MESOSCALE PRECIPITATION DISCUSSION 0037
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1034 AM EST SUN JAN 22 2017
 
AREAS AFFECTED...SOUTHERN CA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 221534Z - 230304Z
 
SUMMARY...THE APPROACH OF A STRONG COLD FRONT AND ANOMALOUS
MOISTURE PLUME WILL BRING A HEAVY RAINFALL THREAT TO SOUTHERN
CALIFORNIA TODAY.

DISCUSSION...AS OF 15Z AN OCCLUDED LOW WAS LOCATED OFF THE PACIFIC
NORTHWEST COAST...WITH A FRONTAL BOUNDARY EXTENDING SOUTH FROM THE
LOW INTO PORTIONS OF THE CENTRAL/NORTHERN CA COAST...SOUTHWESTWARD
TO OFF THE SOUTHERN CA COAST. AN IMPRESSIVE ATMOSPHERIC RIVER OF
MOISTURE IS LOCATED TO THE SOUTH OF THIS BOUNDARY...WITH DEEP
SOUTHWESTERLY FLOW RESULTING IN A LONG MOISTURE FETCH TRACING BACK
TO NEAR HAWAII PER LATEST WATER VAPOR IMAGERY AND THE CIRA LAYERED
PRECIPITABLE WATER PRODUCT. 

RECENT GPS OBSERVATIONS SHOW PRECIPITABLE WATER VALUES AS HIGH AS
AROUND 1.1" ALONG THE CA COAST AS OF 13Z. EVEN HIGHER MOISTURE
VALUES ARE LOCATED OFFSHORE...AND SHOULD MOVE INTO THE SOUTHERN CA
COAST AFTER 18Z AND DRIFT SOUTH THROUGH THE AFTERNOON HOURS.
VALUES THIS HIGH ARE AROUND CLIMATOLOGICAL RECORDS. GIVEN THE
IMPRESSIVE WIND FIELDS WITHIN THE MOISTURE PLUME...IT IS ALSO NOT
SURPRISING THAT INTEGRATED MOISTURE FLUX IS ALSO AT OR ABOVE
CLIMATOLOGICAL RECORD VALUES. SOUTH SOUTHWESTERLY 850 MB WINDS
ABOVE THE 99TH PERCENTILE WILL HAVE A LARGE COMPONENT
PERPENDICULAR TO THE TERRAIN...RESULTING IN UPSLOPE ENHANCEMENT TO
THE RAINFALL. A LACK OF INSTABILITY SHOULD KEEP RAINFALL RATES
FROM GETTING TOO EXTREME...ALTHOUGH THE IMPRESSIVE MOISTURE
PARAMETERS AND UPSLOPE ENHANCEMENT WILL STILL RESULT IN RATES HIGH
ENOUGH TO RESULT IN FLASH FLOOD ISSUES THROUGH THE AFTERNOON
HOURS. 

THERE IS GOOD AGREEMENT AMONGST THE HIGH RES GUIDANCE FOR A BROAD
SHIELD OF HOURLY RAINFALL TOTALS BETWEEN 0.25" AND 0.5" AN HOUR
NEAR SANTA MARIA AND SANTA BARBARA BY 17Z...PROGRESSING SOUTH TO
LAS ANGELES BY 19Z. WITHIN THIS RAIN SHIELD WILL LIKELY SEE
EMBEDDED HEAVIER AMOUNTS APPROACHING 1" AN HOUR...ESPECIALLY IN
THE FAVORED UPSLOPE TERRAIN. THE 18Z TO 00Z TIME FRAME APPEARS TO
HAVE THE HIGHEST PROBABILITY OF SEEING RAINFALL TOTALS REACHING 1"
IN AN HOUR FROM THE SANTA MARIA TO LAS ANGELES AREAS. GIVEN THE
HEAVY ANTECEDENT RAINFALL...RATES THIS HIGH WILL POTENTIALLY CAUSE
RUNOFF AND FLASH FLOOD ISSUES...ESPECIALLY IN URBAN AREAS AND NEAR
BURN SCARS. THROUGH 03Z EXPECT TOTAL RAINFALL AMOUNTS OF 1-3"
ACROSS THE MPD AREA...LOCALLY AS HIGH AS 3-5" OVER THE FAVORED
TERRAIN.

CHENARD

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

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

LAT...LON   34911955 34901893 34671821 34381719 34281656 
            33861636 33091656 32881711 33001829 33451898 
            34042007 34252043 34642067 34832015