MESOSCALE PRECIPITATION DISCUSSION 0294
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1142 PM EDT MON AUG 18 2014
 
AREAS AFFECTED...WESTERN AZ, SOUTHEAST CA, SOUTHERN NV 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 190341Z - 190941Z
 
SUMMARY...AN INCREASE IN LOW-LEVEL INFLOW ACROSS THE REGION
COMBINED WITH REMAINING INSTABILITY AND MOISTURE COULD LEAD TO
THUNDERSTORMS WITH RAIN RATES UP TO 2" AN HOUR.  FLASH FLOODING IS
CONSIDERED POSSIBLE.

DISCUSSION...PRECIPITABLE WATER VALUES OF 1.5-2", 1.5-2.5 STANDARD
DEVIATIONS ABOVE NORMAL FOR MID TO LATE AUGUST, INHABIT PORTIONS
OF THE DESERT SOUTHWEST, COURTESY OF A DEEP-LAYERED CYCLONE
OFFSHORE NORTHWEST CA.  THE LATEST SPC MESOANALYSIS INDICATES
MIXED LAYER CAPES OF 500-2000 J/KG ACROSS THIS REGION, WHICH HAS
BEEN RELATIVELY UNTOUCHED BY AFTERNOON AND EVENING CONVECTION THUS
FAR.

THE LATEST RAP ANALYSES INDICATE THAT 850-700 HPA INFLOW IN THIS
REGION IS EXPECTED TO INCREASE AS THE DEEP LAYERED LOW DROPS
SOUTHWARD, WITH CLOUD-BASED INFLOW INCREASING TO ~20 KTS FROM 07Z
ONWARD.  THIS IS EXPECTED TO LEAD TO A REJUVENATION OF CONVECTIVE
ACTIVITY, WHICH TO SOME DEGREE APPEARS TO BE OCCURRING IN
WEST-CENTRAL AZ AT THIS TIME.  SOME DRY AIR IN THE LOW LEVELS OF
AREA SOUNDINGS INDICATE THAT STORMS SHOULD BE FORWARD PROPAGATING
TO THE NORTH-NORTHEAST AT 15-20 KTS ONCE THEY LEAVE THEIR SOURCE
REGION, THOUGH INCREASING LOW-LEVEL INFLOW COULD ALLOW FOR
BACKBUILDING.  THIS SITUATION COULD LEAD TO CELL TRAINING.  THE
18Z SSEO NEIGHBORHOOD PROBABILITIES OF EXCEEDING THREE HOUR FLASH
FLOOD GUIDANCE INCREASE OVERNIGHT, RISING INTO THE 30-50% RANGE
ACROSS PORTIONS OF SOUTHERN AND WESTERN AZ FROM 07Z ONWARD.  WITH
HOURLY RAIN RATES IN STRONGER STORMS POTENTIALLY REACHING 2",
FLASH FLOODING IS CONSIDERED POSSIBLE.

ROTH

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

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

LAT...LON   371411393 365011276 354711200 340511134 328311122 
            316911159 318911306 322911434 325511509 325411621 
            325711626 336011666 356011678 366011676 371111562