MESOSCALE PRECIPITATION DISCUSSION 0065
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
421 PM EDT TUE MAR 20 2018

AREAS AFFECTED...CENTRAL/SOUTHERN CA COASTAL RANGES

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

VALID 202015Z - 210800Z

SUMMARY...HEAVY RAINS TO BEGIN OVERSPREADING THE CENTRAL/SOUTHERN
CA COASTAL RANGES LATE THIS AFTERNOON AND THROUGH THIS EVENING AS
ATMOSPHERIC RIVER CONDITIONS COMMENCE. SOME RUNOFF CONCERNS WILL
BE POSSIBLE ALONG WITH ISOLATED FLASH FLOODING CONCERNS NEAR ANY
BURN SCAR AREA.

DISCUSSION...THE LATEST GOES-15 WV IMAGERY SHOWS A WELL-DEFINED
CLOSED LOW CENTERED NEAR 32N 139W WHICH WILL BEGIN TO GRADUALLY
WEAKEN THIS EVENING AND ADVANCE OFF TO THE EAST-NORTHEAST
UNDERNEATH MUCH STRONGER NORTHERN STREAM HEIGHT FALLS/TROUGHING
DIGGING SOUTH DOWN OFF THE PACIFIC NORTHWEST. THIS WILL ALLOW DEEP
LAYER SOUTHWEST FLOW WITH A WELL-DEFINED WARM CONVEYOR BELT AND
TRANSPORT OF SUBTROPICAL MOISTURE TO TAKE AIM ON THE COASTAL
RANGES OF CENTRAL/SOUTHERN CA THIS EVENING WHICH WILL ALLOW
ATMOSPHERIC RIVER CONDITIONS TO ENSUE.

ALREADY THE LATEST GOES-15 IR IMAGERY HAS BEEN SHOWING A GENERAL
COOLING OF CLOUD TOPS OFFSHORE WITHIN A REGION OF INCREASINGLY
DIFFLUENT FLOW ALOFT AROUND THE EASTERN FLANK OF THE UPPER LOW.
MEANWHILE...850/700 MB MOISTURE FLUX ANOMALIES ARE INCREASING AS
THE LOW/MID LEVEL WIND FIELD INCREASES AND THE 12Z GFS FORECASTS A
LARGE AREA OF MOISTURE FLUX ANOMALIES EXCEEDING 4-5 STANDARD
DEVIATIONS ABOVE NORMAL OFFSHORE CENTRAL/SOUTHERN CA BY 00Z. THE
LATEST BLENDED-TPW DATA DEPICTS AN AXIS OF 1.5 TO 1.6 INCH PWATS
ALIGNED WITH THE DEEP LAYER SOUTHWEST FLOW AND THIS MOISTURE
TRANSPORT AIMING TOWARD THE COAST WILL PROMOTE AN INCREASE IN
RAINFALL EFFICIENCY WITH TIME.

EXPECTING A BROAD AREA OF MODERATE TO LOCALLY HEAVY RAIN TO BEGIN
OVERSPREADING THE COASTAL RANGES OVER THE NEXT SEVERAL
HOURS...WITH A GRADUAL INCREASE IN RAINFALL RATES THROUGH THE
EVENING AS A COMBINATION OF GREATER RAINFALL EFFICIENCY...FORCING
ALOFT AND FAVORABLY ORTHOGONAL LOW LEVEL FLOW SETS UP OVER THE
TERRAIN.

AN EARLIER AMSU PASS BACK AT 1550Z WAS ALREADY SHOWING POCKETS OF
0.50 INCH/HR RAIN RATES OFFSHORE...AND SINCE THAT PASS...THE CLOUD
TOPS HAVE BEEN COOLING FURTHER WHICH SUGGESTS AT LEAST AN UPTICK
IN LARGER SCALE FORCING AND AN EXPANSION OF HEAVIER RAINS.

THE LATEST RUNS OF THE HRRR AND THE 12Z ARW/NMMB GUIDANCE SUGGESTS
THAT AS MUCH AS 1 TO 2 INCHES OF RAIN MAY FALL BETWEEN NOW AND
06Z...WITH MUCH HEAVIER RAIN AFTER THIS PERIOD GOING INTO VERY
EARLY WEDNESDAY. HOWEVER...THERE IS SOME POTENTIAL FOR THE
GUIDANCE TO BE A TAD UNDERDONE OVER THE UPSLOPE AREAS OF THE
TRANSVERSE RANGE GIVEN THE DEEPENING SUBTROPICAL CONNECTION AND
ORTHOGONAL WIND COMPONENT. THE SATELLITE TRENDS TEND TO FAVOR A
WETTER SHORT-TERM OUTLOOK...AND THUS THE CONCERN IS FOR HEAVIER
RAINFALL RATES THAT COULD BE AS MUCH AS 0.25 TO 0.50 INCH/HR EVEN
BY 06Z WHICH WOULD BE A BIT WETTER THAN THE GUIDANCE.

GIVEN THE SENSITIVITY TO THE BURN SCAR AREAS IN PARTICULAR...IT
WILL NOT NECESSARILY TAKE VERY HEAVY RAINFALL TO ENCOURAGE RUNOFF
PROBLEMS AND POTENTIAL FOR DEBRIS FLOWS OR FLASH FLOODING. WILL
CONTINUE TO MONITOR AND UPDATE ACCORDINGLY.

ORRISON

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

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

LAT...LON   36762152 35902059 35312010 35091967 34881913
            34511873 34101866 33961898 34131923 34251982
            34442049 34802081 35292098 36222200