Significant Weather Observer Program

 WEEKLY WEATHER DISCUSSION

written: 12/17/14 

Here is the latest U.S. Hazards Outlook issued by the Climate Prediction Center (CPC): http://www.cpc.ncep.noaa.gov/products/predictions/threats/hazards_d3_7_contours.png

A predominantly west-to-east zonal flow pattern across North America has kept all the bitterly cold air locked up well to the north across the Arctic over the past couple of weeks; however, signs are beginning to strongly suggest a piece of this cold airmass will be dropping southward into the United States after Christmas.  In the meantime, near-normal temperatures in the 30s can be expected through the middle of next week.  Several weak weather systems will impact the region over this time period as well, with the first arriving on Thursday, December 18th.  This system will be interacting with an initially dry airmass across central Illinois, so precipitation amounts are expected to remain quite light.  While it will be cold enough for snow, accumulations will be limited to less than 1 inch along and southwest of a Rushville...to Taylorville...to Olney line.  A second disturbance will track across the southern Plains into the Tennessee River Valley on Saturday, December 20th.  While most of the precipitation associated with this feature will remain southeast of central Illinois, light snow will likely spread as far north as the I-70 corridor Friday night into Saturday.  Little or no accumulation is anticipated.  A much stronger system will drop into the Midwest next week.  With southerly flow ahead of the wave, precipitation will initially be in the form of rain on Monday, December 22nd.  As the upper low gradually amplifies and lifts northeastward into the Great Lakes, colder air will spill into the region and change the rain to light snow by next Tuesday into Wednesday.  By the time the atmosphere cools enough to support snow, the bulk of the precipitation will have exited the region, so a significant snow accumulation is not expected.  Even still, the potential exists for a little snow on the ground for Christmas Day.  After that, medium range computer models have been consistently building an upper-level ridge of high pressure over western North America, with a corresponding downstream trough of low pressure over central Canada and the central U.S.  This will allow the much colder air currently in place over the Arctic to spill southward, resulting in below normal temperatures for the last week of December.               

NOTE: This weather discussion will be updated weekly (usually on Wednesday or Thursday) 


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.