MESO: Mesoscale Discussion 2115

ACUS11 KWNS 172042
SWOMCD
SPC MCD 172042
WAZ000-ORZ000-172245-

MESOSCALE DISCUSSION 2115
NWS STORM PREDICTION CENTER NORMAN OK
0342 PM CDT SAT OCT 17 2009

AREAS AFFECTED...WRN WASHINGTON INTO PARTS OF NW OREGON

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 172042Z - 172245Z

THE RISK FOR SEVERE STORMS DURING THE MID/LATE AFTERNOON HOURS MAY
NOT BE NEGLIGIBLE. WHILE THE NEED FOR A SEVERE WEATHER WATCH IS NOT
CURRENTLY ANTICIPATED...TRENDS WILL CONTINUE TO BE MONITORED.

LATEST RUC GUIDANCE AND OBSERVATIONAL DATA INDICATE THAT A BROADLY
CYCLONIC 70+ KT 500 MB JET STREAK IS NOW IN THE PROCESS OF NOSING
ACROSS THE OREGON COAST INTO THE CASCADES. JUST OFF THE SURFACE
THROUGH THE LOWER TROPOSPHERE...WIND FIELDS ARE INCREASING AS WELL
...BUT VEERING TO A MORE SOUTHWESTERLY COMPONENT ABOVE NEAR SURFACE
FLOW THAT REMAINS SOUTH/SOUTHEASTERLY TO THE WEST OF THE CASCADE
MOUNTAINS...AS A SURFACE LOW CENTER MIGRATES TOWARD VANCOUVER
ISLAND. RESULTANT HODOGRAPHS ARE CHARACTERIZED BY STRONG DEEP LAYER
SHEAR...AND APPEAR SUPPORTIVE OF UPDRAFT ROTATION AS THUNDERSTORMS
DEVELOP THIS AFTERNOON.

ALTHOUGH CLOUD COVER HAS INHIBITED SURFACE HEATING...RAINFALL
ASSOCIATED WITH THE CONVECTIVE FRONTAL BAND HAS MOISTENED THE
BOUNDARY LAYER...WITH SURFACE DEW POINTS NOW GENERALLY RANGING FROM
THE MID 50S TO AROUND 60F. WITH THE ONSET OF STRONGER MID-LEVEL
COOLING /500 TEMPS AOB -20C/ AND FORCING FOR UPWARD VERTICAL
MOTION...DESTABILIZATION SUFFICIENT FOR AT LEAST THE INITIATION OF
SCATTERED THUNDERSTORMS APPEARS LIKELY DURING THE 21-23Z TIME FRAME.
SEVERE POTENTIAL REMAINS MORE UNCERTAIN DUE TO THE LIKELIHOOD OF
MIXED LAYER CAPE PEAKING AROUND 500 J/KG OR LESS. BUT...THIS STILL
COULD BE SUFFICIENT FOR AN ISOLATED LOW TOPPED SUPERCELL OR TWO. IF
THIS OCCURS...MARGINALLY SEVERE HAIL WILL BE POSSIBLE ACROSS THE
COASTAL RANGES INTO THE WESTERN SLOPES OF THE CASCADES. AN ISOLATED
TORNADO IS NOT OUT OF THE QUESTION FROM NEAR PUGET SOUND
SOUTHWARD...PERHAPS AS FAR AS THE LOWER WILLAMETTE VALLEY.

..KERR.. 10/17/2009

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

ATTN...WFO...SEW...PQR...

LAT...LON 48052415 48702368 48952245 48742157 47602152 46442198
45642217 44972260 44912327 45422387 46292389 47212421
47702421 48052415

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