MESOSCALE DISCUSSION 0239
   NWS STORM PREDICTION CENTER NORMAN OK
   1151 AM CDT SUN MAR 20 2011
   
   AREAS AFFECTED...THE SIERRAS OF E-CNTRL CA AND EXTREME W-CNTRL NV
   
   CONCERNING...HEAVY SNOW 
   
   VALID 201651Z - 202045Z
   
   THE THREAT FOR SNOWFALL WITH RATES OF 2 IN/HR WILL SHIFT SWD ACROSS
   THE SIERRAS OF E-CNTRL CA AND EXTREME W-CNTRL NV THIS AFTERNOON.
   
   A ZONE OF STRONG SLY TO SWLY FLOW IN THE BASE OF DEEP-LAYER LOW
   PRESSURE AND AN ATTENDANT TROUGH OVER THE ERN PACIFIC IS IMPINGING
   ON THE SIERRAS /500-MB SWLY WINDS OF 70 TO 80 KT ACCORDING TO RUC
   ANALYSIS/. THE COMBINATION OF AMPLE MOISTURE TRANSPORT AND UPSLOPE
   FLOW CONTINUES DRIVING PRECIPITATION OVER THIS AREA. SUFFICIENTLY
   COLD TEMPERATURES OVER THE HIGHER ELEVATIONS...PARTICULARLY AOA 4000
   FT ABOVE MSL...WILL SUPPORT HEAVY SNOW INTO THIS AFTERNOON ACROSS
   THE CNTRL/SRN SIERRAS. MORNING SFC OBSERVATIONS INDICATE HEAVY SNOW
   IN THE LAKE TAHOE AREA...LIKELY IN ASSOCIATION WITH A SLOWLY MOVING
   COLD FRONT. THE AFOREMENTIONED DEEP-LAYER TROUGH IS FORECAST TO
   AMPLIFY TODAY...SHUNTING THE ZONE OF STRONGEST DEEP-LAYER FLOW AND
   ASCENT SWD. THIS WILL GRADUALLY DRIVE THE FOCUS FOR POTENTIAL
   2-IN/HR SNOWFALL RATES SWD...AS SUGGESTED BY THE 14Z RUN OF THE
   EXPERIMENTAL HRRR. HOWEVER...AS MID-LEVEL LAPSE RATES STEEPEN ACROSS
   NRN PORTIONS OF THE DISCUSSION AREA THIS AFTERNOON...THE
   PRECIPITATION COULD ASSUME MORE OF A CONVECTIVE NATURE WITH AN
   ATTENDANT THREAT FOR 2-IN/HR RATES. THESE CONDITIONS ARE EXPECTED TO
   PERSIST INTO THE EARLY EVENING HOURS...BEFORE TAPERING OFF TONIGHT.
   
   ..COHEN.. 03/20/2011
   
   
   ATTN...WFO...VEF...REV...HNX...STO...
   
   LAT...LON   35691811 35921856 36771911 37631983 38402040 39022051
               39462038 39432009 38961980 38261937 37601879 36251796
               35691811