MESOSCALE PRECIPITATION DISCUSSION 0066
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
931 AM EDT WED MAR 21 2018

AREAS AFFECTED...SOUTH-CENTRAL CA COAST AND TRANSVERSE RANGES

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

VALID 211330Z - 211900Z

SUMMARY...A WAVE OF HEAVIER RAINFALL MAY ARRIVE ALONG THE
SOUTH-CENTRAL CALIFORNIA COAST LATER THIS MORNING. RAIN RATES IN
EXCESS OF 0.5 IN/HR WILL BE POSSIBLE WHICH MAY LEAD TO FLASH
FLOODING...PARTICULARLY NEAR RECENT BURN SCARS AND IN SMALL BASINS
BOUNDED BY STEEP TERRAIN GRADIENTS.

DISCUSSION...THE LATEST BLENDED PRECIPITABLE WATER PRODUCTS FROM
CIRA CONTINUE TO HIGHLIGHT A PROMINENT ATMOSPHERIC RIVER IMPACTING
THE SOUTH-CENTRAL CALIFORNIA COAST WITH A PLUME OF TOTAL PW VALUES
AROUND OR IN EXCESS OF 1.5 INCHES JUST OFFSHORE. ALTHOUGH A 12Z
SOUNDING FROM VANDENBERG AFB WAS NOT AVAILABLE AS THIS DISCUSSION
WAS BEING COMPOSED...SEVERAL GPS OBSERVATIONS MEASURED 1.2 TO 1.3
INCH PWS ON THE COAST OF SAN LUIS OBISPO AND SANTA BARBARA
COUNTIES. THESE ARE AROUND THE HIGHEST PW VALUES THAT HAVE BEEN
OBSERVED ON UPPER-AIR SOUNDINGS AT VBG IN THE MONTH OF MARCH...AND
THE 1.5 INCH PW VALUES JUST OFF THE COAST ARE OUTSIDE THE MARCH
HISTORICAL RANGE IN THE CFSR CLIMATOLOGY. THESE HIGHLY ANOMALOUS
MOISTURE VALUES SHOULD PERSIST THROUGH THE DAY AS STRONG
NORTHEASTWARD MOISTURE FLUX CONTINUES THROUGH A DEEP LAYER. THE
LAYER PW PRODUCTS FROM CIRA SHOW THE SUBSTANTIAL RIBBON OF
MOISTURE EXTENDS VERTICALLY INTO THE MID-UPPER LEVELS OF THE
ATMOSPHERE AND THAT SHOULD ENABLE RELATIVELY EFFICIENT
PRECIPITATION PROCESSES WHEN RAIN DOES OCCUR.

LOOKING OFFSHORE...SEVERAL CONVECTIVE CLUSTERS HAVE BEEN MAKING
STEADY PROGRESS NORTHEAST WITH COOLING CLOUD TOPS ON IR SATELLITE
SINCE AROUND 09Z...AND THE LEADING EDGE OF THE INITIAL CLUSTER
REACHED AROUND 32N/124W BY 13Z. SOME LIGHTNING HAS BEEN DETECTED
IN THESE CONVECTIVE CLUSTERS...SUGGESTING AT LEAST MARGINAL
INSTABILITY AND POTENTIALLY HIGHER RAIN RATES. THE GLOBAL
HYDRO-ESTIMATOR RAIN RATES IN THESE CONVECTIVE CLUSTERS ARE
GENERALLY AROUND 5-8 MM/HR (0.25-0.30 IN/HR). THAT IS A LITTLE
HIGHER THAN SOME POLAR-ORBITER PASSES EARLIER IN THE NIGHT...BUT
RECENT COOLING OF CLOUD TOPS AND THE PRESENCE OF LIGHTNING
SUGGESTS THAT AT A MINIMUM 0.25 IN/HR RAIN RATES WOULD BE
ACHIEVABLE. AS THESE RAIN BANDS AND STORMS ARRIVE ONSHORE...WE CAN
TYPICALLY EXPECT AT LEAST A DOUBLING OF THE OFFSHORE RATES IN
SATELLITE ESTIMATES IN FAVORED UPSLOPE REGIONS ON LAND. THIS WOULD
MEAN THAT RAIN RATES ON THE ORDER OF 0.5 TO 0.7 IN/HR WOULD BE
ACHIEVABLE PROVIDED THAT THESE AREAS OF RAIN ARE MAINTAINED TO A
REASONABLE DEGREE. GOES-WEST IR SATELLITE ALSO SHOWED A NEW BURST
OF CONVECTION CLOSER TO THE COAST AROUND 34N/122W AS OF 13Z...AND
LIGHTNING HAD BEEN OBSERVED WITH THAT AS WELL. THIS MAY ARRIVE A
LITTLE SOONER THAN THE OTHER CONVECTIVE CLUSTERS...AND
EXTRAPOLATING CURRENT TRENDS WOULD TAKE IT INTO THE COAST AROUND
BETWEEN 14Z AND 15Z.

THEREFORE...SATELLITE TRENDS ARE HIGHLY SUGGESTIVE THAT THE FLASH
FLOOD THREAT WILL BEGIN TO INCREASE IN THE NEXT SEVERAL HOURS. THE
06Z HREF ALIGNS WITH THESE TRENDS...SHOWING A NOTABLE INCREASE IN
THE PROBABILITY FOR 0.5 IN/HR RAIN RATES IN AND AROUND SANTA
BARBARA COUNTY BETWEEN 15Z AND 18Z...AND A SIMILAR ENHANCEMENT IN
THE BLENDED MEAN 1-HR RAIN RATES.

THE GREATEST THREAT FOR FLASH FLOODING WILL EXIST IN AND NEAR
RECENT LARGE BURN SCARS IN PORTIONS OF SANTA BARBARA AND VENTURA
COUNTIES...AS WELL AS SMALLER DRAINAGE BASINS EMBEDDED IN THE
MOUNTAIN RANGES AND STEEPER TERRAIN GRADIENTS. IF THE HEAVIER RAIN
RATES IN EXCESS OF 0.5 IN/HR CAN ALIGN WITH SOME OF THE LARGER
BURN SCAR FOOTPRINTS...LIFE-THREATENING AND DANGEROUS DEBRIS FLOWS
AND FLASH FLOODING WOULD BE INCREASINGLY LIKELY.

LAMERS

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

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

LAT...LON   35392086 35382029 35011973 34871913 34471886
            34101925 33861979 33832046 34532085 35092100