MESO: Mesoscale Discussion 2121

ACUS11 KWNS 201852
SWOMCD
SPC MCD 201851
ORZ000-WAZ000-202045-

MESOSCALE DISCUSSION 2121
NWS STORM PREDICTION CENTER NORMAN OK
1251 PM CST TUE NOV 20 2012

AREAS AFFECTED...COASTAL SWRN WA AND COASTAL NWRN ORE

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 201851Z - 202045Z

PROBABILITY OF WATCH ISSUANCE...5 PERCENT

SUMMARY...A STRONG CONVECTIVE BAND AROUND 30 NM OFFSHORE IS FORECAST
TO REACH THE COAST AROUND 1930Z AND POSE A THREAT FOR STRONG
WINDS...PERHAPS LOCALLY REACHING SVR THRESHOLDS. A WW IS NOT NEEDED
OWING TO ONLY SPORADIC SVR COVERAGE...AT MOST.

DISCUSSION...AS A FRONT CONTINUES APPROACHING THE COAST...COMING
INTO CLOSER RANGE OF THE LANGLEY HILL WA RDA...RADAR DATA REVEAL A
NEARLY CONTINUOUS ARC OF LOW-TOPPED FRONTAL CONVECTION OFF THE COAST
FROM GRAYS HARBOR COUNTY TO TILLAMOOK COUNTY. MORE
DISCRETE/SEMI-DISCRETE CONVECTION LIES JUST NORTH OF THE LINE AND
AHEAD OF THE LINE. CURRENT EXTRAPOLATION TAKES THE PRIMARY
CONVECTIVE ARC TO THE COAST AROUND 1930Z...AS IT POSSESSES A
SHOREWARD COMPONENT OF MOTION AROUND 30 KNOTS. VISIBLE SATELLITE
IMAGERY INDICATES A NARROW AXIS OF MODEST INSOLATION AHEAD OF THE
CONVECTIVE ARC TO SUPPORT MARGINAL DESTABILIZATION. MODIFIED NAM
SOUNDINGS WHERE INSOLATION-DRIVEN BOUNDARY-LAYER HEATING HAS
OCCURRED SUGGEST THAT MLCAPE VALUES OF 400-800 J/KG WILL IMPACT
INLAND LOCATIONS. AND...WITH LANGLEY HILLS VWP DATA INDICATING
STRONG LOW-LEVEL AND DEEP SHEAR /E.G. 0-1-KM BULK SHEAR MAGNITUDE OF
44 KT/...CONVECTIVE ORGANIZATION WILL LIKELY BE MAINTAINED TO THE
COAST. WHILE SOME VEERING OF THE NEAR-SFC FLOW WILL OCCUR AS THE
FRONT APPROACHES...DOWNWARD MOMENTUM TRANSFER IN CONVECTION MAY
SUPPORT AN ISOLATED SVR WIND GUST OR TWO.

..COHEN/BUNTING.. 11/20/2012

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

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

LAT...LON 44732400 45432402 46272408 47372424 47032376 46152353
45252351 44502374 44392402 44732400

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