MESO: Mesoscale Discussion 2240

ACUS11 KWNS 181810
SWOMCD
SPC MCD 181810
CAZ000-182245-

MESOSCALE DISCUSSION 2240
NWS STORM PREDICTION CENTER NORMAN OK
1210 PM CST TUE DEC 18 2007

AREAS AFFECTED...CA SIERRA MOUNTAINS

CONCERNING...HEAVY SNOW

VALID 181810Z - 182245Z

HEAVY SNOW WILL DEVELOP SOUTHWARD ACROSS THE SIERRA MOUNTAINS
THROUGH MID/LATE AFTERNOON...WITH SNOW LEVELS LOWERING TO AROUND
4500-5000 FT. SNOW RATES OF 2 IN/HR ARE LIKELY IN THE HIGHER
ELEVATIONS...ESPECIALLY ABOVE 5500-6000 FT.

WATER VAPOR IMAGERY FEATURES SHORTWAVE TROUGH/COLD FRONT NEARING THE
NORTHERN CA COAST LATE THIS MORNING. IN ASSOCIATION WITH THIS
TROUGH...HEAVY SNOW IS EXPECTED TO INCREASE ACROSS THE SOUTHERN
SIERRA OWING TO LARGE SCALE FORCING FOR ASCENT/OROGRAPHIC
LIFT...WITH A MODEST LOWERING /500 FT TO PERHAPS 1000 FT / OF SNOW
LEVELS EXPECTED VIA THE ARRIVAL OF COLDER AIR ALOFT /REFERENCE -22C
AT 500 MB PER 12Z MEDFORD ORE OBSERVED RAOB/. LATEST MESONET/RAWS
OBSERVATIONS SUGGEST SNOW LEVELS ARE GENERALLY RUNNING AROUND
4500-5000 FT ACROSS THE NORTHERN SIERRA...WITH 5500-6000 FT ACROSS
THE SOUTHERN SIERRA. SIMILAR TO THIS MORNINGS /12Z/ OBSERVED RAOB AT
OAKLAND /0.85 INCHES/...GPS-DERIVED PRECIPITABLE WATER DATA DEPICT A
RELATIVELY MOIST PLUME OF 0.80-1.00 INCH PW VALUES SPREADING ACROSS
NORTHERN/CENTRAL CA. ALONG WITH AMPLE LIFT AND A RELATIVELY MOIST
AIRMASS...STEEPENING MID LEVEL LAPSE RATES WILL FURTHER PROMOTE
HEAVY SNOW...WITH 2 IN/HR RATES LIKELY IN THE HIGHEST ELEVATIONS OF
THE SOUTHERN SIERRA THIS AFTERNOON.

.GUYER.. 12/18/2007

..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...

39802086 38461959 37801895 37451873 36741849 36111825
35861844 35891874 36611891 37041916 37281945 37801995
38632057 39422115

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