MESO: Nws Weather Prediction Center College Park Md

AWUS01 KWNH 242116
FFGMPD
UTZ000-AZZ000-NVZ000-CAZ000-250014-

MESOSCALE PRECIPITATION DISCUSSION 0222
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
515 PM EDT SAT AUG 24 2013

AREAS AFFECTED...SOUTHWEST UT...SOUTHERN NV...NORTHWEST
AZ...SOUTHEAST CA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 242114Z - 250014Z

SUMMARY...SCATTERED THUNDERSTORMS WITH HOURLY RAIN RATES EXCEEDING
TWO INCHES IN STRONGER CELLS COULD CAUSE REGIONAL FLASH FLOODING.

DISCUSSION...SCATTERED THUNDERSTORMS ACROSS THE DESERT SOUTHWEST
HAVE SHIFTED WESTWARD THROUGH THE DAY TODAY. SOME OF THIS SHIFT
IS DUE TO AN OUTFLOW BOUNDARY FROM EARLIER ACTIVITY IN AZ, WHILE
THE REMAINDER OF THE SHIFT COULD BE DUE TO A SLIGHT WESTWARD SHIFT
IN RIDGING ACROSS THE WEST CAUSED BY A SHORTWAVE OVERTOPPING THE
RIDGE ACROSS THE NORTHERN ROCKIES. RECENT HRRR RUNS HAVE HAD A
HARD TIME DEPICTING THE THUNDERSTORM EVOLUTION, WITH ITS 15Z RUN
FAVORING UT AND ITS 18Z RUN SUPPORTING THE AREAS AROUND THE DESERT
SOUTHWEST.

AN UPPER-LEVEL TROUGH NEAR THE WEST COAST HAS LED TO DIVERGENCE
ALOFT ACROSS THE REGION. THE UPPER LEVEL JET ACROSS NEVADA IS
EXPECTED TO STRENGTHEN THROUGH THE AFTERNOON INTO THE EVENING
HOURS NEAR THE NV/UT BORDER, ENHANCING DIVERGENCE WITH TIME.
PRECIPITABLE WATER VALUES OF 1-2 INCHES ACROSS THE REGION EXCEED
THE 85TH PERCENTILE OF LATE AUGUST IN THE LAS VEGAS NV AREA,
SUPPORTIVE OF GREATER THAN NORMAL PRECIPITATION EFFICIENCY. RADAR
IMAGERY SUGGEST THAT THE STRONGEST THUNDERSTORMS ARE DROPPING OVER
TWO INCHES OF RAIN PER HOUR, WHICH IS CAUSING LOCAL FLASH
FLOODING. RECENT SATELLITE IMAGERY INDICATES THAT MOISTURE VALUES
ARE INCREASING ACROSS THE REGION DUE TO DECENT LOW-LEVEL INFLOW
FROM THE GULF OF CALIFORNIA, WHICH IS EXPECTED TO DECREASE WITH
TIME. THE 850-250 HPA MEAN WIND SUPPORTS A GENERAL NORTHWARD
MOVEMENT TO THUNDERSTORM CELLS OF 10-15 KNOTS, THOUGH RECENT
WESTWARD PROPAGATION CAUSED BY SHIFTING MID-LEVEL RIDGING AND
OUTFLOW BOUNDARIES FROM EARLIER ACTIVITY ARE RESULTING IN A MORE
NORTHWEST MOTION.

THUNDERSTORMS APPEAR TO BE MIGRATING TOWARD A LOW-LEVEL MOISTURE
GRADIENT ACROSS CA AND NV ROUGHLY ALIGNED WITH A TROUGH VISIBLE AT
850 HPA. CAPE VALUES THROUGH THE DAY TODAY HAVE BEEN IN THE 2000+
J/KG RANGE. RECENT RUNS OF THE ARW INDICATE THE CAPE -- BOTH
SURFACE BASED AND MUCAPE -- SHOULD ERODE THROUGH THIS AFTERNOON
AND EVENING FROM NORTHEAST TO SOUTHWEST, PARTICULARLY AFTER 00Z.
REGIONAL FLASH FLOODING IS CONSIDERED POSSIBLE UNTIL 00Z, WHICH
ACTIVITY BECOMES MORE ISOLATED IN NATURE.

ROTH

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

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

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

LAT...LON 35911428 34651445 33231379 32021255 32191497 32791609
34171707 35831710 36801668 37811582 38671413 38711229
38251186 36631367 35911428

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