MESO: Mesoscale Discussion 0234

ACUS11 KWNS 220230
SWOMCD
SPC MCD 220230
CAZ000-NVZ000-220830-

MESOSCALE DISCUSSION 0234
NWS STORM PREDICTION CENTER NORMAN OK
0930 PM CDT SAT MAR 21 2009

AREAS AFFECTED...SIERRA NEVADA MOUNTAINS

CONCERNING...HEAVY SNOW

VALID 220230Z - 220830Z

SNOWFALL RATES ARE EXPECTED TO STEADILY INCREASE TONIGHT ALONG THE
WRN SLOPES OF THE SIERRA NEVADA MOUNTAINS.

SATELLITE IMAGERY AND 00Z UPR AIR OBS REVEAL A SHARP DIGGING TROUGH
LOCATED OFF OF THE W COAST. ONE S/W TROUGH HAS ALREADY PASSED EAST
ACROSS THE REGION EARLIER DURING THE AFTERNOON...WITH ANOTHER POISED
TO ENTER CENTRAL CA BETWEEN 06-12Z/SUN. DCVA AHEAD OF THIS FEATURE
WILL RESULT IN STRENGTHENING LARGE SCALE ASCENT...SUPPORTING A
SYNOPTIC SCALE REGIME FAVORABLE FOR WIDESPREAD PRECIPITATION. IN
ADDITION...SOME INCREASE IN SOUTHWESTERLY LOW TO MID LEVEL FLOW IS
LIKELY DURING THE NIGHT...RESULTING IN STRENGTHENING OROGRAPHIC
ASCENT AND HIGHER PRECIPITATION RATES ALONG THE WRN SLOPES OF THE
SIERRA NEVADA MOUNTAINS.

COLD AIR IS FORECAST TO BE INJECTED INTO THE REGION AS THE UPPER
TROUGH AXIS APPROACHES CA. THIS WILL RESULT IN SNOW LEVELS...WHICH
BASED ON 00Z RAOBS WERE ESTIMATED TO BE AROUND 5000-6000 FT MSL...TO
FALL TOWARDS 3000-4000 FT MSL BY 12Z. THE COLD AIR WILL ALSO AID IN
STEEPENING LAPSE RATES...SUPPORTING THE PRODUCTION OF A FEW HUNDRED
J/KG OF MUCAPE AND POSSIBLE THUNDERSNOW. THEREFORE...SNOWFALL RATES
SHOULD GENERALLY RESIDE AROUND 1 IN/HR AS THE NIGHT PROGRESSES DUE
TO THE PERSISTENT MOIST UPSLOPE REGIME...WITH LOCALLY HIGHER RATES
OF 2-3 IN/HR POSSIBLE WITH CONVECTIVELY ENHANCED SNOWBURSTS.

..GARNER.. 03/22/2009

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...VEF...REV...HNX...STO...

LAT...LON 40122170 40732171 40482101 40022050 39242014 38621971
37981912 37241844 36571811 36251816 36091828 36111872
36451881 36731906 37121940 37571985 37922017 38102035
38512046 38892064 39432100 39712132 39922159 40122170

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