MESOSCALE PRECIPITATION DISCUSSION 0803
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
142 AM EDT MON SEP 11 2017

AREAS AFFECTED...SOUTH-CENTRAL CALIFORNIA COAST

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

VALID 110541Z - 110911Z

SUMMARY...DEVELOPING CONVECTION OVER THE TERRAIN ALONG THE
SOUTH-CENTRAL CALIFORNIA COAST MAY LEAD TO LOCALLY HEAVY RAINFALL
AND FLASH FLOODING THROUGH 09-10Z.

DISCUSSION...RAP ANALYSIS AND WATER VAPOR SATELLITE LOOPS PLACED
AN UPPER LEVEL LOW NEAR 30N/124W. TO THE NORTH OF THIS CONVECTION
WAS BEGINNING TO BLOSSOM AROUND 05Z WITH COOLING CLOUD TOPS
OBSERVED ON IR SATELLITE. THIS APPEARED TO BE OCCURRING AROUND A
PV LOBE THAT IS ROTATING TO THE WEST AROUND THE NORTH SIDE OF THE
SAME UPPER LEVEL LOW...EVIDENT ON GOES-16 AIR MASS RGB IMAGERY AND
ALSO IN RAP 500MB VORTICITY FIELDS. GIVEN FAVORABLE UPPER LEVEL
SUPPORT...CONVECTION SHOULD CONTINUE TO DEVELOP IN THIS CORRIDOR
OF FOCUSED ASCENT TO THE NORTH OF THE SANTA BARBARA CHANNEL ON THE
CENTRAL CALIFORNIA COAST.

THUNDERSTORMS CURRENTLY HAVE ACCESS TO ABOUT 500-1000 J/KG OF
MUCAPE...BUT THIS SHOULD BE A LIMITING FACTOR WITH TIME AS THE AIR
MASS BECOMES CONVECTIVELY OVERTURNED. MODEL FORECASTS SUGGEST THAT
THIS SHOULD OCCUR IN SEVERAL HOURS...LIKELY BY 09Z. HOWEVER...THIS
STILL SETS UP A SHORT WINDOW IN WHICH DEVELOPING CONVECTION SHOULD
HAVE ENOUGH VIGOR TO TRAIN OVER SIMILAR CORRIDORS OF FAVORED
ASCENT (PERHAPS ALONG TERRAIN FEATURES) AND COULD LEAD TO LOCALLY
HEAVY RAINFALL AND FLASH FLOODING.

PRECIPITABLE WATER VALUES ARE ANOMALOUSLY HIGH IN THIS AREA...AND
EXCEED 1.5 INCHES ALONG PORTIONS OF THE CALIFORNIA COAST NEAR
SANTA BARBARA AS SUGGESTED BY BOTH RAP ANALYSIS AND SEVERAL GPS
OBSERVATIONS. THE 00Z SOUNDING FROM VANDENBERG AFB ALSO HAD 1.49
INCH PWAT. THIS IS MORE THAN SUFFICIENT TO SUPPORT HEAVY RAINFALL.
CLOUD BEARING LAYER MEAN WINDS ARE AROUND 125 DEGREES AT 30 KNOTS
PER THE VBG SOUNDING...WHICH IS CLOSE TO PARALLEL TO THE
ORIENTATION OF BANDS OF CONVECTION THAT HAVE ALREADY DEVELOPED.
THE POTENTIAL FOR TRAINING CELLS IS ANOTHER FACTOR THAT WOULD
FAVOR A FLASH FLOOD THREAT IN THE NEAR TERM. MRMS AND KHNX DUAL
POL ESTIMATES ALREADY SUGGEST RAIN RATES APPROACHING 1
IN/HR...WHICH IS CLOSE TO FFG VALUES ACROSS PORTIONS OF THE
REGION. ADDITIONAL RAINFALL MAY PUSH SOME OF THESE AREAS OVER FFG
VALUES IN THE NEXT HOUR OR TWO.

LAMERS

ATTN...WFO...HNX...LOX...MTR...

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

LAT...LON   36002132 35952051 35811994 35621937 35391829
            35141794 34881784 34421793 34101824 33981861
            33991901 34211936 34361963 34402021 34422049
            34632077 34952077 35232094 35462124 35472124
            35812151