Although March 1 is considered to be the start of meteorological spring, it was expected to continue the snowy trend of the previous few months. The forecast was very challenging, but it was appearing that a significant winter storm was on tap for the Midwest for the weekend of March 1-2. In the left image below, the forecast from Friday morning shows that a corridor of 8 to 10 inches of snow was anticipated across the central third of Illinois. The center image is the forecast from Saturday morning, lowering snow totals, but still indicating a significant amount of snow. In the right image, the total snowfall over the weekend is show, with a large part of central and southeast Illinois only seeing an inch or two.
There is no way to sugarcoat this; while southeast Illinois did see significant amounts of sleet and some freezing rain, as well as a couple inches of snow, overall the forecast was obviously quite bad. So what exactly happened?
All week, it was appearing that there would be a significant storm system. However, the computer model agreement was poor, which is not that unusual when a storm system is several days away. This clip is from the forecast discussion our office issued on Monday afternoon, and references the two main computer models (GFS and ECMWF) that are used at that forecast range:
A strong southern stream system is projected to roll out of Texas and move up the Ohio Valley Sunday and Sunday night. The path of that low will be critical to how far north the moisture and warm air extend. The GFS is advertising several inches of snow across the southern 2/3rds of our forecast area, while the ECMWF brings much warmer
air into our southeast counties. The warmer scenario could produce rain in the SE with a band of freezing rain and sleet in central areas, with all snow on the northern edges. Forecast changes are likely as the models try to resolve the track of this system before it is anywhere near the West Coast.
Resolution of systems over the Pacific is generally a challenge. Weather balloons are launched twice a day in order to analyze the conditions in the upper atmosphere, but these types of observations are very sparse over the Pacific Ocean. Some analysis is available using weather sensors on long-distance aircraft flights, tracking features (clouds, etc.) over the ocean and calculating the winds, plus other analyses using satellite data.
Warm air aloft, in the lower levels of the atmosphere, also is a significant concern when forecasting winter weather. Freezing rain and sleet occurs when temperatures aloft are warmer than 32 degrees, but there is still a shallow layer near the surface that is below 32 degrees. The amount and duration of the warm air aloft dictates how much of the precipitation falls as these types, which increases the amount of icing on trees and power lines but decreases how much snow would occur. A stronger storm system is more likely to pump the warmer air into the area. The graphic at left illustrates the process (click to enlarge).
Wednesday afternoon, there was still quite a bit of uncertainty as to this warmer air, but the overall trend was for temperatures to be colder, diminishing the freezing rain/sleet threat some and pointing more toward snow. By Thursday, confidence was starting to increase about mixed precipitation being limited to just around I-70 and south, with several inches of snow elsewhere, but falling over an extended period (Saturday night into early Monday). Friday morning, a Winter Storm Watch was issued from about Rushville to Bloomington southward, due to the heavy snow potential across central Illinois, and the mixture of snow and freezing rain (up to 1/4 inch) in southeast Illinois. The potential for some isolated thunderstorms was there, which would locally enhance the precipitation totals. A conference call was held with area emergency managers and broadcast media on Friday, to discuss the upcoming storm and the challenges it would present.
By this point, the storm system in the Pacific was getting close to coming onshore, which happened on Saturday:
|Water vapor satellite image from Friday morning, February 28, showing the center of the storm system west of San Francisco. Image courtesy of the Space Science and Engineering Center, University of Wisconsin.||Upper air analysis at 500 mb from Saturday morning, showing the upper low about to come onshore.|
The strength of this system remained a key player in how the weather would evolve. As it started moving across the western U.S., it weakened and increased in speed. This meant much less energy would be available to enhance the precipitation in our area. Even on Saturday morning, confidence was lower than normal on the temperatures aloft and thus the precipitation types, and a general consensus was that the heaviest snow area would be a bit lower and further south. There was also a concern that although the expected snow totals would normally meet the criteria for a Winter Storm Warning, it would fall over too long of a duration (criteria in our area is at least 6" in 12 hours or 8" in 24 hours). However, several computer models were also starting to indicate the precipitation would potentially end sooner than first thought. A Winter Weather Advisory was issued for areas from about Taylorville to Bloomington northwest, and the Winter Storm Watch was kept in place further east into the afternoon, before being converted to a warning during the afternoon. These concerns were included in the afternoon Area Forecast Discussion, as well as in an update to the daily Weather Story, which was posted to our homepage, Facebook and Twitter during the afternoon (see image at right).
Saturday evening, several areas from Bloomington northwest were starting to see some freezing drizzle, which preceded the snow. The precipitation reached near the I-72 corridor by midnight, but rapidly started to fade as sunrise approached. A broad 3 to 4 inches of snow did occur by early Sunday morning across the north half of Illinois. Across the south half of the state, freezing rain and sleet were more of a concern, spreading into the area from Missouri, with the precipitation continuing into mid morning Sunday. During the afternoon, another band of sleet and snow tracked east along the I-72 corridor, and more freezing rain, sleet and snow moved into southern Illinois out of Missouri. The bulk of the precipitation moved out of the area by late Sunday evening.
