Significant Weather Observing Program


November 23-24, 2004

     The first snow of the 2004-2005 winter season occurred across parts of central Illinois on Wednesday, November 24th...as a vigorous storm system tracked just south of the region.  The precipitation began as rain in most locations, but changed over to wet snow during the day of the 24th.  Accumulations of between 3 and 8 inches were common along and west of I-55...with lower amounts further east and south.

     An area of low pressure developed across Texas...then began tracking northeastward toward the Ohio Valley on Tuesday, November 23rd.  Brisk easterly winds ahead of the system kept temperatures on the cool side on Tuesday...however conditions remained predominantly dry for most of the day...with only patchy drizzle and fog noted from time to time.  Light rain developed and spread across much of the area by Tuesday night as the low began to approach.  By Wednesday, the low continued to intensify as it tracked into the Ohio Valley.  Strong northerly winds on the backside of the system advected colder air into central Illinois...resulting in the rain changing to snow from northwest to southeast across the area.  Snow began during the morning across locations along and northwest of the Illinois River and around midday along and west of I-55.  Further east, snow did not commence until later in the afternoon.  Winds increased to 20 to 30 mph...with gusts of between 40 and 50 mph at times.  As a result of the strong winds and wet snow...tree branches and power lines were downed...causing numerous power outages across the area.

     At upper levels of the atmosphere, a "coupled jet structure" led to enhanced divergence and rising motion across central Illinois, helping maintain the precipitation for several hours.  In addition, ample moisture was drawn northward into the system from the Gulf of Mexico, leading to snowfall rates of between one to two inches per hour at times.  Due to the strong dynamics in place, thunder occasionally accompanied the snow, particularly in heavier snow bands.  The snow gradually tapered off across much of central Illinois by late evening.  Areas along and south of I-70 remained warm enough for all rain during the day Wednesday...with only a few flurries noted as the precipitation came to an end. 

     Snowfall totals around the region were quite impressive...with areas along and west of I-55 receiving the most.  Two distinct bands of heavier snowfall occurred.  The first band developed west of the Illinois River...where 4 to 5 inch amounts were common from near Galesburg southward to Brooklyn in Schuyler County.  A second band formed between the Illinois River and I-55...where a general 4 to 6 inch snowfall occurred.  Many locations across Tazewell...Woodford...and McLean counties picked up as much as 7 to 8 inches before this band of snow diminished late Wednesday evening. 



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