MESOSCALE PRECIPITATION DISCUSSION 0003
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1001 AM EST TUE JAN 09 2018

AREAS AFFECTED...TERRAIN OF SOUTHERN CA

CONCERNING...HEAVY RAINFALL

VALID 091500Z - 100110Z

SUMMARY...A DEEP LAYER CYCLONE IS EXPECTED TO LEAD TO HOURLY RAIN
RATES OCCASIONALLY BREACHING 0.5" WITH LOCAL AMOUNTS UP TO 3"
POSSIBLE THROUGH 01Z.

DISCUSSION...A DEEP LAYER CYCLONE IS SPLITTING INTO TWO CENTERS
PER 14Z OBSERVATIONS.  THE SOUTHERN 1000 HPA CENTER IS JUST WEST
OF BURBANK WITH ITS TRAILING FRONTAL BOUNDARY LOCATED EAST OF THE
CHANNEL ISLANDS.  INFRARED SATELLITE IMAGERY SHOWS SOME UNSTABLE
CLOUDINESS NEAR THE UPPER LEVEL LOW SOUTHWEST OF POINT CONCEPTION.
 THE LATEST SPC MESOANALYSES INDICATE MUCAPE VALUES SLOWLY
FALLING, DOWN IN THE 250-500 J/KG RANGE OFFSHORE SOUTHERN CA. 
PRECIPITABLE WATER VALUES ARE 0.8-1.4" PER GPS DATA WHILE INFLOW
AT 850 HPA IS CYCLONIC AT 30-40 KTS PER VAD WIND PROFILES.  THE
SOUNDING FROM SAN DIEGO/NKX SHOWS A SUBSTANTIALLY WARM ATMOSPHERE,
WITH WET BULB ZERO HEIGHTS ABOVE 11,000 FEET.  A COUPLE SITES NEAR
CUCAMONGA AND BORREGO DESERT STATE PARK HAVE REPORTED OVER 0.5" IN
THE PAST HOUR.

AS THE CYCLONE MOVES EASTWARD, ONSHORE FLOW SHOULD SHIFT FROM
SOUTHERLY TO MORE WESTERLY WITH TIME WITH SIMILAR MAGNITUDE.  BY
00Z, THE LOW-LEVEL INFLOW IS EXPECTED TO EXCEED THE MEAN 850-400
HPA, WHICH MAY AID RAINFALL EFFICIENCY.  WHILE WET BULB ZERO
HEIGHTS SHOULD LOWER WITH TIME, NEAR THE UPPER CENTER ITSELF, AN
ADDED BURST OF HEAVY RAINFALL IS POSSIBLE DUE TO A COLD POOL ALOFT
IN AN AREA OF DEEPLY CYCLONIC AND UNIDIRECTIONAL FLOW. 

RAIN RELATED ISSUES SHIFT FROM THE TRANSVERSE RANGES TO THE
COASTAL RANGES NEAR AND NORTH OF SAN DIEGO OVER THE NEXT SEVERAL
HOURS.  TWO BOUTS OF HEAVY RAINFALL ARE FORECAST: ONE THROUGH 20Z
WITH THE ADVANCING FRONT AND NEAR THE UPPER LEVEL LOW FROM 21Z
ONWARD. BURN SCARS FROM 2017 FIRES EXIST IN THIS REGION, WHICH ARE
A CONCERN AS RECENT ARW AND HRRR RUNS SUGGEST 0.5"+ AN HOUR RAIN
TOTALS ARE POSSIBLE UNTIL ~01Z, SO USED THIS AS THE HORIZON FOR
THE MPD.  LOCAL RAINFALL OF 2-3" IS POSSIBLE THROUGH 01Z.  THIS
RAINFALL COULD LEAD TO ISSUES NEAR ANY BURN SCARS. 

ROTH

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

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

LAT...LON   34951896 34561734 33311655 32491668 32521725
            33221772 33741858 34081906 34491926