More information on the subject of forecast uncertainty is available at this link.
Specific snow, sleet and freezing rain amounts:
|SNOW REPORTS LISTED BY AMOUNT...
INCHES LOCATION COUNTY TIME
------ ------------------- ----------- -------
4.20 ELMWOOD PEORIA 0905 AM
4.20 HOPEWELL MARSHALL 0720 AM
4.10 ST. DAVID FULTON 0700 AM
3.70 GALESBURG KNOX 0700 AM
3.70 ALTONA KNOX 0600 AM
3.60 MORTON TAZEWELL 0700 AM
3.50 E BLOOMINGTON MCLEAN 0918 AM
3.50 1 N MACKINAW TAZEWELL 0700 AM
3.50 1 SSE MORTON TAZEWELL 0700 AM
3.40 BLOOMINGTON MCLEAN 0739 AM
3.40 1 S MORTON TAZEWELL 0700 AM
3.30 GILSON KNOX 0813 AM
3.20 PEORIA INTERNATIONAL PEORIA 0600 AM
3.10 5 NNE ARENZVILLE CASS 0700 AM
3.10 3 SE DUNLAP PEORIA 0900 AM
3.00 3 NE BLOOMINGTON MCLEAN 1109 AM
3.00 HAVANA MASON 0700 AM
3.00 3 W FARMER CITY DE WITT 0700 AM
3.00 5 W BLOOMINGTON MCLEAN 0600 AM
2.80 HENRY MARSHALL 0931 AM
2.80 5 NW PEORIA PEORIA 0800 AM
2.80 EUREKA WOODFORD 0728 AM
2.80 LEXINGTON MCLEAN 0700 AM
2.70 ROANOKE WOODFORD 0700 AM
2.40 1 ENE LINCOLN LOGAN 0600 AM
2.30 MINONK WOODFORD 0745 AM
2.30 1 SE LINCOLN LOGAN 0700 AM
2.30 2 NE MAHOMET CHAMPAIGN 0700 AM
2.30 PANA CHRISTIAN 1100 PM
2.20 3 W BISMARCK VERMILION 0851 AM
2.20 3 SSE HENNING VERMILION 0700 AM
2.10 4 SSE CHAMPAIGN CHAMPAIGN 0700 AM
2.10 ROSAMOND CHRISTIAN 0800 PM
2.00 2 NE ARROWSMITH MCLEAN 0740 AM
2.00 MOUNT PULASKI LOGAN 0700 AM
2.00 7 W ATLANTA LOGAN 0700 AM
2.00 2 N HOMER CHAMPAIGN 0700 AM
2.00 4 NE NORMAL MCLEAN 0500 AM
2.00 SULLIVAN MOULTRIE 0800 PM
1.80 1 SSW ST. JOSEPH CHAMPAIGN 0800 AM
1.80 4 NNE WHITE HEATH PIATT 0800 AM
1.80 4 SSW LE ROY MCLEAN 0800 AM
1.80 2 SSE CHAMPAIGN CHAMPAIGN 0500 AM
1.70 SHERMAN SANGAMON 0700 AM
1.70 2 NNW CHAMPAIGN CHAMPAIGN 0700 AM
1.70 OGDEN CHAMPAIGN 0600 AM
1.60 URBANA CHAMPAIGN 0800 AM
1.60 3 S CHAMPAIGN CHAMPAIGN 0800 AM
1.60 1 S MANITO MASON 0742 AM
1.60 1 N SIDNEY CHAMPAIGN 0700 AM
1.50 1 N SAVOY CHAMPAIGN 0700 AM
1.50 4 ESE URBANA CHAMPAIGN 0700 AM
1.50 1 E CHATHAM SANGAMON 1100 PM
1.30 2 NNW DEWEY CHAMPAIGN 0800 AM
1.30 EFFINGHAM EFFINGHAM 0800 PM
1.10 HOMER CHAMPAIGN 0735 AM
1.00 4 NNE MONTICELLO PIATT 0630 AM
|SLEET AND FREEZING RAIN REPORTS LISTED BY AMOUNT...
INCHES LOCATION COUNTY TIME TYPE
------ ------------------- ----------- ------- -----
1.25 6 SSE CLAY CITY CLAY 1100 PM SLEET
0.80 EFFINGHAM EFFINGHAM 0811 PM SLEET
0.50 1 E TRIMBLE CRAWFORD 0840 AM SLEET
0.05 EFFINGHAM EFFINGHAM 0811 PM FRZ RAIN