MESO: Mesoscale Discussion 1799

ACUS11 KWNS 221743
SWOMCD
SPC MCD 221743
AZZ000-UTZ000-NVZ000-CAZ000-221945-

MESOSCALE DISCUSSION 1799
NWS STORM PREDICTION CENTER NORMAN OK
1243 PM CDT WED AUG 22 2012

AREAS AFFECTED...SOUTHEAST CA/SOUTHERN NV TO NORTHWEST AZ/FAR
SOUTHWEST UT

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 221743Z - 221945Z

PROBABILITY OF WATCH ISSUANCE...20 PERCENT

SUMMARY...LOCALLY VERY HEAVY/PERSISTENT RAINFALL WILL LIKELY BE THE
PRIMARY CONCERN...BUT ISOLATED DAMAGING WINDS/MAINLY SUB-SEVERE HAIL
WILL BE POSSIBLE ACROSS PORTIONS OF SOUTHEAST CA/SOUTHERN NV INTO
NORTHWEST AZ/FAR SOUTHWEST UT THROUGH THIS AFTERNOON.

DISCUSSION...A SOUTHERN STREAM UPPER LOW CONTINUES TO SHIFT SLOWLY
EAST-SOUTHEASTWARD OVER SOUTH-CENTRAL CA AT MIDDAY PER WATER VAPOR
IMAGERY...WITH AN ASSOCIATED DIFFLUENT/UPPER JET EXIT REGION
OVERSPREADING MUCH OF NV TO NEAR THE UT/AZ BORDERS. A VERY MOIST
AIRMASS IS IN PLACE ACROSS THE REGION...ESPECIALLY EVIDENT BY AN
EXTREMELY HIGH CLIMO-RELATIVE PRECIPITABLE WATER VALUE OF 1.69
INCHES PER THE 12Z LAS VEGAS OBSERVED SOUNDING...IN ADDITION TO
RECENT GPS-DERIVED PW VALUES AROUND 2-INCHES ACROSS SOUTHEAST CA.

THE DEGREE OF MOISTURE/PW...ALONG WITH INCREASING LARGE
SCALE/OROGRAPHIC ASCENT AND DIURNAL HEATING WILL CONTRIBUTE TO
INCREASING NUMBER/VIGOR OF TSTMS THROUGH THIS AFTERNOON. LOCALLY
VERY HEAVY/PERSISTENT RAINFALL WILL LIKELY BE THE PRIMARY
IMPACT...ALTHOUGH SUFFICIENT VERTICAL SHEAR AND THERMODYNAMIC
CHARACTERISTICS WILL EXIST FOR SOME SEMI-SUSTAINED STORMS/MULTICELLS
CAPABLE OF WIND DAMAGE AND MAINLY SUB-SEVERE HAIL. SAMPLING THE
RELATIVELY STRONG SOUTHWESTERLY MID-HIGH LEVEL WINDS AHEAD OF THE
ADVANCING UPPER LOW...THE 12Z OBSERVED SOUNDING AND MORE RECENT
WSR-88D VWP DATA FROM LAS VEGAS ARE INDICATIVE OF AROUND 25 KT OF
0-6 KM SHEAR...WHILE THERMODYNAMIC ADJUSTMENTS TO THE 12Z VEF
SOUNDING SUGGEST AS MUCH AS 1000-1500 J/KG MLCAPE EXISTS ACROSS THE
LOWER CO RIVER VALLEY WHERE MORE AGGRESSIVE HEATING IS OCCURRING.

..GUYER/MEAD.. 08/22/2012

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

ATTN...WFO...FGZ...SLC...PSR...VEF...LKN...SGX...

LAT...LON 33371647 35131580 37001592 38151500 37911361 37321304
35751243 34801351 33271469 33371647

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