What Happened?

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:

1. Surface temperature: When at or below freezing (32F or 0C), snow, freezing rain, freezing drizzle and sleet are all possible. When the temperatures is above freezing, rain, drizzle, or even some snow (if not too warm) are possible.
2. Just above the surface: If there is an above freezing layer of air (called a "warm layer") above the surface, any ice falling through this layer (i.e., snow) could melt into a liquid drop, or partially melt in an ice-liquid mix. If the surface temperature is at or below freezing, this precipitation could then reach the ground as freezing rain, or refreeze just above the surface and land as sleet (a "ball" of ice, also called an ice pellet).
3. Coldest cloud temperature: This coldest temperature within the cloud layer is important as well since it dictates whether there will be any ice created in the cloud when the precipitation process begins. Generally, temperatures need to be at least -10C for ice growth to begin in the cloud. If temperatures within the cloud are between 0C and -9C, the cloud will likely not have ice but rather any precipitation created within the cloud will be in the form of liquid (super cooled droplets or liquid drops with a temperature below freezing).

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).   

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.

 Ice Reports

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