MESO: Mesoscale Discussion 2251

ACUS11 KWNS 151147
SWOMCD
SPC MCD 151146
WAZ000-ORZ000-151545-

MESOSCALE DISCUSSION 2251
NWS STORM PREDICTION CENTER NORMAN OK
0546 AM CST TUE DEC 15 2009

AREAS AFFECTED...SRN WA AND NRN ORE

CONCERNING...FREEZING RAIN

VALID 151146Z - 151545Z

FREEZING RAIN WITH RATES UP TO OR EXCEEDING 0.05 IN/3HRS IS EXPECTED
TO PERSIST THROUGH THE MORNING AND POSSIBLY INTO THE EARLY AFTERNOON
ACROSS PORTIONS OF SRN WA/NRN ORE.

EARLY MORNING SURFACE OBSERVATIONS COMBINED WITH REGIONAL RADAR
IMAGERY SHOW A PERSISTENT SHIELD OF PRECIPITATION SPREADING EWD
ACROSS WA/ORE WITHIN WARM CONVEYOR BELT CLOUD BAND. MEANWHILE...THE
NEXT IN A SERIES OF UPPER IMPULSES WILL BEGIN TO APPROACH THE PAC NW
DURING THE LATE MORNING-AFTERNOON...WITH SUBTLE DOWNSTREAM RIDGING
TAKING PLACE. THE RRQ OF A 120+ KT UPPER LEVEL JET WILL BE EMBEDDED
WITHIN THE S/W RIDGE...WHICH MAY MAINTAIN FAVORABLE UPPER
DIVERGENCE/LARGE SCALE ASCENT ACROSS THE REGION FOR CONTINUED
PRECIPITATION PRODUCTION. FURTHERMORE...SHORT TERM MODEL GUIDANCE
SUGGESTS LOW LEVEL JET WILL REINTENSIFY THROUGH THE
MORNING...FOCUSING WAA OVER THE COLUMBIA GORGE. AS THIS TAKES
PLACE...MODEL FORECAST SOUNDINGS SUGGEST SURFACE AIRMASS WITHIN
COLUMBIA/SNAKE RIVER VALLEYS WILL REMAIN AT OR BELOW FREEZING WHILE
ELEVATED WARM NOSE ASSOCIATED WITH WAA ABOVE THE VALLEYS BECOMES
INCREASINGLY MORE PROMINENT. THIS THERMODYNAMIC STRUCTURE WILL FAVOR
FREEZING RAIN...WITH RATES UP TO AT LEAST 0.05 IN/3HRS...AS
SUGGESTED BY RECENT SURFACE OBS WITHIN THE DISCUSSION AREA.

..GARNER.. 12/15/2009

...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...OTX...PDT...PQR...

LAT...LON 45942174 46042042 47071813 46871765 45651843 45232046
45612193 45942174

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