MESOSCALE PRECIPITATION DISCUSSION 0417
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
414 PM EDT SAT JUL 07 2018

AREAS AFFECTED...MOUNTAINS OF SOUTHERN CALIFORNIA

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

VALID 072013Z - 072330Z

SUMMARY...SCATTERED THUNDERSTORMS HAVE DEVELOPED OVER THE MOUNTAIN
RANGES IN SOUTHERN CALIFORNIA AND ARE NEARLY STATIONARY. THESE
STORMS MAY DISSIPATE IN A FEW HOURS...BUT UNTIL THEY DO THEY MAY
PRODUCE LOCALLY HEAVY RAINFALL UP TO 1-2 IN/HR AND LOCALIZED FLASH
FLOODING.

DISCUSSION...KEYX AND KYUX RADARS SHOWED NEARLY STATIONARY
CONVECTION ALONG PORTIONS OF THE SAN GABRIEL...SAN
BERNARDINO...AND SAN JACINTO RANGES IN SOUTHERN CALIFORNIA...JUST
INLAND FROM THE COAST. THE 850-300MB MEAN FLOW WAS RELATIVELY WEAK
OVERALL (AROUND 10 KNOTS) AND WAS GENERALLY ORIENTED ALONG THE
TERRAIN FEATURES. THUS ANY STORM MOTION WAS TENDING TO REMAIN OVER
THE AREAS OF HIGHER TERRAIN. RADAR DUAL POL ESTIMATES FROM KEYX
AND KYUX HAVE SHOWN LOCALIZED RATES UP TO 1-2 IN/HR...AND THIS
WOULD BE SUFFICIENT TO CAUSE LOCALIZED FLASH FLOODING. RAP
ANALYSIS SHOWS LIMITED INSTABILITY (ONLY AROUND 500-1000
J/KG)...WHICH SUGGESTS THAT THE STORMS MAY NOT BE ABLE TO LAST
MORE THAN A FEW HOURS ONCE THEY HAVE EXHAUSTED THE SUPPLY OF
AVAILABLE INSTABILITY. HOWEVER...GPS-PW OBSERVATIONS AND RAP
ANALYSIS ALSO SHOW PRECIPITABLE WATER VALUES GENERALLY 1.2 TO 1.6
INCHES...WHICH ARE GENERALLY ABOVE THE 90TH PERCENTILE BASED ON
THE CSFR CLIMATOLOGY FOR JULY. THUS...SUFFICIENT MOISTURE EXISTS
FOR LOCALLY HEAVY RAINFALL AND A FEW HOURS OF FLASH FLOOD THREAT
BEFORE STORMS DISSIPATE.

LAMERS

ATTN...WFO...HNX...LOX...PSR...SGX...VEF...

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

LAT...LON   34881813 34671625 33291602 32381591 32471676
            33461727 33901772 34321853