Flash Flood Guidance
Issued by NWS

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000
AWUS01 KWNH 171002
FFGMPD
CAZ000-171400-

MESOSCALE PRECIPITATION DISCUSSION 0434
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
501 AM EST WED DEC 17 2014

AREAS AFFECTED...SRN CA

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

VALID 171000Z - 171400Z

SUMMARY...ORGANIZED THUNDERSTORMS ARE LIKELY TO CONTINUE THIS
MORNING AHEAD OF A SHARP MID LEVEL TROUGH. RAIN RATES OF AT LEAST
0.75 INCHES IN AN HOUR ARE POSSIBLE. THE ISOLATED LOCATIONS WHICH
EXPERIENCE THESE HEAVY RAIN RATES MAY ALSO EXPERIENCE FLASH
FLOODING.

DISCUSSION...WATER VAPOR SATELLITE IMAGERY INDICATED THE PRESENCE
OF A SHARPLY DEFINED MID LEVEL SHORTWAVE TROUGH DIGGING TOWARD
SOUTHERN CA. STRONG FORCING...DIFLUENT UPPER FLOW AND LOW LEVEL
WARM ADVECTION...SHOULD SUSTAIN ISOLATED THUNDERSTORMS FROM THE
LOS ANGELES BASIN SOUTHWARD. THE RAP AND HRRR FORECAST STRONG LIFT
AND SUFFICIENT INSTABILITY...AT AROUND 500-700 J/KG OF CAPE...DOWN
TO THE LATITUDE OF SAN DIEGO THROUGH 14Z. THE CANADIAN GEM
REGIONAL MODEL HAS HANDLED QPF WELL ALONG THE CA COAST IN RECENT
NIGHTS...AND INDICATES A PRECIPITATION MAXIMUM WITH AREAL AVERAGE
OF 0.75 INCHES NORTH OF SAN DIEGO THROUGH 15Z. THE NOTION OF
THUNDERSTORM MAINTENANCE AND MODERATELY HEAVY RAINFALL OVER THIS
AREA HAS SUPPORT FROM THE 08Z HRRR AND 00Z SSEO MEAN.

INITIAL STORMS FORMING WITHIN THE WARM ADVECTION REGIME MAY BE
FOLLOWED BY THE PASSAGE OF A SMALL SCALE...YET
ORGANIZED...MESOSCALE CONVECTIVE SYSTEM JUST AHEAD OF THE MID
LEVEL TROUGH. SOME LOCATIONS WILL EXPERIENCE MODERATE AND BRIEFLY
HEAVY RAIN DUE TO BOTH FORCING MECHANISMS. RADAR DERIVED RAINFALL
ACCUMULATION ESTIMATES ARE GENERALLY 0.50 TO 0.75 INCHES PER
HOUR...BUT MAY REACH 1 INCH PER HOUR WHEREVER BRIEF TRAINING
OCCURS. GIVEN THESE RATES...A NUMBER OF OTHER FACTORS INCLUDING
BURN SCARS AND RECENT WET WEATHER...INCREASE THE POSSIBILITY OF
FLASH FLOODING.

BURKE

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

ATTN...WFO...LOX...SGX...

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

LAT...LON   34451789 34271699 33901669 33221651 32691630
            32511632 32321676 32351725 33151766 33601821
            33881876 34321868


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