Quantitative Precipitation Forecast
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FOUS30 KWBC 161443
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
942 AM EST THU NOV 16 2017

...VALID 15Z THU NOV 16 2017 - 12Z FRI NOV 17 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 WNW MRY 45 SW SQL 30 SW SQL SFO PAO HWD LVK 10 E LVK
20 ESE LVK 15 WSW MOD 10 ESE SCK 20 NNE SCK 15 WSW AUN 15 NE OVE
30 NE CIC 45 NE CIC 60 N BLU 40 N BLU 10 ENE BLU 30 SW TVL
55 NNE MCE 40 NNE FAT 40 NE VIS 35 ENE PTV 30 W IYK 35 ENE BFL
25 SE PTV 10 E VIS FAT 15 W MAE 20 SSW MCE 35 SSW MCE 45 E SNS
30 W NLC 30 NE PRB 20 E PRB 10 NE PRB PRB 15 SSE PRB SBP
10 W SBP 25 WNW SBP 15 S 87Q.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NE VIS 15 ENE MAE 15 NE MCE 20 NE MOD 20 ESE MHR 20 NE BAB
35 ENE OVE 35 N BLU 20 N BLU 20 SSE BLU 45 SSW TVL 45 ENE MCE
30 NNE FAT 35 NE VIS 30 ENE VIS 20 NE VIS.


...CALIFORNIA...

IN THE 15Z UPDATE WE TRIMMED BACK THE COVERAGE OF MARGINAL RISK
PROBABILITIES IN THE CENTRAL VALLEY AND THE BAY AREA. THE OBSERVED
RAIN RATES SIMPLY ARE NOT THERE THIS MORNING...AND MAY INCREASE
ALONG THE COAST SOUTH OF MONTEREY THIS AFTERNOON AS ANOTHER
INFLECTION POINT IN THE UPPER FLOW ARRIVES.
OVERALL...HOWEVER...THE GREATER THREAT OF IMPACTS FROM HEAVY
RAINFALL SHOULD BE IN THE UPSLOPE REGIONS OF THE SIERRAS...WHERE A
SLIGHT RISK OF EXCESSIVE RAINFALL IS MAINTAINED.

PREVIOUS DISCUSSION...A POTENT HEAVY RAINFALL EVENT CONTINUES TO
EVOLVE OVER ESPECIALLY THE UPSLOPE REGION OF THE SIERRAS...BUT
ALSO ALONG THE COAST IN NORTHERN CALIFORNIA. PER THE ESRL ARDT
ANALYSIS AND 6 HOURLY GFS-BASED FORECAST...AN ATMOSPHERIC RIVER
WITH AN ATTENDANT ANOMALOUS PLUME OF INTEGRATED WATER VAPOR
TRANSPORT (IVT) WAS DIRECTED INTO NORTHERN CA... COINCIDENT WITH
THE 125+ KT UPPER LEVEL JET STREAK AND 500-300 MB LAYER PW VALUES
~0.25".

THE ELONGATED NATURE OF THIS AR...SAFELY TUCKED UNDERNEATH THE
LONGWAVE UPPER LOW AND FURTHER FUELED BY ADDITIONAL
DAMPENED...FAST-MOVING SHORTWAVE ENERGY ASSOCIATED ALONG THE NEXT
UPPER JET STREAK...WILL CONTINUE TO SUPPLY ANOMALOUS PWS OF
1-1.25+ INCHES AS IT SLOWLY DIPS SOUTH THROUGH CENTRAL CA IN
RESPONSE TO THE TROUGH PASSAGE/HEIGHT FALLS ACROSS THE PAC NW AND
NORTHERN CA. PW STANDARDIZED ANOMALIES PER BOTH THE SREF AND GEFS
ENSEMBLE MEANS CONTINUE TO BE A ROBUST +2 TO +3...THROUGH THE
PERIOD...TO GO ALONG WITH EQUALLY HEALTHY 850-700 MB MOISTURE FLUX
ANOMALIES OF +3 TO +4 WITHIN THE EXPANSIVE WSW-ENE ORIENTED WARM
CONVEYOR BELT. MOREOVER...MUCAPES OF 100-200 J/KG (BOTH OBSERVED
PER THE LATEST SPC MESOANALYSIS AND PROGGED PER THE GFS/ECMWF WILL
CERTAINLY HELP THE ALREADY-HEALTHY 1-3 HOUR RAINFALL RAINS IN
AREAS OF FORCED OROGRAPHIC ASCENT -- I.E. FROM THE BAY AREA INTO
THE CENTRAL CA COASTAL RANGES AND ESPECIALLY THE WESTERN SLOPES OF
THE SIERRA WITH THE ORTHOGONAL W-WSW LOW-LEVEL FLOW. HOURLY
RAINFALL RATES OF 0.50-0.75" AND 3HR RATES UP TO 1.5" CONTINUE TO
BE NOTED ACROSS THESE AFOREMENTIONED AREAS PER THE BEVY OF
AVAILABLE HIGH-RESOLUTION CAM GUIDANCE. FORECAST AVERAGE
EVENT-TOTAL RAINFALL AMOUNTS OF 1.5 TO 3.0 INCHES (LOCALLY
GREATER) ACROSS THE COASTAL RANGES AND 3-6+ INCHES OVER MUCH OF
THE SIERRA. THESE AMOUNTS WERE SUPPORTED BY THE LATEST GEFS-BASED
RE-FORECAST DATA...AND COULD LEAD TO FLASH FLOODING.

HURLEY/BURKE

$$





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