Mild Winter With Little Snow...Why?

Winter 2011-2012 produced less than 50% of normal snowfall for much of Central and Southeast Illinois, while temperatures averaged 4-6 degrees above normal.  This is in stark contrast to the past three winters which produced above normal snowfall and below normal temperatures for much of the area.  The maps below from the Midwestern Regional Climate Center highlight the relatively warm and snow free weather.  While snowfall was well below normal, precipitation total (including rain) was near normal.

Below is a chart showing snowfall amounts this winter through January 31,
snowfall amounts last year, and normals for Springfield, Peoria, Champaign
and Effingham, IL.

Snowfall amounts from several cities in central Illinois through January 31


Below is a list of central IL cities snowfall amounts and departures from normals from Oct 1, 2011 
through Mar 9, 2012.
  Between 6 and 12 inches of snow has fallen over central IL with 3-7 inches 
over southeast IL so far this winter.  Most of that
snow fell in a two week period from Jan 12-28, 2012.
The biggest snowfall so far this winter season was 3-5 inches over central IL Jan 12-13th with strong
west winds causing 1-3 foot drifts.  Any snow that fell would melt in 2 to 4 days so much of this winter 
central and southeast Illinois did not have snow on
the ground.  Snowfall amounts so far this winter are
generally running 6 to 12 inches below normal.  Paris is running 15 inches below normal for snowfall
this winter.  Hoopeston in northern Vermilion county is one of the few cities in central IL that is closer 
to normal for snowfall so far this winter with 13.4 inches and 9 inches
of that falling from Jan 12-21, 2012.

...SNOWFALL OCT 1, 2011 - MAR 9, 2012...

Peoria......................11.5 inches of snow  (10.9 inches below normal)
Springfield................ 9.2 inches of snow  (9.9 inches below normal)
Lincoln, IL................. 7.0 inches of snow (12.3 inches below normal)
Champaign/Urbana.... 11.4 inches of snow (9.7 inches below normal)
Bloomington/Normal... 8.3 inches of snow  (10.2 inches below normal)
Morton............................... 10.1 inches of snow  (11.2 inches below normal)
Knoxville........................... 10.5 inches of snow  (12.1 inches below normal)
Danville..................... 8.4 inches of snow  (7.7 inches below normal)
Hoopeston................. 13.4 inches of snow (3.2 inches below normal)
Tuscola..................... 7.8 inches of snow  (9.0 inches below normal)
Paris......................... 6.3 inches of snow  (15.2 inches below normal)
Sullivan...................... 5.5 inches of snow (7.7 inches below normal)
Jacksonville............... 5.6 inches of snow  (10.2 inches below normal)
Rushville.................... 6.3 inches of snow  (8.7 inches below normal)
Charleston................. 6.3 inches of snow  (9.6 inches below normal)  
Mattoon..................... 6.9 inches of snow  (6.7 inches below normal)
Effingham.................. 7.1 inches of snow  (7.7 inches below normal)
Palestine.......................... 8.0 inches of snow   (4.8 inches below normal)
Flora......................... 3.0 inches of snow  (8.7 inches below normal)
Olney................................. 4.2 inches of snow  (7.0 inches below normal)
Lawrenceville............. 4.8 inches of snow  (9.8 inches below normal)



Many people have been asking why we are experiencing abnormal conditions this winter.  The answer lies in the position of the jet stream, which is a relatively narrow band of strong wind in the upper levels of the atmosphere, often around 30,000 feet above ground.  Wind blows from west to east in jet streams but often the flow has a northward or southward component.  Jet streams move around the hemisphere in response to the location of cold and warm air masses, and help to drive storm systems.  Two jet streams typically affect weather patterns across the central U.S. during winter: The Polar Jet Stream and the Subtropical Jet Stream.

    

The Polar Jet Stream periodically digs south from Canada and delivers rounds of Arctic air masses to the area during winter.  Meanwhile, the Subtropical Jet Stream varies in position from the Mexico/Gulf of Mexico region, north into the Southern and Central U.S.  This jet stream delivers relatively warm and moist air north into the mid-latitudes.  Several times per winter these two jet streams merge to produce large and powerful winter storms capable of producing everything from heavy rain severe weather in the Southern U.S. to heavy snow and blizzards across the Central and Northern U.S. 

The unusual aspect of this winter has been the tendency of the Polar Jet Stream to remain anchored well north across Central Canada, therefore infrequently dipping south into the Central U.S. to bring frigid temperatures.  Since Arctic air has been lacking, most of our weather systems have been driven by the warmer Subtropical Jet Stream, bringing rain and above normal temperatures.  It is interesting to note that while our winter has been mild, the dominant position of the Polar Jet Stream has brought a brutally cold and snowy winter to the other side of the hemisphere, including much of Eastern Europe east through Asia and even parts of Alaska. 

Ongoing climate research has shown a driving force for the location and movement of the Polar Jet Stream to be a naturally occurring pattern of climate variability.  This pattern of intra-seasonal variability is called the Arctic Oscillation (AO), with a closely related index termed the North Atlantic Oscillation (NAO).  These oscillations describe how air circulates around the northern part of the Northern Hemisphere, and where the "Polar Vortex", or heart of the Arctic air, sets up.  When the values of these indices are negative, cold air tends to spill into the Central and Eastern U.S. and Canada, and the North Atlantic Ocean.  When these indices turn positive, those same areas tend to be warmer than normal while the cold air is shoved to the other side of the hemisphere (Europe and Asia).  The past few winters featured frequent negative AO/NAO patterns which resulted in numerous cold air outbreaks across our region.  So far this winter, the indices have been dominated by positive values.  As this is an area of ongoing research, predictability of these values is currently available in the one to two week time range.  The Climate Prediction Center tracks each index and provides a forecast which is shown below.

Check out a comparison of snowfall in the US from this winter to last winter 



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