled northward ahead of the winter storm, which actually caused temperatures to rise through Saturday evening. Many locations across the region recorded their high temperatures for the day just before midnight!
At the onset of the precipitation, cold air was still in place, and snow fell as a result. By mid morning, warmer air above the surface had started to force its way into northeast Iowa, yet the surface temperatures were in the 20s. As the snow fell into this warm layer, it partially melted, but then refroze as it dropped through the colder air near the surface. This is how sleet forms, and this warm layer gradually advanced northward into southeast Minnesota and west central Wisconsin in the afternoon, changing the snow there over to sleet also. Normally, when sleet occurs, it doesn't last long, changing over to either snow, rain, or perhaps freezing rain, as the air cools or warms. In this case, the strong and steady flow of warm air, coupled with the sufficiently cold surface and near surface temperatures, provided the perfect temperature profile for an extended period of sleet.
This warming continued to advance north. By late afternoon and evening, the warm air was completely melting the snow as it fell. At the surface, temperatures also started to climb, and the near surface cold layer was not as deep. As a result, there was not enough time for the precipitation to refreeze, so it fell as a liquid...freezing rain or freezing drizzle. Luckily, the bulk of the heavy precipitation was shifting east by this time, thus preventing what could have been some significant glaze ice accumulations. As it was though, an ice glaze from a trace to a few tenths of an inch was common for most locations south of a Lake City, MN to Neillsville, WI line.
As the evening wore on, the warm air increased, surface temperatures quickly climbed above freezing, leading to more drizzle...and lessening the threat for additional ice accumulations.
The links below will open graphics depicting the temperature structure at different times during the storm. These graphics are cross-sections from the ground to about 30,000 feet. They run from north to south (Eau Claire, WI on the far left, Cedar Rapids, IA on the far right). Basically, its a way to look at what the temperatures are doing above the surface, along a horizontal line. As you go through the images, you will notice how the warm air (above 0 degrees Celsius) in the low layers of the atmosphere pushes northward, yet it stays cold at the surface (below 0 degrees Celsius). This leads to changes in precipitation type. The most "probable" precipitation types, based on the temperature, are annotated on the graphics (a rough estimate, not necessarily what was occurring).
Due to the difficultly in differentiating between what fell as snow, and what fell as sleet, the totals below are for snow and sleet combined.
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE LA CROSSE WI
