MESOSCALE PRECIPITATION DISCUSSION 0092...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EDT SAT APR 07 2018

CORRECTED FOR TYPOS

AREAS AFFECTED...NORTHERN & CENTRAL CA

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

VALID 070553Z - 071753Z

SUMMARY...A COLD FRONT INTERACTING WITH THE PINEAPPLE EXPRESS AND
AVAILABLE INSTABILITY IS EXPECTED TO LEAD TO HOURLY RAIN TOTALS OF
0.5-1.0" INTO THE MORNING HOURS, WITH LOCAL AMOUNTS TO 4" THROUGH
18Z.

DISCUSSION...THE PINEAPPLE EXPRESS FORM OF AN ATMOSPHERIC RIVER
CONTINUES INVADING CALIFORNIA FROM THE PACIFIC.  A COLD FRONT IS
POISED TO MAKE LANDFALL IN NORTHWEST CA AT THIS TIME. 
PRECIPITABLE WATER VALUES PER GPS VALUES ARE 1.8" JUST NORTH OF
THE BAY AREA, AND REGIONALLY EXCEED 1.3".  WET BULB ZERO HEIGHT
WAS A TROPICAL 13,000 FEET PER THE 00Z OAKLAND SOUNDING.  MU CAPE
VALUES PER RECENT SPC MESOANALYSES ARE QUITE HIGH FOR OUTSIDE THE
CENTER OF A DEEP LAYER PACIFIC CYCLONE; 100-250 J/KG ALONG THE
IMMEDIATE COAST AND OVER 500 J/KG NOT FAR OFFSHORE.  INFLOW AT 850
HPA IS WEST TO SOUTHWEST AT 20-40 KTS PER THE EUREKA CA SOUNDING
AND RAP FORECASTS.  EFFECTIVE BULK SHEAR IS 25-40 KTS PER SPC
MESOANALYSES.

BOUNDARY LAYER MOISTURE CONVERGENCE ASSOCIATED WITH A COLD FRONT
MOVES INLAND SHORTLY AND IS EXPECTED TO EFFECTIVELY WRING OUT THE
AVAILABLE MOISTURE, EVEN AFTER MU CAPE IS TAPPED AND FADES BEYOND
08Z.  RULES OF THUMB REGARDING MOISTURE AND INSTABILITY SUPPORT
HOURLY RAIN TOTALS OF 0.75" IN THIS ENVIRONMENT, WHICH IS
FORECAST/INDICATED BY THE 00Z HREF PROBABILITIES OF REACHING THE
0.5" AND 1" AN HOUR THRESHOLDS.  WHEN COMBINED WITH THE FLOOD
ISSUES/DEBRIS FLOWS WHICH HAVE OCCURRED SO FAR IN CENTRAL CA,
FLASH FLOODING BECOMES AN INCREASING CONCERN.  THE MESOSCALE
GUIDANCE INDICATES TOTAL AMOUNTS THROUGH 18Z UP TO 4", WHICH COULD
LEAD TO EVENT TOTALS OF 9"+ FOR CENTRAL CA.

ROTH

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

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

LAT...LON   42712437 42262301 41042127 39171984 36641797
            36211774 35811778 35381843 35592060 36082172
            36192189 36772232 37642282 38382361 39212404
            39792410 40352454 41492446 41742464