An area of low pressure moved out of the Southern Plains, tracked across Iowa and Illinois on Saturday the 3rd, and then moved into the eastern Great Lakes on Sunday January 4th. A trough of low pressure extended north of the low, sliding across the Upper Mississippi River Valley as the system transitioned east. Out ahead of the low, southerly low level winds brought warm air northward, which helped fuel areas of precipitation north of the low center, and also resulted in a wintery mix of precipitation.
Initially, areas of freezing drizzle developed across western/central Iowa and southern Minnesota early Saturday afternoon, spreading to the Mississippi River, and then east of it by mid to late afternoon. The freezing drizzle would continue for much of the evening, but freezing rain also mixed in for a few hours, with snow much farther north (across northern sections of Minnesota and Wisconsin). Some light snow would eventually fall across the local area overnight Saturday, but accumulations were generally only a dusting to 1/2 inch. The main impact from the storm was the icing, with many locations reporting 1 to 2 tenths of an inch from the freezing drizzle/freezing rain. The icing made driving very treacherous, which continued into Sunday morning.
Why All the Freezing Precipitation?
To better understand why freezing drizzle was the dominant precipitation type from this event, the interaction of warm air on precipitation and the depth of the cloud layer (i.e., the amount of moisture/saturation in the atmosphere) need to be examined.
When it comes to temperature, there are three basic considerations that will impact what type of precipitation will fall:
The depth of the cloud is a contributing factor to whether there will be a chance for accumulating precipitation (such as rain or snow) or lighter precipitation like drizzle. Generally, the deeper the cloud layer is, the better chance that a precipitation process can be initiated (in meteorology these processes are called the collision-coalescence process and the ice-crystal process [Bergeron-Findeisen process]). If the cloud layer is more shallow, typically it supports light precipitation in the form of drizzle or perhaps flurries - if any precipitation occurs at all.
For this weather system, both the temperature and the cloud depth were important to the freezing precipitation. The cloud depth was relatively shallow across much of the local area from Saturday afternoon through Saturday night. This favored light precipitation as the deeper cloud was located farther to the northwest and north. The lack of cloud depth and cloud temperatures warmer than -10C supported drizzle formation. Because surface temperatures were at or below 32F or 0C, the drizzle would freeze on objects creating an ice coating.
In areas of deeper cloud, temperatures at higher altitudes were important. The deeper cloud, with temperatures colder than -10C, began the ice or snow growth process at heights above 5-10 thousand feet. These ice crystals began to fall vertically toward the surface. However, a warm layer of air with temperatures above freezing had pushed northward into the region just above the surface. The ice encountered the warm layer on its descent and melted back into liquid drops. Because the surface temperature remained below freezing, the liquid would then freeze on objects creating an ice coating. However, because of the deeper cloud, the drops were larger and the accumulation was slightly greater. This slightly higher precipitation rate and drop size is then termed freezing rain.
Below are links to cross-sections of the lower atmosphere showing the moisture and temperature profile above the surface throughout the storm. The cross-sections run northwest to southeast across the local area, with La Crosse as the center point. As you move through the images, note how the cloud depth (e.g., relative humidity over 80 percent, darker green and red) remains shallow and warmer than -10C for much of the area. While these cross-sections cannot depict all the smaller scale features (as there were periods during the evening where the cloud depth became deep enough for freezing rain), they do show why freezing drizzle was the dominant precipitation type. Clearly seen in the temperature cross-sections is the layer of above freezing temperatures (warmer than 0C) above the surface (yellow, orange, red).
|Relative Humidity:||Noon||3 pm||6 pm||9 pm||Midnight||3 am|
|Temperature:||Noon||3 pm||6 pm||9 pm||Midnight||3 am|
Below is an analysis loop of the warmest temperature through the lowest 20,000 feet of the atmosphere (left), and the regional radar imagery (right). The above freezing temperatures (left loop, see reds and yellows) depict where the atmosphere would support a mix of either sleet or freezing rain, and the blue area depicts where snow is more probable. Note that much of the local area is covered by above freezing temperatures above the surface.