945 AM CST SUN DEC 2 2007
...24 HOUR SNOWFALL/SLEET TOTALS AROUND THE AREA...
LOCATION SNOWFALL TIME LAT/LON
LANSING 4SE 5.7 0700 AM 43.32N 91.16W
POSTVILLE 5.0 0700 AM 43.09N 91.56W
DORCHESTER 4.3 0700 AM 43.47N 91.51W
N. WASHINGTON 3.0 0800 AM 43.12N 92.42W
IONIA 2W 2.8 0700 AM 43.03N 92.50W
STRAWBERRY POINT 5.0 0700 AM 42.68N 91.54W
VOLGA 1NE 4.0 0700 AM 42.81N 91.52W
ELKADER 6SSW 2.3 0700 AM 42.79N 91.45W
FAYETTE 3.4 0700 AM 42.85N 91.82W
CHARLES CITY COOP 3.0 0700 AM 43.08N 92.67W
CRESCO 6.0 0700 AM 43.39N 92.09W
SAINT ANSGAR 3.6 0700 AM 43.38N 92.92W
OSSIAN 4.0 0733 AM 43.15N 91.77W
KENDALLVILLE 4.0 0311 AM 43.43N 92.03W
KENDALLVILLE 4N 2.8 0441 AM 43.50N 92.01W
MANTORVILLE 3E 4.0 0700 AM 44.07N 92.72W
SPRING VALLEY 3SW 6.0 0700 AM 43.66N 92.43W
SPRING VALLEY 3E 6.0 0700 AM 43.68N 92.31W
HOUSTON 5E 5.5 0700 AM 43.76N 91.49W
RENO 3SW 4.8 0700 AM 43.53N 91.33W
GRAND MEADOW 4.6 0530 AM 43.71N 92.56W
GRAND MEADOW 6SE 3.3 0700 AM 43.62N 92.49W
ORONOCO 4.2 0700 AM 44.16N 92.54W
BYRON 3.6 0700 AM 44.03N 92.65W
ELGIN 2SSW 3.4 0700 AM 44.10N 92.27W
ROCHESTER 2NE 3.0 0700 AM 43.93N 92.48W
ROCHESTER INTL AIRPORT 3.0 0600 AM 43.91N 92.50W
THEILMAN 1SSW 5.5 0700 AM 44.28N 92.19W
WABASHA 5.0 0700 AM 44.39N 92.05W
ZUMBRO FALLS 3.2 0700 AM 44.29N 92.43W
WINONA DAM 5A 6.0 0600 AM 44.09N 91.67W
ST CHARLES 5N 4.0 0700 AM 44.04N 92.07W
FRIENDSHIP 6.1 0700 AM 43.98N 89.83W
ARKDALE 6.0 0700 AM 44.03N 89.88W
BIG FLATS 6.0 0800 AM 44.12N 89.80W
MONROE CENTER 5.5 0700 AM 44.11N 89.92W
COCHRANE 6.3 0800 AM 44.22N 91.82W
MONDOVI 6S 5.5 0600 AM 44.48N 91.67W
NEILLSVILLE 4NNE 7.0 0800 AM 44.61N 90.56W
OWEN 2N 6.8 0700 AM 44.98N 90.55W
NEILLSVILLE 3SW 6.2 0630 AM 44.53N 90.64W
GAYS MILLS 4.8 0700 AM 43.32N 90.85W
MT ZION 2NW 2.0 0830 AM 43.29N 90.87W
CUBA CITY 2NW 4.0 0800 AM 42.63N 90.46W
LANCASTER 3.5 0630 AM 42.85N 90.71W
HATFIELD 7.5 0700 AM 44.42N 90.73W
TAYLOR 7.2 0600 AM 44.32N 91.12W
MERRILLAN 6.0 0800 AM 44.45N 90.84W
NEW LISBON 2N 5.8 0900 AM 43.90N 90.17W
NECEDAH 2SE 4.9 0700 AM 44.00N 90.04W
...LA CROSSE COUNTY...
WEST SALEM 1W 6.5 0600 AM 43.89N 91.09W
FRENCH ISLAND 4.7 0700 AM 43.87N 91.27W
LA CROSSE AIRPORT 4.8 0600 AM 43.88N 91.26W
LA CROSSE NATL WEATHER SVC 3.9 0700 AM 43.82N 91.19W
LA CROSSE 5SE 5.1 0600 AM 43.77N 91.15W
FOUR CORNERS 5.0 0700 AM 44.07N 90.92W
WARRENS 5WSW 6.9 0700 AM 44.10N 90.59W
RICHLAND CENTER 4W 6.0 0630 AM 43.33N 90.46W
GOODRICH 1E 8.0 0600 AM 45.15N 90.07W
MEDFORD 7.0 0700 AM 45.13N 90.34W
PERKINSTOWN 5E 6.0 0900 AM 45.20N 90.51W
ARCADIA 6.0 0600 AM 44.25N 91.49W
OSSEO 6.0 0700 AM 44.58N 91.22W
BLAIR 5.5 0700 AM 44.29N 91.22W
ETTRICK 5SE 5.0 0600 AM 44.11N 91.22W
WHITEHALL 2W 4.0 0620 AM 44.37N 91.36W
INDEPENDENCE 3NE 4.0 0700 AM 44.39N 91.39W
HILLSBORO 4.0 0700 AM 43.65N 90.33W
ONTARIO 4.1 0600 AM 43.72N 90.60W
VIROQUA 4.2 0700 AM 43.56N 90.88W
WESTBY 3ENE 7.8 0700 AM 43.67N 90.81W
OBSERVATIONS ARE COLLECTED FROM A VARIETY OF SOURCES WITH VARYING
EQUIPMENT AND EXPOSURE. NOT ALL DATA LISTED IS CONSIDERED OFFICIAL.
