SWOMCD
SPC MCD 151725
WAZ000-ORZ000-152130-
MESOSCALE DISCUSSION 2252
NWS STORM PREDICTION CENTER NORMAN OK
1125 AM CST TUE DEC 15 2009
AREAS AFFECTED...COLUMBIA RIVER GORGE/BASIN
CONCERNING...FREEZING RAIN
VALID 151725Z - 152130Z
FREEZING RAIN IS LIKELY TO CONTINUE INTO AT LEAST THE 18-21Z PERIOD
OVER THE COLUMBIA GORGE/BASIN REGION WITH PRECIP RATES 0.05-0.15
INCH/3 HOURS EXPECTED--HEAVIEST PRECIP ACCUMULATIONS LIKELY NEAR
DLS. A DECREASING TREND IN PRECIP RATES IS THEN EXPECTED IN THE
LATE AFTERNOON HOURS.
A LEAD UPPER DISTURBANCE AND ASSOCIATED UVV/S AHEAD OF MORE POTENT
FEATURE 400 MI WSW SEA WILL MOVE NEWD ACROSS THE REGION EARLY THIS
AFTERNOON AS THE AREA RESIDES IN A RIGHT ENTRANCE REGION OF
ANTICYCLONIC UPPER JET OVER SRN ALBERTA. THESE FEATURES ALONG WITH
ISENTROPIC LIFT WILL SUPPORT LARGE SCALE ASCENT JUXTAPOSED WITHIN A
SEASONABLY MOIST AXIS /PWATS AOA 0.75 INCH OVER WRN ORE ACCORDING TO
MORNING GPS DATA/ PROTRUDING NEWD FROM THE ERN PACIFIC TO THE
COLUMBIA BASIN.
EARLY MORNING SURFACE ANALYSIS PLACES A WELL-DEFINED SURFACE RIDGE
EXTENDING WWD FROM THE COLUMBIA BASIN TOWARDS THE COLUMBIA GORGE.
THIS WILL LIKELY CONTINUE TO BE MAINTAINED THROUGH THE AFTERNOON
HOURS AS A SHALLOW ELY LOW LEVEL FETCH DOWN THE COLUMBIA RIVER
VALLEY AIDS IN MAINTAINING A SHALLOW SUB-FREEZING NEAR-SURFACE LAYER
INTO THE MID AFTERNOON HOURS. STRONG AGREEMENT IN MORNING MODEL
OUTPUT INDICATES THE HEAVIEST PRECIP RATES WILL LIKELY OCCUR DURING
THE EARLY AFTERNOON HOURS /18Z-21Z/...THEN DECREASE IN THE LATE
AFTERNOON /21Z-00Z/ AS LARGE SCALE SUPPORT WANES. FORECAST
SOUNDINGS /ETAKF-NAM-RUC/ SHOW WARMING IN THE 925MB-800MB LAYER BUT
GENERALLY MAINTAIN A VERY SHALLOW NEAR SURFACE LAYER AOB 32 DEG F.
THESE THERMODYNAMIC PROFILES WOULD SUPPORT A CONTINUATION OF
PRIMARILY FREEZING RAIN THROUGH AT LEAST THE MID AFTERNOON HOURS.
..SMITH.. 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 45401859 45202051 45472135 45782165 46232109 46892058
47091934 46861795 46361780 45401859
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