December 22-27, 2009

A major winter storm impacted the Midwest from December 22nd through December 27th, bringing a wide variety of wintry precipitation.  The first stage of the storm brought freezing rain to northern Illinois, while heavy rain and flooding occurred further south across Missouri and the remainder of Illinois. On the cold side of the system, heavy snow and blizzard conditions were common across the Plains into the Upper Midwest.  As the colder air eventually pushed eastward, several periods of snow fell across much of Iowa as well as the central and northern sections of Illinois.  By the time the storm finally lifted out of the region on December 27th, a thick blanket of snow and ice was in place across most locations along and north of the I-70 corridor.  Below you will find a map showing the track of the storm.

The storm began its life cycle on December 22nd, as a 150mph upper-level jet streak dropped southward along the backside of a deep upper trough centered over the Rockies.  As the jet rounded the base of the trough, a piece of energy ejected northeastward ahead of the main system, spreading the first round of precipitation into the Midwest by the night of December 22nd into the 23rd.  A cold low-level airmass was initially in place across Illinois: however, as winds aloft gradually became southerly ahead of the approaching wave, temperatures aloft quickly rose above freezing.  Surface temperatures were just warm enough across central Illinois to support all rain: however, areas north of the I-74 corridor hovered near or slightly below freezing, which allowed the precipitation to fall in the form of freezing rain.  Temperatures slowly warmed by the afternoon of December 23rd, resulting in rain spreading as far north as a Moline to Kankakee line.  Further north across northern Illinois, freezing rain continued, with ice accumulations exceeding one quarter of an inch in many locations.

As the upper low shifted into the southern Plains, surface low pressure rapidly deepened over the Lower Mississippi River Valley on December 24th.  A continuous stream of Gulf moisture flowing northward ahead of the system produced periods of moderate to occasionally heavy rain across central Illinois.  A few rumbles of thunder occurred from time to time as well, as warmer air flowing in aloft produced increasing amounts of elevated instability. 

A wedge of dry air wrapping around the system brought an end to the steady rain during the night of December 24th, with widespread storm-total rainfall amounts of 1 to 3 inches across central Illinois.  The heaviest rain during this period was found along and southwest of a Effingham line...where a few SWOP observers recorded in excess of 3 inches.

By Christmas morning, the entire storm had become occluded, with the upper low and surface low now co-located with one another over northern Missouri.  Up to this point, central Illinois had been on the warm side of the system: however, cold air to the north and west began to wrap around the circulation center on Christmas Day.  This eventually resulted in a rather unusual event, as cold air spilled into the region from the south.  To illustrate this, please see the two maps below.  The top image shows the 850mb temperatures at 6 AM December 25th, while the bottom image shows the 850mb temperatures at 6 PM December 25th.  The purple color represents readings of -8C (-18F) or lower at an altitude of about 5000ft.  Typically cold air arrives from the north or west, but this time around, it came into the area from the south!  This allowed lingering rain showers to change to snow showers as the day progressed, although the ground remained warm and wet enough to prevent much in the way of accumulation.


The entire system, now fully in the occlusion stage, slowed down considerably and wobbled over Iowa, Missouri, and Illinois from December 25th through 27th.  Several waves of energy rotated around the parent low during this time frame, with the first wave arriving during the morning of December 26th.  Enough lift was triggered by this particular disturbance to produce a period of moderate to heavy snow across the Illinois River Valley northward into northern Illinois.  Due to favorable temperature and moisture conditions aloft, very large snow flakes were produced, resulting in impressive snowfall rates.  By the time the wave departed the area by the late afternoon of December 26th, a wide swath of 3 to 6 inch snow accumulations had occurred along and west of the Illinois River.  Further east, snowfall totals were considerably less, with most locations north of I-70 picking up 1 to 2 inches.

Another piece of energy rotated around the upper low into central Illinois during the morning of December 27th, producing yet another round of light to moderate snow.  This particular feature was not quite as strong as the previous one, so snowfall was a bit lighter.  Once the snow ended by around midday on December 27th, nearly all of central and southeast Illinois had snow on the ground.  The highest snow totals were observed across the Illinois River Valley, where 7 to 9 inch amounts were common.  The snowfall gradually tapered off further south and east, with areas south of I-70 picking up 1 inch or less.
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