MESO: Mesoscale Discussion 0009

ACUS11 KWNS 181803
SWOMCD
SPC MCD 181803
CAZ000-182200-

MESOSCALE DISCUSSION 0009
NWS STORM PREDICTION CENTER NORMAN OK
1203 PM CST MON JAN 18 2010

AREAS AFFECTED...SRN SIERRA MOUNTAINS OF CA

CONCERNING...HEAVY SNOW

VALID 181803Z - 182200Z

SNOWFALL RATES AOA 2 IN/HR WILL BECOME MORE PROBABLE BY MID TO LATE
AFTERNOON ACROSS MAINLY THE SRN SIERRA MOUNTAINS OF CA.

MORNING WV IMAGERY DEPICTS A S/W TROUGH QUICKLY APPROACHING
CA...WITH AN EXPANSIVE SHIELD OF PRECIPITATION OCCURRING WITHIN A
ZONE OF LARGE SCALE ASCENT AHEAD OF THE UPPER DISTURBANCE. THIS AREA
OF PRECIPITATION WILL OVERSPREAD THE SIERRA MOUNTAINS AND FOOTHILLS
THROUGH THE MORNING AND INTO THE AFTERNOON...THOUGH SNOW LEVELS WILL
INITIALLY RESIDE ABOVE 5500-6000 FT MSL.

THE MORE SUBSTANTIAL THREAT FOR HEAVY SNOWFALL RATES /AOA 2 IN PER
HR/ IS FORECAST TO DEVELOP DURING THE 21-00Z TIME PERIOD AS THE
UPPER IMPULSE MOVES ACROSS THE REGION. DYNAMIC FORCING FOR ASCENT
WITH THIS FEATURE...ALONG WITH INCREASING LOW-MID LEVEL WIND SPEEDS
/LEADING TO INCREASING UPSLOPE FLOW/ WILL OCCUR IN CONJUNCTION WITH
MID LEVEL COOLING...RESULTING IN STEEPENING LAPSE RATES AND THE
DEVELOPMENT OF MODEST CONVECTIVE INSTABILITY WITHIN THE DENDRITIC
GROWTH ZONE. IN ADDITION...THOUGH RECENT TRENDS IN SURFACE
OBSERVATIONS SHOW WARMING IS TAKING PLACE ACROSS THE HIGHER
TERRAIN...MODEL GUIDANCE IS CONSISTENT IN INDICATING FALLING SNOW
LEVELS BY 00-03Z ATTENDANT WITH THE COOLER MID LEVEL AIRMASS
/POSSIBLY AS LOW AS 4500-5000 FT MSL/. AFTER 03Z...FORCING FOR
ASCENT ALONG WITH MOISTURE AND INSTABILITY ARE FORECAST TO SLOWLY
BEGIN TO WANE...WHICH SHOULD RESULT IN A DECREASING PROBABILITY FOR
2 IN/HR SNOWFALL RATES.

..GARNER.. 01/18/2010

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

...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 38052014 38732042 38912004 38141926 37731887 36951836
36411811 36151828 36261869 36881886 38052014

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