MESOSCALE PRECIPITATION DISCUSSION 0938
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1227 AM EST MON NOV 20 2017

AREAS AFFECTED...WESTERN OR & NORTHWEST CA

CONCERNING...HEAVY RAINFALL

VALID 200526Z - 201726Z

SUMMARY...ONSHORE FLOW COMBINED WITH LIMITED INSTABILITY AND
SIGNIFICANT DIVERGENCE ALOFT SHOULD LEAD TO HOURLY RAINFALL TOTALS
OF 0.5"+ WITHIN FAVORED TOPOGRAPHY AND LOCAL AMOUNTS TO 5" THROUGH
18Z.

DISCUSSION...PER A RECENT NESDIS/SAB BRIEFING, A BROADLY DIVERGENT
AREA ALOFT EXISTS BETWEEN THE NORTHERN STREAM JET ACROSS
SOUTHWESTERN CANADA AND THE SOUTHERN STREAM JET STREAKING BY THE
OR/CA BORDER.  WATER VAPOR IMAGERY SHOWS A MOISTURE PLUME
EMANATING FROM SOUTHEAST OF HAWAI'I MAKING SUBSTANTIAL PROGRESS
INTO THE NORTHWEST AND TOWARDS NORTHERN CA, WHICH SHOWS UP
STRONGLY IN THE SURFACE-850 HPA LAYER PER THE CIRA LAYERED
PRECIPITABLE WATER IMAGERY.  SPC MESOANALYSES SHOW SLOWLY
INCREASING MU CAPE VALUES, NOW >100 J/KG OFFSHORE OR.  INFLOW AT
850 HPA WAS OUT OF THE SOUTHWEST AT 30-60 KTS PER VAD WIND
PROFILES, WHILE 700 HPA FLOW WAS MORE WESTERLY AT 40-60 KTS. 
PRECIPITABLE WATER VALUES ARE 0.7-1.1" PER GPS VALUES ACROSS
WESTERN WA, OR, AND NORTHERN CA.

HEAVY RAINFALL HAS BEEN SEEN ACROSS PORTIONS OF THE WA COASTAL
RANGES AND CASCADES WITH THE INITIAL SURGE OF MOISTURE MOVING IN
AHEAD OF A COLD FRONT, WITH HOURLY RAIN TOTALS OF 0.3-0.6" NOTED
IN AREA MESONETS.  TO THE SOUTH, SHOWERS WERE STARTING TO BECOME
MORE CONCENTRATED NEAR THE SOUTHERN STREAM JET, MOVING INTO
SOUTHWEST OR AND NORTHERN CA AND HOURLY RAIN TOTALS WERE CLOSE TO
0.1".

WITH TIME, THE 850 HPA AND 700 HPA FLOW BECOME MORE ALIGNED OUT OF
THE WEST, AND BOUNDARY LAYER MOISTURE CONVERGENCE AIDS IN
PRECIPITATION PRODUCTION ACROSS WESTERN OR AND NORTHWEST CA. 
UPSTREAM PRECIPITABLE WATER VALUES OF 1.5"+ EXIST PER RAP
FORECASTS AND SATELLITE DERIVED VALUES.  WET BULB ZERO HEIGHTS
WILL BE PARTICULARLY HIGH NEAR THE CA/OR BORDER WHERE 700 HPA
TEMPERATURES WILL BE NEAR 0C.  INFLOW AT 850 HPA SLOWLY WEAKENS
WHILE 700 HPA INFLOW INCREASES SLIGHTLY AS A SYSTEM OFFSHORE THE
NORTHWEST MOVES INTO VANCOUVER ISLAND.  SREF MEAN MASS FIELDS
INDICATE THAT THE LEVEL OF INSTABILITY ACROSS THE REGION SHOULD BE
FAIRLY CONSTANT THROUGH 15Z BEFORE POSSIBLY FADING. 

THE INGREDIENTS AVAILABLE SUGGEST HOURLY RAIN RATES TO ~0.5"
SHOULD BE POSSIBLE.  THE MESOSCALE GUIDANCE AGREES, WITH THE 00Z
HREF PROBABILITIES OF EXCEEDING 0.5" AN HOUR TOTALS GREATER THAN
50% SHIFTING SOUTHWARD THROUGH OR WITH TIME WHILE THE SOUTHERN
AREA IN SOUTHWEST OR/NORTHWEST CA EXHIBITS A SIMILAR THREAT BEYOND
07Z.  THE MESOSCALE GUIDANCE HAS A GOOD SIGNAL FOR LOCAL AMOUNTS
IN THE 4-5", WHICH WOULD BE MOST PROBLEMATIC NEAR ANY EXISTING
BURN SCARS AND IN STEEP TERRAIN PRONE TO ROCK SLIDES.

ROTH

ATTN...WFO...EKA...MFR...PDT...PQR...SEW...STO...

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

LAT...LON   47212419 47192355 46792283 46142187 45162123
            44842121 42802122 41582183 40862235 40172313
            40022335 40002421 40372463 41052427 41932451
            43102461 43902436 45302414 46512410