MESOSCALE PRECIPITATION DISCUSSION 0788
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
942 PM EDT FRI SEP 08 2017

AREAS AFFECTED...SOUTHEAST CA...SOUTHERN NEVADA...WESTERN AZ

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

VALID 090142Z - 090442Z

SUMMARY...SHOWERS AND THUNDERSTORMS COULD PRODUCE HEAVY RAINFALL
WITH THE POTENTIAL FOR FLASH FLOODING FOR A FEW MORE HOURS.

DISCUSSION...SHOWERS AND THUNDERSTORMS PERSISTS AS WAVES OF
SHORTWAVE ENERGY ROTATE AROUND AN UPPER LOW IN SOUTHERN CA INTO AN
AXIS OF CONFLUENCE NEAR THE CA/NV AND CA/AZ BORDER. 

THE MODEL SOUNDINGS FROM SOUTHEAST CA TO WESTERN AZ SHOW ABOVE
NORMAL MOISTURE...SUPPORTED BY CONTINUITY FROM   THE 18Z YUMA
SOUNDING INDICATING 1.6 INCHES OF PRECIPITABLE WATER.  THESE
VALUES ARE 1-2 STD DEVIATIONS ABOVE NORMAL.  THE ABOVE NORMAL
MOISTURE COMBINED WITH CAPE OF 1000-2000 J/KG INDICATE REMAINING
AVAILABLE INSTABILITY.

THERE WERE EARLIER SHOWERS AND STORMS IN SOUTHEAST CA TO THE AZ
BORDER AND ALSO SOUTHERN NV. ADDITIONAL SHOWERS DEVELOPING IN THE
AREA COULD LEAD TO FLOODING PROBLEMS NOW THAT SOILS HAVE BECOME
WETTER.

THE OBSERVED 00Z AND FORECAST SOUNDINGS ACROSS SOUTHEAST
CA/WESTERN AZ/SOUTHERN NV INDICATE DEEP SOUTHERLY FLOW.  THIS
PROMOTES SOUTH TO NORTH MOVEMENT.

GIVEN LITTLE DIRECTIONAL WIND SHEAR...
PERIODS OF TRAINING CELLS ARE POSSIBLE.  WHERE TRAINING
OCCURS...AMOUNTS OF 1-2 INCHES ARE POSSIBLE....SUPPORTED BY THE
12Z WRF ARW AND NSSL WRF. THUS SOME FLASH FLOODING WILL BE
POSSIBLE. AS THE NIGHT PROGRESSES...LOSS OF INSTABILITY WILL LEAD
TO A REDUCTION IN SHOWER COVERAGE/INTENSITY.

PETERSEN

ATTN...WFO...PSR...SGX...VEF...

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

LAT...LON   36931537 36151393 33971384 33301477 33031530
            33141599 33671619 34321641 35201653 36441638