MESOSCALE PRECIPITATION DISCUSSION 0070
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
117 PM EDT THU MAR 22 2018

AREAS AFFECTED...PORTIONS OF SOUTHERN CALIFORNIA

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

VALID 221716Z - 230500Z

SUMMARY...STEADY RAIN ASSOCIATED WITH THE WANING ATMOSPHERIC RIVER
WILL PUSH EAST AND BEGIN TO DIMINISH...BUT MORE CONVECTIVE RAIN IS
EXPECTED DURING THE AFTERNOON AND EVENING HOURS IN ADVANCE OF THE
SYNOPTIC COLD FRONT. EVEN SHORT BURSTS OF HEAVY RAIN COULD BE
SUFFICIENT TO PRODUCE FLASH FLOODING...ESPECIALLY IN AND AROUND
RECENT BURN SCARS.

DISCUSSION...AS OF 17Z REGIONAL RADARS SHOWED A STEADIER
CONTIGUOUS AREA OF RAIN BECOMING INCREASINGLY CONFINED TO THE EAST
OF 119W LONGITUDE. THIS WAS ASSOCIATED WITH THE LINGERING
ATMOSPHERIC RIVER THAT WILL BE WANING THROUGH THE REST OF THE DAY
IN OVERALL INTENSITY. WHERE THE MOIST SSWLY FLOW WAS INTERCEPTING
THE COASTLINE...GPS OBSERVATIONS SHOWED PRECIPITABLE WATER VALUES
GENERALLY AROUND 1.1 TO 1.2 INCHES. SUCH VALUES ARE CLOSE TO MARCH
MAXIMUM OBSERVED VALUES ON AREA SOUNDING CLIMATOLOGIES. EMBEDDED
CLUSTERS AND BANDS OF HEAVIER RAIN WITHIN THE LARGER AREA OF RAIN
WERE PRODUCING RAINFALL RATES APPROACHING 0.25 IN/HR OUTSIDE AREAS
OF TERRAIN AND OCCASIONALLY EXCEEDING 0.50 IN/HR IN FAVORED
UPSLOPE REGIONS. THE HEAVIER RAIN RATES HAVE RECENTLY (15-17Z)
AFFECTED THE THOMAS FIRE BURN SCAR AREA IN VENTURA COUNTY. AS THIS
SHIFTS EAST THROUGH THE REST OF THE LATE MORNING HOURS AND INTO
THE EARLY AFTERNOON...OTHER BURN SCARS IN LOS ANGELES COUNTY MAY
BE AFFECTED.

FURTHER TO THE WEST...RAIN WAS BECOMING MORE CONVECTIVE IN A
REGION AHEAD OF THE SYNOPTIC COLD FRONT BUT BEHIND THE PRIMARY
AXIS OF THE ATMOSPHERIC RIVER. A COMBINATION OF SOME LINGERING
LOW-LEVEL MOISTURE AND INCREASING INSTABILITY WAS NOTED...WITH
PRECIPITABLE WATER VALUES ON THE CIRA BLENDED TPW PRODUCT STILL
AROUND 1 INCH AND OBJECTIVE RAP ANALYSIS INDICATING MARGINAL
INSTABILITY (100-500 J/KG MUCAPE). GOES-WEST VISIBLE SATELLITE
SHOWED A PATCH OF INCREASING CLEARING IN THIS REGION WITH A
CONVECTIVE CUMULUS LINE ALONG THE COLD FRONT OVER THE PACIFIC
OCEAN. PER WFO LOX...A SMALL CONVECTIVE BAND NEAR COASTAL SANTA
BARBARA COUNTY ALREADY PRODUCED RAIN RATES AROUND 0.4 IN/HR IN 30
MINUTES ON THE LEADING EDGE OF THIS TRANSITION TO A MORE
CONVECTIVE ENVIRONMENT...RIGHT AROUND THE SHERPA AND WHITTIER BURN
SCARS.

HI-RES MODELS SHOW A SIGNAL FOR INCREASINGLY CONVECTIVE RAIN INTO
THE AFTERNOON AND EVENING HOURS...WITH SMALLER BANDS. HOWEVER THEY
ALSO CONTINUE TO SHOW THE POTENTIAL FOR 0.5 INCHES OF RAIN IN
SHORT PERIODS OF TIME...SO THE THREAT OF FLASH FLOODING IS
EXPECTED TO CONTINUE...PARTICULARLY FOR RECENT BURN SCARS. SOME OF
THE CONVECTIVE SHOWERS (OR EVEN THUNDERSTORMS) MAY ONLY AFFECT A
GIVEN LOCATION FOR 15-30 MINUTES WITH THEIR SMALL SIZE...BUT THE
COMBINATION OF 1 INCH PWS AND MARGINAL INSTABILITY SHOULD BE
SUFFICIENT TO PRODUCE 0.25 TO 0.50 INCHES OF RAIN IN THAT SHORT
PERIOD OF TIME.

FLASH FLOODING AND DEBRIS FLOWS WILL THEREFORE BE POSSIBLE IN THE
VICINITY OF RECENT BURN SCARS...PARTICULARLY IN PORTIONS OF SANTA
BARBARA...VENTURA AND LOS ANGELES COUNTIES. WITH TIME THIS WILL
BECOME INCREASINGLY DEPENDENT ON THE SPECIFIC LOCATION AND MOTION
OF SMALL CONVECTIVE SHOWERS AND STORMS...BUT IF THEY ALIGN WITH
LARGER BURN SCARS (AND PARTICULARLY AREAS THAT HAVE ALREADY
RECEIVED HEAVY RAINFALL EARLIER TODAY) THEN MORE SIGNIFICANT FLASH
FLOODING COULD RESULT.

LAMERS

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

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

LAT...LON   35901853 35621826 35261829 34971842 34721839
            34521815 34471772 34391723 34291683 33861663
            33491663 33221690 33181742 33561820 33661893
            33661957 33772035 34162065 34792074 35292094
            35522036 35191996 34961953 35381919 35811892