PRELIMINARY LOCAL STORM REPORT...SUMMARY NATIONAL WEATHER SERVICE LA CROSSE WI 1214 PM CST SUN JAN 04 2009 ..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON... ..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE.... ..REMARKS.. 0417 PM FREEZING RAIN AUSTIN 43.67N 92.97W 01/03/2009 E0.06 INCH MOWER MN BROADCAST MEDIA 0425 PM FREEZING RAIN 5 WSW WARRENS 44.10N 90.59W 01/03/2009 E0.10 INCH MONROE WI CO-OP OBSERVER 0526 PM FREEZING RAIN TAYLOR 44.32N 91.12W 01/03/2009 E0.10 INCH JACKSON WI CO-OP OBSERVER 0634 PM FREEZING RAIN THORP 44.96N 90.80W 01/03/2009 E0.06 INCH CLARK WI TRAINED SPOTTER LIGHT ACCUMULATIONS SO FAR 0635 PM FREEZING RAIN 4 SE SPRING GROVE 43.52N 91.58W 01/03/2009 E0.00 INCH HOUSTON MN BROADCAST MEDIA ICE ACCUMULATION BETWEEN SPRING GROVE AND EITZEN. REPORTED BY KNEI RADIO. 0655 PM FREEZING RAIN WINONA 44.05N 91.66W 01/03/2009 E0.13 INCH WINONA MN EMERGENCY MNGR VERY SLICK ROADS 0720 PM FREEZING RAIN GRAND MEADOW 43.71N 92.57W 01/03/2009 E0.13 INCH MOWER MN CO-OP OBSERVER GLAZE COATING EXPOSED SURFACES 0815 PM FREEZING RAIN 7 ENE DECORAH 43.34N 91.66W 01/03/2009 E0.13 INCH WINNESHIEK IA PUBLIC ICE COVERED ROADS THROUGHOUT THE AREA 0825 PM FREEZING RAIN 3 NE STODDARD 43.69N 91.18W 01/03/2009 E0.13 INCH VERNON WI NWS EMPLOYEE 0840 PM FREEZING RAIN 2 N LA CRESCENT 43.86N 91.30W 01/03/2009 E0.10 INCH WINONA MN NWS EMPLOYEE GLAZE COVERING EXPOSED SURFACES 0945 PM FREEZING RAIN 3 NW WABASHA 44.40N 92.09W 01/03/2009 E0.10 INCH WABASHA MN AMATEUR RADIO REPORTED FROM 1 MILE WEST OF READS LANDING 1147 PM FREEZING RAIN 3 E LA CROSSE 43.83N 91.17W 01/03/2009 E0.10 INCH LA CROSSE WI NWS EMPLOYEE THIN COATING OF ICE AT NWS LA CROSSE 1159 PM FREEZING RAIN 1 WNW OELWEIN 42.68N 91.93W 01/03/2009 M0.10 INCH FAYETTE IA PUBLIC 0200 AM FREEZING RAIN 1 S PETERSON 43.77N 91.83W 01/04/2009 U0.00 INCH FILLMORE MN PUBLIC 0.1 INCH ICE COVERED WITH 0.3 INCH SNOW 0200 AM FREEZING RAIN 1 NW VALLEY 43.66N 90.55W 01/04/2009 U0.00 INCH VERNON WI TRAINED SPOTTER 1/10 INCH ICE GLAZE 0200 AM FREEZING RAIN 6 S MONDOVI 44.48N 91.67W 01/04/2009 M0.16 INCH BUFFALO WI TRAINED SPOTTER 0.3 INCHES OF ICE GLAZE 0200 AM FREEZING RAIN EDGEWOOD 42.65N 91.40W 01/04/2009 M0.16 INCH CLAYTON IA CO-OP OBSERVER 0200 AM FREEZING RAIN GOODVIEW 44.07N 91.71W 01/04/2009 U0.00 INCH WINONA MN PUBLIC 0.4 INCHES OF SNOW ATOP 0.2 INCHES OF ICE 0200 AM FREEZING RAIN 3 NE INDEPENDENCE 44.39N 91.38W 01/04/2009 U0.00 INCH TREMPEALEAU WI PUBLIC 0.2-0.3 INCHES ICE COATING EXPOSED SURFACES TOPPED WITH 0.5 INCHES SNOW 0230 AM FREEZING RAIN GALESVILLE 44.08N 91.36W 01/04/2009 U0.00 INCH TREMPEALEAU WI NWS EMPLOYEE 0.3 INCHES OF SNOW ATOP 0.2 INCHES OF ICE GLAZE 0300 AM FREEZING RAIN TAYLOR 44.32N 91.12W 01/04/2009 M0.09 INCH JACKSON WI TRAINED SPOTTER 0.25 INCH OF ICE GLAZE 0300 AM FREEZING RAIN MAUSTON 43.80N 90.08W 01/04/2009 U0.00 INCH JUNEAU WI PUBLIC 1/8 INCH ICE ON ALL EXPOSED SURFACES 0300 AM FREEZING RAIN TUNNEL CITY 44.00N 90.57W 01/04/2009 M0.13 INCH MONROE WI CO-OP OBSERVER 0.15 INCH OF ICE GLAZE