MESO: Nws Weather Prediction Center College Park Md

AWUS01 KWNH 251949
FFGMPD
AZZ000-UTZ000-NVZ000-CAZ000-260017-

MESOSCALE PRECIPITATION DISCUSSION 0223
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
348 PM EDT SUN AUG 25 2013

AREAS AFFECTED...WESTERN AZ...SOUTHEAST CA...AND SOUTHERN NV

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 251947Z - 260017Z

SUMMARY...SCATTERED THUNDERSTORMS WITH RAINFALL RATES OF 2"+ PER
HOUR ARE INCREASING IN COVERAGE AND ORGANIZATION, WHICH ARE
EXPECTED TO MOVE SLOWLY AND POTENTIALLY MERGE/TRAIN OVER THE NEXT
SEVERAL HOURS. FLASH FLOODING IS LIKELY IN THIS REGION.

DISCUSSION...TO THE WEST OF AN AREA OF STRATIFORM RAIN ACROSS
WESTERN AZ ASSOCIATED WITH TROPICAL DEPRESSION IVO, DIURNAL
HEATING HAS LED TO CAPE VALUES AROUND 1000 J/KG FROM THE LOWER
COLORADO RIVER VALLEY WESTWARD ACROSS THE DESERT SOUTHWEST.
LOW-LEVEL INFLOW, PER THE 18Z RAP RUN, IS EXPECTED TO INCREASE
ACROSS THE REGION THROUGH THE AFTERNOON AND EVENING HOURS TOWARDS
20-30 KNOTS OUT OF THE SOUTH. THE LOW-LEVEL WIND SHEAR ACROSS THE
REGION IS NEAR THE BORDERLINE FOR ORGANIZED CONVECTION -- BEYOND
WHAT WAS SEEN THIS TIME YESTERDAY -- WHICH INCREASES THE RISK OF
FLASH FLOODING. DIVERGENCE ALOFT IS BEING AIDED BY AN UPPER LEVEL
JET ACROSS SOUTH-CENTRAL CA AND NV. THE STORMS SHOULD BE
EFFICIENT RAIN MAKERS, AS PRECIPITABLE WATER VALUES OF 1.75-2.50
INCHES LIE ACROSS THE REGION, WHICH IS WELL ABOVE NORMAL, BEYOND
THE 90TH PERCENTILE FOR LATE AUGUST, AND ABOVE THAT SEEN THIS TIME
YESTERDAY. WHILE 850-400 HPA MEAN WINDS IMPLY CELL MOTION OF
15-20 KNOTS TO THE NORTH, CORFIDI VECTORS ARE EVEN LIGHTER, ON THE
ORDER OF 5 KNOTS, WHICH COULD BE MORE RELEVANT PER THE EXPECTATION
OF ORGANIZED CONVECTION. SOME WESTWARD DRIFT IS EXPECTED SIMILAR
TO THIS TIME YESTERDAY, WITH MOVEMENT TOWARDS THE CURRENT
LOW-LEVEL MOISTURE GRADIENT NEAR THE 117TH MERIDIAN EXPECTED,
SIMILAR TO YESTERDAY. THE 12Z AFWA, 12Z EXREF MEAN, 06Z ARW, AND
06Z NMM SEEM TO HAVE THE BEST HANDLE OF THE CURRENT CONVECTIVE
DEVELOPMENT. CELL MERGERS/TRAINING COULD LEAD TO LOCAL RAINFALL
AMOUNTS OF 2-4 INCHES WITHIN A SHORT TIME FRAME, WHICH IS CLOSE TO
THE ANNUAL AVERAGE RAINFALL FOR THE REGION. CONFIDENCE IN
REGIONAL FLASH FLOODING IS AS HIGH AS IT HAS BEEN FOR THIS REGION
DURING THIS YEAR'S MONSOON SEASON. REGIONAL FLASH FLOODING IS
CONSIDERED LIKELY.

ROTH

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

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

ATTN...RFC...CBRFC...CNRFC...

LAT...LON 33111671 33741706 34981706 36381655 37171546 37081362
35791322 33051393 32441489 32601614 33111671

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