MESOSCALE DISCUSSION 0020
   NWS STORM PREDICTION CENTER NORMAN OK
   1028 AM CST WED JAN 20 2010
   
   AREAS AFFECTED...SIERRA NEVADA MOUNTAINS OF CA
   
   CONCERNING...HEAVY SNOW 
   
   VALID 201628Z - 202030Z
   
   SNOWFALL RATES AOA 2 IN/HR ARE EXPECTED TO DEVELOP OVER THE SIERRA
   NEVADA MOUNTAINS OF CA BY EARLY AFTERNOON.
   
   A S/W TROUGH OBSERVED IN WV IMAGERY WAS LOCATED OFF THE COAST OF CA
   THIS MORNING...AND IS FORECAST TO MOVE ACROSS THE STATE THIS
   AFTERNOON/EVENING. AS A SHIELD OF WARM CONVEYOR BELT PRECIPITATION
   LOCATED AHEAD OF THE TROUGH CONTINUES TO OVERSPREAD THE REGION
   THROUGH THE LATE MORNING...SNOW LEVELS WILL GENERALLY REMAIN AOA
   4000-5000 FT MSL. HOWEVER...MID LEVEL TEMPERATURES ARE FORECAST TO
   COOL BY LATE AFTERNOON WITH THE APPROACHING UPPER TROUGH...WHICH
   WILL AID IN LOWERING SNOW LEVELS AS WELL AS STEEPENING LAPSE RATES.
   IN ADDITION...MID LEVEL SWLY WIND SPEEDS ARE EXPECTED TO INCREASE BY
   21Z...BECOMING ORIENTED NEARLY PERPENDICULAR WITH THE SIERRA NEVADA
   MOUNTAINS /PARTICULARLY THE SRN SIERRAS/. THE COMBINATION OF
   STRENGTHENING UPSLOPE FLOW AND LARGE SCALE ASCENT ASSOCIATED WITH
   THE TROUGH...IN ADDITION TO THE REDUCTION IN STABILITY...WILL FAVOR
   INCREASING SNOWFALL RATES /AOA 2 IN PER HR/. MODEL GUIDANCE THEN
   INDICATES THE S/W TROUGH WILL EXIT THE REGION BY 06Z...WHICH SHOULD
   RESULT IN A GRADUAL REDUCTION IN SNOWFALL RATES DURING THE OVERNIGHT
   HOURS.
   
   ..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...VEF...REV...HNX...STO...
   
   LAT...LON   35991807 35961845 36301879 37231947 38022023 39332084
               40022159 40332150 39572039 38691993 38051918 37221841
               35991807