MESOSCALE PRECIPITATION DISCUSSION 0001
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1013 PM EST MON JAN 08 2018

AREAS AFFECTED...CENTRAL AND SOUTHERN CA COASTAL RANGES

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

VALID 090310Z - 090930Z

SUMMARY...HEAVY RAINS WILL BECOME MORE WIDESPREAD OVERNIGHT WITH
INCREASING CONCERNS FOR ENHANCED RUNOFF AND POSSIBLE FLASH
FLOODING. BURN SCAR AREAS GENERATED FROM RECENT WILDFIRE ACTIVITY
WILL BE PARTICULARLY SUSCEPTIBLE.

DISCUSSION...GOES-16 1-MINUTE WV IMAGERY SUITE SHOWS IMPRESSIVE
NORTHERN STREAM HEIGHT FALLS ADVANCING SOUTH DOWN OFFSHORE
SOUTHERN CA AS MULTIPLE SHORTWAVE IMPULSES DIGGING DOWN THE WEST
SIDE OF THE AMPLIFYING TROUGH DIG TOWARD THE BASE OF IT. GOES-16
RGB AIRMASS IMAGERY ALSO SHOWS A VERY IMPRESSIVE PV ANOMALY
SIGNATURE ORIENTED NNW/SSE WITH AN ANOMALY CORE CENTERED NEAR
33.0N 127.7W. THIS STRONG VORT CENTER IS SEEN MOVING EAST AND WITH
ADDITIONAL ENERGY DIGGING INTO THE BASE OF THE TROUGH...A
WELL-DEFINED CLOSED MID LEVEL LOW CENTER WILL EVOLVE LATER THIS
EVENING OFFSHORE SOUTHERN CA.

VERY STRONG FORCING ALOFT WITH ENHANCED UPPER LEVEL DIVERGENCE
AROUND THE EASTERN FLANK OF THIS MID LEVEL LOW WILL OVERSPREAD
SOUTHERN CA THIS EVENING AND OVERNIGHT...AND WILL COUPLE WITH AN
ANOMALOUS NORTHEASTWARD TRANSPORT OF SUBTROPICAL MOISTURE WEST OF
THE BAJA PENINSULA FOR RATHER WIDESPREAD HEAVY RAINFALL AND
ENHANCED RAINFALL RATES.

MOISTURE FLUX ANOMALIES IN THE 850/700MB LAYER ARE FORECAST TO
INCREASE TO NEAR 5 SIGMAS ABOVE NORMAL BY 06Z ACROSS THE
TRANSVERSE RANGES OF SOUTHERN CA. THIS WILL BE AIDED BY A
SOUTHWEST LOW LEVEL JET OF 40 TO 45 KTS WHICH WILL BE ORIENTED
ORTHOGONAL TO THE COASTAL TERRAIN...AND WILL ALSO BE FACILITATING
PRECIPITABLE WATER VALUES OF NEAR 1.1 TO 1.25 INCHES WHICH IS
ABOUT 3 SIGMAS ABOVE NORMAL FOR THIS TIME OF YEAR.

THIS WILL FAVOR RELATIVELY EFFICIENT RAINFALL PROCESSES WHICH WILL
TEND TO ENHANCE THE RAINFALL RATES. HOWEVER...THE ORTHOGONAL
NATURE OF THE LOW TO MID LEVEL WINDS RELATIVE TO THE COASTAL
RANGES AHEAD OF THE OFFSHORE CLOSED LOW WILL FAVOR STRONG
OROGRAPHIC ASCENT FOR NOTABLY HEAVIER RAINFALL RATES LATER TONIGHT.

THE LAST COUPLE OF AVAILABLE AMSU PASSES SHOWED OFFSHORE RAINFALL
RATES EXCEEDING A 0.50 INCH/HR IN SEVERAL LOCATIONS...AND THESE
HEAVIER RATES SHOULD BEGIN TO ARRIVE IN EARNEST ACROSS THE CENTRAL
AND SOUTHERN CA COASTAL RANGES BETWEEN 06Z AND 09Z.

ALREADY THE 12Z ARW/NMMB SOLUTIONS AND TO AN EXTENT THE HRRR
GUIDANCE HAS BEEN GENERALLY SLOW TO HANDLE THE INITIAL ARRIVAL OF
MODERATE TO HEAVY RAIN THIS AFTERNOON ASSOCIATED WITH THE ARRIVAL
OF SOUTHERN STREAM SHORTWAVE ENERGY WEST OF THE BAJA PENINSULA.
THE LATEST GOES-16 IR/WV AND RGB IMAGERY CLEARLY DEPICTS THIS
ENERGY AND A VORT CIRCULATION ADVANCING RAPIDLY INLAND OVER
INTERIOR SOUTHERN CA.

THE INITIAL WAVE OF HEAVIER RAINS HAVE HELPED TO PRIME THE SOIL
CONDITIONS FOR THIS NEXT SURGE OF HEAVY RAIN AND ALREADY PARTS OF
SANTA BARBARA...VENTURA AND LOS ANGELES COUNTIES HAVE SEEN OVER 1
INCH OF NEW RAIN.

MOVING THROUGH ABOUT 09Z...EXPECT ADDITIONAL RAINFALL TOTALS OF AS
MUCH AS 1.5 INCHES FOR THE CENTRAL AND SOUTHERN CA COASTAL
RANGES...WITH ISOLATED HEAVIER TOTALS OVER THE TRANSVERSE RANGES
GIVEN BETTER OROGRAPHIC FORCING HERE.

THESE AMOUNTS IN CONJUNCTION WITH THE EARLIER RAINFALL WILL BEGIN
TO CAUSE CONCERN FOR ENHANCED RUNOFF AND POSSIBLE FLASH FLOODING
WITH THE DOMINANT CONCERN NOTED AROUND AREAS WHERE RECENT WILDFIRE
ACTIVITY HAS GENERATED BURN SCARS. PERHAPS ONE OF THE MORE
SUSCEPTIBLE AREAS WILL BE THE TERRAIN INVOLVING THE LARGE THOMAS
BURN SCAR. THE THREAT OF DEBRIS FLOWS AND MUDSLIDES WILL BECOME
ELEVATED...AND WITH ADDITIONAL HEAVY RAIN EXPECTED INTO TUESDAY
MORNING...THIS THREAT WILL ONLY GROW FURTHER. EXPECT ADDITIONAL
MPDS TO UPDATE THIS CONCERN AS THIS EVENT CONTINUES TO EVOLVE.

ORRISON

ATTN...WFO...HNX...LOX...MTR...SGX...STO...VEF...

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

LAT...LON   37632192 36772079 35511985 34521795 34061657
            33561633 32971667 32831742 33111789 33831873
            34151951 34372053 34772102 35522153 36432245
            37402270