MESO: Mesoscale Discussion 1307

ACUS11 KWNS 292006
SWOMCD
SPC MCD 292006
WAZ000-292200-

MESOSCALE DISCUSSION 1307
NWS STORM PREDICTION CENTER NORMAN OK
0306 PM CDT SAT JUN 29 2013

AREAS AFFECTED...PORTIONS OF WA

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 292006Z - 292200Z

PROBABILITY OF WATCH ISSUANCE...5 PERCENT

SUMMARY...AS CONVECTION DEVELOPS THROUGH THE AFTERNOON OVER THE
CASCADES...AN ISOLATED SEVERE THUNDERSTORM OR TWO CANNOT BE RULED
OUT. HOWEVER...THE ISSUANCE OF A WW IS NOT NEEDED.

DISCUSSION...TERRAIN-DRIVEN CIRCULATIONS ARE ENCOURAGING
THUNDERSTORM DEVELOPMENT OVER CNTRL/ERN PARTS OF THE WASHINGTON
CASCADES WHILE DEEPER FORCING FOR ASCENT IS BEING WEAKLY BOLSTERED
IN ASSOCIATION WITH A LOW-AMPLITUDE MID-LEVEL SHORTWAVE IMPULSE
CROSSING THE AREA. WITH RELATIVELY RICH TROPOSPHERIC MOISTURE IN
PLACE -- PW VALUES AROUND ONE INCH PER GPS DATA AND SFC DEWPOINTS IN
THE 50S TO LOWER 60S -- AT LEAST PATCHES OF INSOLATION/SFC DIABATIC
HEATING ARE CONTRIBUTING TO AROUND 300-1000 J/KG OF MLCAPE ACROSS
THE AREA. THE ENHANCED MID-LEVEL HEIGHT GRADIENT AND FLOW ASSOCIATED
WITH THE IMPULSE ARE REFLECTED BY OTX VWP DATA THAT INDICATE AT
LEAST 40-KT SWLY/S AOA 5 KM AGL. THIS WILL SUPPORT SUFFICIENT
EFFECTIVE BULK SHEAR -- I.E. AROUND 35-50 KT -- FOR A FEW ORGANIZED
CONVECTIVE STRUCTURES/POSSIBLE WEAK SUPERCELL STRUCTURES.

AN ISOLATED INSTANCE OF SVR HAIL AND/OR SVR WINDS CANNOT BE RULED
OUT WITH THIS ACTIVITY AS IT SPREADS NNEWD/NEWD ACROSS THE MCD AREA.
SOME OF THIS ACTIVITY COULD MOVE OFF THE HIGHER TERRAIN AND IMPACT
PORTIONS OF THE COLUMBIA BASIN LATER TODAY AS THE IMPULSE MAKES
ADDITIONAL NEWD PROGRESS AND AS CONVECTION-RELATED COLD POOLS
POTENTIALLY DESCEND TOWARD LOWER ELEVATIONS. THE PRESENCE OF ONLY
WEAK DEEP-LAYER FORCING FOR ASCENT...AND GENERALLY WEAK MID-LEVEL
LAPSE RATES ACROSS THE MCD AREA...ARE EXPECTED TO PREVENT A MORE
SUBSTANTIAL SVR THREAT FROM EVOLVING.

..COHEN/MEAD.. 06/29/2013

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

ATTN...WFO...OTX...PDT...SEW...

LAT...LON 49031889 48511877 47681949 46892012 45932069 46212141
47512140 48942024 49031889

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