MESOSCALE PRECIPITATION DISCUSSION 0087
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
225 AM EDT FRI APR 06 2018

AREAS AFFECTED...THE BAY AREA & NORTH COAST OF CA

CONCERNING...HEAVY RAINFALL

VALID 060624Z - 061824Z

SUMMARY...AN INCOMING SHORTWAVE INTERACTING WITH AN ATMOSPHERIC
RIVER AND A FRONTAL ZONE SHOULD RAMP UP HOURLY RAIN TOTALS ABOVE
0.5" BEYOND 14Z.

DISCUSSION...AN ATMOSPHERIC RIVER FLOWING FROM THE
PAPAHANAUMOKUAKEA MARINE NATIONAL MONUMENT (NORTHWEST OF THE MAIN
HAWAIIAN ISLANDS) IS POISED TO MAKE LANDFALL ALONG THE CA COAST. 
GPS DATA SHOW PRECIPITABLE WATER VALUES OF 1-1.3" ALONG THE CA
COAST WITH HIGHER VALUES OF 2"+ WELL OFFSHORE PER BOUNDARY LAYER
TOTAL PRECIPITABLE WATER SATELLITE ESTIMATES.  A 04Z RAP-BASED
SOUNDING INDICATES WET BULB ZERO HEIGHTS NEAR 10,400 FEET NEAR SAN
FRANCISCO AT THE PRESENT TIME.  A SHORTWAVE IS APPROACHING THE
132ND MERIDIAN WEST NEAR AND NORTH OF THE MOISTURE PLUME. 
LOW-LEVEL INFLOW IS MODEST OUT OF THE SOUTHWEST, WITH 15-20 KT
VALUES REPORTED IN VAD WIND PROFILES FROM NORTHWEST CA NEAR 850
HPA. 

THE RAP GUIDANCE HAS BEEN SHOWING A FRONTAL ZONE SHARPENING
OFFSHORE THE NORTH-CENTRAL COAST OF CA WITH TIME, LIKELY DUE TO
THE APPROACHING SHORTWAVE.  INFLOW AT 850 HPA RAMPS UP AFTER 10Z,
WHEN PRECIPITABLE WATER VALUES ALONG THE COAST RISE TO 1.50"+,
WITH 50 KT VALUES EXPECTED JUST OFFSHORE THE NORTHWEST CA COAST AS
EARLY AS 13Z.  WET BULB ZERO HEIGHTS ARE FORECAST TO RESEMBLE THE
TROPICS, WITH THE RAP ADVERTISING HEIGHTS CLOSE TO 13,300 FEET --
EFFICIENT WARM RAIN PROCESSES ARE EXPECTED.  THE 12Z AND 00Z HREF
PROBABILITIES HAVE BEEN INDICATING 50%+ CHANCES OF 0.50" AN HOUR
TOTALS IN THE 14-17Z TIME FRAME ASSOCIATED WITH THE ABOVE FACTORS.
 ANY SMALL BUBBLES OF 50-100 J/KG MU CAPE WOULD ASSURE THESE
HOURLY TOTALS.  THE MESOSCALE GUIDANCE INDICATES LOCAL AMOUNTS OF
3"+ THROUGH 18Z IN THIS AREA, WHICH SEEMS REALISTIC.

ROTH

ATTN...WFO...EKA...MTR...STO...

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

LAT...LON   40832424 40662343 39992266 39042213 38272181
            36942179 37122230 37862294 38522350 38952388
            39762394 40232452