MESOSCALE PRECIPITATION DISCUSSION 0067
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
227 PM EDT WED MAR 21 2018

AREAS AFFECTED...CTRL/SC CA COAST...SRN SAN JOAQUIN VALLEY...SRN
SIERRAS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 211830Z - 220530Z

SUMMARY...ANOTHER WAVE OF HEAVY RAIN IS EXPECTED TO ARRIVE ON THE
SOUTH-CENTRAL CALIFORNIA COAST THIS AFTERNOON. RAIN RATES IN
EXCESS OF 0.5 IN/HR ARE LIKELY...WHICH MAY LEAD TO AREAS OF FLASH
FLOODING. SOME FLOODING WILL EVEN BE POSSIBLE OUTSIDE OF IMMEDIATE
BURN SCAR LOCATIONS.

DISCUSSION...AS OF 18Z A CONVECTIVE CLUSTER HAD PUSHED EAST FROM
THE CHANNEL ISLANDS AND SANTA BARBARA COUNTY INTO VENTURA AND LOS
ANGELES COUNTIES. AT LEAST SEVERAL MESONET SITES HAVE MEASURED 0.5
IN/HR RAIN RATES...PARTICULARLY IN UPSLOPE REGIONS OF HIGHER
TERRAIN. A LULL IN COVERAGE AND INTENSITY OF PRECIPITATION HAD
DEVELOPED BEHIND THIS CONVECTIVE CLUSTER. HOWEVER...A PLUME OF
MODERATE TO OCCASIONALLY HEAVY CONVECTIVE SHOWERS AND CLUSTERS WAS
SITUATED IN THE PLUME OF HIGHEST PRECIPITABLE WATER (PW) VALUES
OFFSHORE FROM AROUND 33.5N/123W SOUTHWEST TO AROUND 27N/128W. THIS
HAS BEEN DETECTED IN MORNING MICROWAVE (AMSU AND SSMIS) SATELLITE
PASSES AND IS BEING TRACKED BY THE GLOBAL HYDRO-ESTIMATOR. PER
DISCUSSION WITH NESDIS SAB...RAIN RATES FROM THE MICROWAVE
INSTRUMENTS AND GHE ARE 5-10 MM/HR (0.2-0.4 IN/HR) IN THE
INDIVIDUAL CONVECTIVE CLUSTERS...WITH SOME ISOLATED MAXIMA AS HIGH
AS 0.7 IN/HR. THE EXPECTATION IS THAT THIS PLUME OF ENHANCED RAIN
RATES SHOULD CONTINUE TO ADVANCE NORTHEAST AND ARRIVE ALONG THE
CALIFORNIA COASTLINE IN SEVERAL HOURS. EXTRAPOLATION OF CURRENT
TRENDS SUGGESTS SOMETIME AROUND 20-23Z IS MOST LIKELY.

TERRAIN ENHANCEMENT WILL BE A FACTOR AS AREAS OF RAIN REACH THE
COAST...WITH BROAD SOUTHWESTERLY FLOW AROUND 40-50 KNOTS AT 700MB
DRIVING NEAR-RECORD MARCH PW VALUES INTO COASTAL TERRAIN FEATURES.
GOES-WEST WATER VAPOR LOOPS CONTINUE TO SHOW A DIGGING WAVE AROUND
40N/140W THAT MAY LEAD TO A DEEPER TROUGH AND ALSO CONTRIBUTE TO
BACKING FLOW INTO THE AFTERNOON AND EVENING HOURS IN THE
ATMOSPHERIC RIVER. AN INCREASING SOUTHERLY COMPONENT TO THE FLOW
MAY PUSH SOME OF THE HEAVIER RAIN RATES FURTHER UP THE
COAST...ALTHOUGH THE MORNING RAIN ENDED UP A BIT FURTHER EAST THAN
MODELS HAD BEEN SHOWING...SO ITS POSSIBLE THAT THE ATMOSPHERIC
RIVER WILL REMAIN FOCUSED AROUND POINT CONCEPTION. ITS WORTH
NOTING THAT IF THE RAIN CONTINUES TO AFFECT SANTA BARBARA AND
VENTURA COUNTIES WITH INCREASINGLY BACKED FLOW...THE OROGRAPHIC
LIFT WOULD LIKELY BE INCREASED AS THE FLOW WOULD BE INTERCEPTING
THE TRANSVERSE RANGES AT A MORE ORTHOGONAL ANGLE.

RAIN RATES IN FAVORED UPSLOPE REGIONS APPEAR LIKELY TO EXCEED 0.5
IN/HR. THIS WOULD BE ACHIEVED BY THE RULE-OF-THUMB OF DOUBLING THE
OFFSHORE RAIN RATES...BUT IS ALSO INDICATED BY HI-RES ENSEMBLE
GUIDANCE (AND HAS ALREADY BEEN OBSERVED ACROSS THE AREA...SO THE
ENVIRONMENT CAN CLEARLY SUPPORT SUCH RAIN RATES). THE 12Z HREF HAS
PROBABILITIES OF 0.5 IN/HR RAIN RATES AROUND 60-80 PERCENT IN
PARTS OF SAN LUIS OBISPO...SANTA BARABARA...AND VENTURA COUNTIES
FROM 00-03Z...ALTHOUGH THE TIMING MAY BE SLIGHTLY DELAYED FROM
WHAT WILL ACTUALLY OCCUR. ADDITIONALLY...IT SHOWS SIMILARLY HIGH
PROBABILITIES SPILLING INTO THE SOUTHERN SAN JOAQUIN VALLEY AROUND
HANFORD AND VISALIA AND PERHAPS INTO THE SOUTHERN EXTENT OF THE
SIERRA NEVADA RANGE JUST TO THE EAST. THESE VALUES WOULD BE LIKELY
TO PRODUCE FLASH FLOODING IN AND NEAR BURN SCAR LOCATIONS. IN THE
SAME AREAS THE HREF ALSO SHOWS A SMALL (10-20 PERCENT) PROBABILITY
OF REACHING 1 IN/HR RAIN RATES WHICH WOULD BE EVEN MORE LIKELY TO
PRODUCE FLASH FLOODING...INCLUDING LOCATIONS OUTSIDE OF BURN SCAR
LOCATIONS.

GIVEN ANTECEDENT RAINFALL FROM THIS MORNING ACROSS MUCH OF THE
AREA...AND THE HIGHER PROBABILITY OF REACHING 0.5 IN/HR RAIN
RATES...FLASH FLOODING SEEMS TO BE INCREASINGLY LIKELY THIS
AFTERNOON AND EVENING IN AND AROUND BURN SCAR LOCATIONS AND IN
FLASHIER DRAINAGE BASINS (SMALLER AND STEEPER TERRAIN GRADIENTS).
WITH RAIN RATES OFFSHORE ALREADY APPROACHING 0.7 IN/HR IN ISOLATED
CASES...HEAVIER RAIN RATES CLOSER TO 1 IN/HR COULD TRIGGER MORE
SUBSTANTIAL AND LIFE-THREATENING DEBRIS FLOWS AND FLASH FLOODING
NEAR BURN SCAR LOCATIONS. FLOODING WILL ALSO BE POSSIBLE OUTSIDE
OF BURN SCAR LOCATIONS AS RAIN RATES INCREASE.

LAMERS

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

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

LAT...LON   37051943 36951903 36851871 36591861 36331844
            36181807 35851795 35521798 35211830 35101890
            34861911 34501903 34281932 33911959 33792036
            34212077 34772086 35152119 35572158 36082181
            36262142 36302092 36342035 36741968