MESOSCALE PRECIPITATION DISCUSSION 0359
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1155 AM EDT SAT JUL 18 2015
 
AREAS AFFECTED...PORTIONS OF THE SOUTHWEST U.S. 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 181600Z - 182200Z
 
SUMMARY...THUNDERSTORMS WITH HEAVY RAINFALL ARE EXPECTED TO
INCREASE IN COVERAGE OVER THE NEXT SEVERAL HOURS.  FLASH FLOODING,
WITH ASSOCIATED DEBRIS FLOWS, IS POSSIBLE.

DISCUSSION...MOISTURE IS BEING FUNNELED INTO THE REGION BETWEEN
TROPICAL STORM DOLORES, AN EXPANDING SUBTROPICAL RIDGE ACROSS THE
SOUTHERN ROCKIES, AND A FRONTAL ZONE DRAPED ACROSS CENTRAL NV. 
THIS HAS LED TO SCATTERED THUNDERSTORMS DEVELOPING NEAR THE
COASTAL RANGES OF FAR SOUTHERN CA AND INCREASING IN COVERAGE. 
RAINFALL RATES HAVE APPROACHED 1" AN HOUR AT RAMONA AIRPORT CA. 
PRECIPITABLE WATER VALUES ARE BETWEEN 1-2" PER GPS INFORMATION
ACROSS SOUTHERN CA, SOUTHERNMOST NV, AND THE SOUTHWESTERN HALF OF
AZ WHICH IS 1.5-3 SIGMAS ABOVE THE MEAN FOR MID- JULY.  INFLOW AT
850 & 700 HPA IS STRONGER THAN NORMAL ACROSS THE REGION, AND FOR
THE FIRST TIME IN DAYS, CO-LOCATED WITH THE HIGHER MOISTURE
VALUES.  MIXED LAYER CAPES HAVE BUILT TO 1000-2000 J/KG EAST OF
LAS VEGAS NV AND ACROSS EASTERN RIVERSIDE AND SAN BERNARDINO
COUNTIES IN CA.

RAP FORECASTS INDICATE THAT INSTABILITY EXPANDS ACROSS SOUTHEAST
CA, SOUTHWEST UT, AND THE SOUTHWEST HALF OF AZ DUE TO DAYTIME
HEATING AND CIN BECOME ELIMINATED BY 17Z.  THE 06Z SSEO
NEIGHBORHOOD PROBABILITIES SUGGEST CONVECTION SHOULD RAMP UP IN
INTENSITY AND COVERAGE BEYOND 19Z.  THERE HAS BEEN AN EXPANSION OF
MID- TO UPPER-LEVEL CLOUDINESS ACROSS PORTIONS OF EASTERN AZ IN AN
AREA WITH MINIMAL CLOUD BASE INFLOW, WHICH COULD HAMPER A BROADER
FLASH FLOOD THREAT ACROSS SOUTHEAST AZ.  ELSEWHERE, THE STRONGEST
700 HPA INFLOW IS EXPECTED MAINLY WEST AND NORTH OF THE AZ/CA
BORDER, WHILE 850 HPA INFLOW IS STRONGEST IN SOUTHERNMOST NV AND
SOUTHWEST AZ TOWARDS 22Z.  THE DEGREE OF INFLOW EXPECTED -- 20-25
KTS -- IS CLOSE TO THE MAGNITUDE OF THE MEAN 850-400 HPA WIND
WHICH SHOULD LEAD TO PRECIPITATION EFFICIENCY.  THE FLOW IS NEARLY
UNIDIRECTIONAL OUT OF THE SOUTHWEST, WHICH RAISES THE SPECTER FOR
TRAINING CONVECTION THIS AFTERNOON AS STORM MOTIONS TO THE
NORTHEAST AT 20-25 KTS ARE EXPECTED INDICATED BY THE MEAN WIND AND
FORWARD PROPAGATING CONVECTIVE VECTORS.  HOURLY RAINFALL RATES IN
TRAINING CONVECTION COULD REACH 1.5".  THE MESOSCALE QPF GUIDANCE
DEPICTS LOCALIZED MAXIMA OF 1.5-2", WHICH COULD FALL QUICKLY AND
LEAD TO FLASH FLOODING/DEBRIS FLOWS IN MOUNTAINOUS AND DESERT
LOCALES.

ROTH

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

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

LAT...LON   38451232 38151194 38291153 38231115 38101055 
            37821069 37001166 35741158 35271146 34231218 
            32581210 31621200 31961339 32461492 32501490 
            32681492 32571628 32621687 33271707 33491718 
            33501749 33861767 33881750 33951735 34081735 
            34071751 34171829 34291862 34331890 34591987 
            35211989 35021882 35841880 36321889 36641886 
            36781823 36611782 36541774 36711741 36761611 
            36831487 37521357