MESOSCALE DISCUSSION 0021
   NWS STORM PREDICTION CENTER NORMAN OK
   1034 AM CST WED JAN 20 2010
   
   AREAS AFFECTED...NERN IA...SWRN WI AND NRN IL
   
   CONCERNING...FREEZING RAIN 
   
   VALID 201634Z - 202030Z
   
   FREEZING RAIN WITH RATES OF 0.02-0.05 IN/HR ARE FORECAST TO SPREAD
   NEWD INTO PORTIONS OF NERN IA...SWRN WI...AND NRN IL THROUGH 21Z.
   THEREAFTER...RATES ARE EXPECTED TO DECREASE IN INTENSITY BY 21-00Z.
   
   A LOW AMPLITUDE S/W TROUGH OBSERVED IN WV IMAGERY HAS BEEN LIFTING
   NEWD INTO THE MID MS VALLEY REGION THROUGH THE MORNING. MODEST
   SURFACE CYCLOGENESIS OVER THE CENTRAL PLAINS ATTENDANT TO THIS UPPER
   FEATURE HAS AIDED IN FOCUSING A ZONE OF LOW LEVEL WAA ACROSS
   CENTRAL/SRN IA INTO WRN IL. FORCING FOR ASCENT ASSOCIATED WITH THESE
   FEATURES HAS RESULTED IN AN ELONGATED BAND OF PRECIPITATION LIFTING
   N-NEWD ACROSS IA AND IL...WITH THE PREDOMINANT PRECIPITATION TYPE
   BEING FREEZING RAIN DUE TO THE PRESENCE OF AN ELEVATED WARM NOSE
   SPREADING NWD ACROSS A SUB FREEZING SURFACE AIRMASS. SUBSTANTIAL ICE
   ACCUMULATIONS HAVE BEEN OBSERVED THROUGH THE EARLY TO MID MORNING
   HOURS ACROSS THE SRN HALF OF IA...AND MODERATE FREEZING RAINFALL
   RATES /UP TO 0.02-0.05 IN PER HR/ WILL LIKELY CONTINUE THROUGH 21Z
   AS THE SHIELD OF PRECIPITATION CONTINUES TO SPREAD NEWD INTO NERN IA
   AND SWRN WI/NRN IL. 
   
   IN ADDITION...LOWERED STABILITY WITHIN A DRY SLOT OBSERVED IN WV
   IMAGERY ADVANCING INTO ERN IA/NWRN IL IS ALSO FAVORING THE
   DEVELOPMENT OF THUNDERSTORMS...WHICH MAY RESULT IN ICE PELLETS WITH
   THE MORE VIGOROUS CONVECTIVE ELEMENTS. SURFACE OBSERVATIONS ALSO
   REVEAL A WARMING TREND WITH THE PASSAGE OF THE MID LEVEL DRY
   SLOT...WITH TEMPERATURES OVER WRN IL INCREASING TO AROUND FREEZING
   IN ITS WAKE. THUS...THE SURFACE FREEZING LINE MAY TEND TO LIFT NWD
   WITH TIME INTO PORTIONS OF CENTRAL-WRN IL AND ERN IA. OTHERWISE...AS
   THE AFTERNOON PROGRESSES...THE S/W TROUGH IS FORECAST TO ENCOUNTER
   AN UPPER RIDGE POSITIONED OVER THE GREAT LAKES REGION...AND LARGE
   SCALE ASCENT IS EXPECTED TO DECREASE SUBSTANTIALLY...RESULTING IN
   LOWERED PRECIPITATION/FREEZING RAINFALL RATES.
   
   ..GARNER.. 01/20/2010
   
   
   ...NOTICE...
   SPC WINTER WEATHER MESOSCALE DISCUSSIONS WILL BE ISSUED ON A
   SCHEDULED BASIS FROM DECEMBER 1 THROUGH FEBRUARY 28...FOUR
   TIMES A DAY...AROUND 00 UTC...06 UTC...12 UTC...AND 18 UTC
   FOR SIGNIFICANT SHORT RANGE WINTER WEATHER EVENTS.
   
   ATTN...WFO...IND...LOT...ILX...MKX...DVN...ARX...MPX...DMX...
   
   LAT...LON   41409060 40398824 40408749 41078729 42098770 43239019
               43539158 43509404 42889408 41409060