MESOSCALE PRECIPITATION DISCUSSION 0804
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
943 PM EST FRI DEC 23 2016
 
AREAS AFFECTED...SOUTHERN CALIFORNIA COASTAL RANGES 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 240241Z - 241200Z
 
SUMMARY...HEAVY RAINS WILL CONTINUE TO OVERSPREAD THE SOUTHERN CA
COASTAL RANGES OVERNIGHT. SOME FLASH FLOODING WILL BE POSSIBLE
OVER THE BURN SCAR AREAS...WITH A RISK OF MUD AND DEBRIS FLOWS.

DISCUSSION...A DEEP UPPER TROUGH CONTINUES TO DIG INTO CENTRAL AND
SOUTHERN CA THIS EVENING WITH SUPPORT OF A 250 MB JET IN EXCESS OF
150 KTS DIGGING DOWN THE WEST SIDE OF THE TROUGH AXIS OFFSHORE. A
WELL-DEFINED COLD FRONT ACCOMPANYING THE TROUGH IS SOUTHEASTWARD
THROUGH THE STATE AND WILL BE ADVANCING THROUGH SOUTHERN CA
OVERNIGHT. 

IR SATELLITE IMAGERY SHOWS A NOTABLE EXPANSION OF COLDER CLOUD
TOPS OVER CENTRAL CA AND EXTENDING OFFSHORE OVER THE LAST COUPLE
OF HOURS...WHICH IS IN RESPONSE TO FAVORABLE 250 MB LEFT-EXIT
REGION JET DYNAMICS ROUNDING THE BASE OF THE TROUGH. THIS ENERGY
IS COUPLING WITH A ROBUST SOUTHWESTERLY LOW LEVEL JET OF 40 TO 50
KTS NOTED AT 850 MB AHEAD OF IT AND IS RESULTING IN AN UPTICK IN
RAINFALL RATES. A 2148Z AMSU PASS HAD ALREADY INDICATED AN UPTICK
IN OFFSHORE RAINFALL RATES...AND COOLER CLOUD TOPS SINCE THEN
SUGGESTS HEAVIER RATES MATERIALIZING.

THE LATEST BLENDED-TPW DATA SHOWS PWATS ON THE ORDER OF 1.0 TO 1.2
INCHES AIMED INTO THE COASTAL RANGES JUST AHEAD OF THE
FRONT...WITH 850 MB MOISTURE FLUX ANOMALIES AS HIGH AS 3 STANDARD
DEVIATIONS ABOVE NORMAL. OBSERVED RAINFALL RATES ARE OCCASIONALLY
REACHING 0.50 TO 0.60 INCHES/HR...AND THESE HEAVIER RAINFALL RATES
WILL CONTINUE OVERNIGHT AND GRADUALLY SETTLE SOUTH DOWN THE
COAST...AND PERHAPS REACHING AS HIGH AS 0.75 INCHES/HR.

A CONSENSUS OF THE HIRES GUIDANCE AND RECENT RUNS OF THE HRRR
FAVOR RAINFALL TOTALS THROUGH 12Z OF 2 TO 3 INCHES...WITH A FOCUS
OVER THE OROGRAPHICALLY FAVORED WEST AND SOUTHWEST-FACING SLOPES
OF THE SOUTHERN CA COASTAL RANGES...STRETCHING FROM THE SANTA ANA
MOUNTAINS DOWN TO THE SAN BERNADINO AND SAN GABRIEL MOUNTAINS.

THESE RAINS WILL BE FALLING OVER SENSITIVE BURN SCAR AREAS AND
WILL FOSTER THE POTENTIAL FOR SOME FLASH FLOODING. MUD AND DEBRIS
FLOWS WILL BE A NOTABLE CONCERN OVERNIGHT FOR THESE AREAS THROUGH
12Z BY WHICH TIME THE HEAVIER RAINS ARE EXPECTED TO BEGIN MOVING
AWAY FROM THE THREAT AREA TO THE EAST.

ORRISON

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

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

LAT...LON   35081978 34561837 34291730 33691659 32801614 
            32441649 32511719 33231771 33541815 33651868 
            33871898 34101980 34272012 34752047