MESOSCALE PRECIPITATION DISCUSSION 0077
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1048 PM EDT MON MAR 20 2017

AREAS AFFECTED...NORTHERN & CENTRAL CA

CONCERNING...HEAVY RAINFALL

VALID 210300Z - 211500Z

SUMMARY...MERGING MOISTURE CHANNELS COMBINED WITH BOUNDARY LAYER
MOISTURE CONVERGENCE, INCREASING INSTABILITY, AND DECENT PACIFIC
INFLOW SHOULD LEAD TO RAIN RATES OF 0.5" WITH LOCAL AMOUNTS UP TO
3" THROUGH 15Z.

DISCUSSION...A DEEP CYCLONE NEAR 40N 135W IS PUSHING A FRONTAL
ZONE/BAROCLINIC TROUGH TOWARDS THE CA COAST.  ALTHOUGH NOT A
CLASSIC ATMOSPHERIC RIVER, TWO MOISTURE CHANNELS -- ONE LEFT
BEHIND BY THE LAST DISTURBANCE AND A FRESH ONE WITH THE INCOMING
FRONT -- HAVE SHOWN SOME MERGER AS OF LATE WITHIN LAYERED
PRECIPITABLE WATER IMAGERY.  VAD WIND PROFILES SHOW LOW-LEVEL
INFLOW OF 30-50 KTS ACROSS CENTRAL AND NORTHERN CA.  PRECIPITABLE
WATER VALUES ARE 0.8-1.2" PER RECENT GPS DATA.  AHEAD OF A
SHORTWAVE MOVING AROUND THE DEEP CYCLONE OFFSHORE, MUCAPES OF
100-300 J/KG LURK JUST OFFSHORE CENTRAL CA AT THIS TIME WITH
RECENT RADAR AND SATELLITE IMAGERY SHOWING SOME SHALLOW CONVECTION
OFFSHORE NORTHERN AND CENTRAL CA.  PER MESONET OBSERVATIONS,
HOURLY RAIN RATES OF 0.3-0.4" HAVE BEEN REPORTED RECENTLY.

INSTABILITY IS EXPECTED TO INCREASE NEAR AND BEHIND THE SURFACE
BOUNDARY MOVING INTO THE STATE, WITH RAP FORECASTS INDICATING
MUCAPES EXCEEDING 500 J/KG SHOWING SOME OVERLAP WITH THE BETTER
LOW-LEVEL INFLOW AND AVAILABLE MOISTURE.  MESOSCALE GUIDANCE
INDICATES THAT HOURLY RAIN RATES COULD EXCEED 0.5" AS EARLY AS
06Z, WHICH SPREAD INLAND ACROSS NORTHERN AND CENTRAL CA WITH TIME.
 ACROSS NORTHERN CA, PRECIPITATION COULD TRANSITION TO SNOW WITHIN
THE SHASTA/SISKIYOU RANGES BEHIND THE FRONTAL ZONE.  LOCAL AMOUNTS
OF 3" ARE POSSIBLE PER THE MESOSCALE GUIDANCE THROUGH 15Z.  THIS
COULD BE PROBLEMATIC IN AREAS OF SNOW MELT AND BURN SCARS.

ROTH

ATTN...WFO...EKA...HNX...LOX...MFR...MTR...REV...STO...

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

LAT...LON   42722410 42252356 41782318 41812154 40862108
            39322042 37172029 35361963 34511991 34542055
            35072079 36142188 37302263 38512359 39942442
            40282453 41422426 42522453