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE LA CROSSE WI
924 PM CST SAT DEC 1 2007
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
0130 PM SLEET VICTORY 43.48N 91.22W
12/01/2007 M1.00 INCH VERNON WI PUBLIC
0202 PM SLEET 4 SE STRAWBERRY POINT 42.64N 91.48W
12/01/2007 M1.50 INCH CLAYTON IA PUBLIC
0257 PM SLEET 3 NE STODDARD 43.69N 91.18W
12/01/2007 M0.70 INCH VERNON WI NWS EMPLOYEE
0313 PM SLEET 8 NW WAUKON 43.35N 91.59W
12/01/2007 M0.50 INCH ALLAMAKEE IA PUBLIC
0330 PM SLEET WATERVILLE 43.21N 91.29W
12/01/2007 M1.50 INCH ALLAMAKEE IA PUBLIC
0350 PM FREEZING RAIN 4 SE STRAWBERRY POINT 42.64N 91.48W
12/01/2007 E0.10 INCH CLAYTON IA PUBLIC
0355 PM SLEET ELMA 43.25N 92.44W
12/01/2007 M1.10 INCH HOWARD IA PUBLIC
0400 PM SLEET ONALASKA 43.89N 91.22W
12/01/2007 M1.00 INCH LA CROSSE WI TRAINED SPOTTER
0420 PM SLEET 2 N HOLMEN 43.99N 91.26W
12/01/2007 M0.50 INCH LA CROSSE WI NWS EMPLOYEE
0450 PM FREEZING RAIN BYRON 44.03N 92.65W
12/01/2007 M0.10 INCH OLMSTED MN TRAINED SPOTTER
0500 PM SLEET WEST SALEM 43.90N 91.08W
12/01/2007 M1.50 INCH LA CROSSE WI NWS EMPLOYEE
0500 PM SLEET 4 NNW LA CROSSE 43.88N 91.26W
12/01/2007 M2.00 INCH LA CROSSE WI NWS EMPLOYEE
0520 PM SLEET 10 W LEWISTON 43.98N 92.07W
12/01/2007 M1.00 INCH WINONA MN PUBLIC
0520 PM SLEET 6 SE GRAND MEADOW 43.64N 92.49W
12/01/2007 M1.00 INCH MOWER MN PUBLIC
0525 PM SLEET 3 SW RENO 43.57N 91.32W
12/01/2007 M1.90 INCH HOUSTON MN PUBLIC
0525 PM SLEET 5 E HOUSTON 43.76N 91.47W
12/01/2007 M0.50 INCH HOUSTON MN PUBLIC
0550 PM SLEET 3 E LA CROSSE 43.83N 91.17W
12/01/2007 M0.90 INCH LA CROSSE WI CO-OP OBSERVER
MEASURED 0.90 INCHES OF SLEET AT THE NWS LA CROSSE
0557 PM FREEZING RAIN BYRON 44.03N 92.65W
12/01/2007 M0.15 INCH OLMSTED MN AMATEUR RADIO
0625 PM FREEZING RAIN OSAGE 43.28N 92.81W
12/01/2007 E0.12 INCH MITCHELL IA EMERGENCY MNGR
3.0 IN. SNOW AND SLEET COMBINED AS WELL. FZDZ CURRENTLY.
0829 PM SLEET VICTORY 43.48N 91.22W
12/01/2007 M3.25 INCH VERNON WI CO-OP OBSERVER
2.8 INCHES OF SNOW EARLIER TODAY PRIOR TO SLEET. NOW
FREEZING RAIN WITH A GLAZING ON ALL SURFACES.
0857 PM FREEZING RAIN 3 SW SPRING VALLEY 43.66N 92.43W
12/01/2007 E0.10 INCH FILLMORE MN AMATEUR RADIO
0858 PM FREEZING RAIN WARRENS 44.13N 90.50W
12/01/2007 M0.25 INCH MONROE WI PUBLIC
0906 PM FREEZING RAIN 1 S HAMMOND 44.21N 92.37W
12/01/2007 M0.25 INCH WABASHA MN PUBLIC
CURRENTLY FREEZING RAIN
0910 PM FREEZING RAIN 3 E MANTORVILLE 44.07N 92.69W
12/01/2007 E0.10 INCH DODGE MN PUBLIC
0913 PM FREEZING RAIN LA CRESCENT 43.83N 91.30W
12/01/2007 E0.20 INCH HOUSTON MN NWS EMPLOYEE
BETWEEN 0.10 AND 0.20 INCHES OF GLAZE ICE.
0917 PM FREEZING RAIN 3 SW RENO 43.57N 91.32W
12/01/2007 E0.20 INCH HOUSTON MN PUBLIC
SINCE 530 PM.
0924 PM FREEZING RAIN 3 E LA CROSSE 43.83N 91.17W
12/01/2007 M0.20 INCH LA CROSSE WI OFFICIAL NWS OBS