Flash Flood Guidance
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000
AWUS01 KWNH 200051
FFGMPD
CAZ000-200933-

MESOSCALE PRECIPITATION DISCUSSION 0058
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
751 PM EST SUN FEB 19 2017

AREAS AFFECTED...CA COAST...SACRAMENTO VALLEY...SIERRAS

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

VALID 192333Z - 200933Z

SUMMARY...MODERATE RAIN RATES BY 06Z BECOMING HEAVY BY 12Z WITH AN
INCREASING THREAT OF FLOODING AND FLASH FLOODING.

DISCUSSION...THE NEXT POTENT PACIFIC STORM INCLUDED A BROAD AND
WELL DEFINED ATMOSPHERIC RIVER EXTENDING FROM HAWAII
NORTHEASTWARD...AND CURVING ANTICYCLONICALLY INTO A WARM FRONTAL
ZONE POSITIONED JUST OFFSHORE OF CALIFORNIA AT 23Z SUNDAY. DEEP
CLOUD LAYERS IN VISIBLE IMAGERY TRACED THE OUTLINE OF THE
RIVER...WHILE ALSO COINCIDING WITH THE SURFACE FRONTAL STRUCTURE
AS ANALYZED BY OPC/WPC. THE RIVER WAS MARKED BY PRECIPITABLE WATER
VALUES 1.00 TO 1.25 INCHES...PERHAPS TOPPING OUT JUST BELOW 1.50
INCHES PER BLENDED TPW IMAGERY.

THE SYNOPTIC SCENARIO ABOUT TO UNFOLD PER THE CONSENSUS OF LARGE
SCALE GUIDANCE...WAS NEARLY IDEAL TO PRODUCE A PROLONGED HEAVY
RAIN EVENT OVER PARTS OF CENTRAL/NORTHERN CA. AN ENERGETIC BUT LOW
AMPLITUDE SHORTWAVE THAT WAS AROUND 140 DEG WEST AT 23Z...WILL
MOVE TO JUST OFF NORTHWEST CA BY 12Z...DRAGGING THE RIVER ONSHORE
BUT NOT BUCKLING THE FLOW SUFFICIENTLY TO MOVE THE FRONTAL SYSTEM
THROUGH. AN UPSTREAM WAVE DIGGING FROM THE GULF OF ALASKA WILL
MAINTAIN THE LARGE SCALE TROUGH OFF CALIFORNIA...ALLOWING
PROLONGED MOIST SOUTHWESTERLY DEEP LAYER FLOW LATE TONIGHT THROUGH
MONDAY NIGHT.

THE MAIN QUESTION MARK HAS BEEN WHICH NARROW CORRIDOR WILL SEE THE
MOST INTENSE RAINFALL. THE CONSENSUS...INCLUDING BIAS CORRECTED
ENSEMBLE INFORMATION...IS FOR THE HEAVIEST RAIN TO FALL DIRECTLY
OVER THE SAN FRANCISCO BAY AREA AND ADJACENT MOUNTAIN
RANGES...EXTENDING DOWNWIND THROUGH SACRAMENTO AND INTO THE
SIERRAS. PER HI-RES GUIDANCE...HOURLY RAIN RATES OF A QUARTER INCH
SHOULD DEVELOP ALONG THE COAST BY 05 OR 06Z TONIGHT...WITH RATES
INCREASING TO AROUND 0.50 INCHES BY 12Z...ESPECIALLY NEAR SAN
FRANCISCO...AND INCREASING TO A QUARTER INCH BY THAT TIME IN THE
WINDWARD SIDE OF THE SIERRAS.

WITH SOUTHWARD EXTENT TOWARD SAN LUIS OBISPO THE MOST LIKELY TIME
FOR HEAVY RAIN WILL BE EARLIER IN THE NIGHT...ALTHOUGH WARM
ADVECTION WILL DOMINATE WHICH USUALLY LEADS TO GENTLER RAIN...AND
RATES MAY STRUGGLE TO APPROACH A HALF INCH PER HOUR. THE HEAVIER
RATES ARE EXPECTED TOWARD MONTEREY / SAN FRANCISCO BY MORNING AS
THE APPROACHING SHORTWAVE INDUCES STRENGTHENING AND VEERING WINDS
WITH A GREATER ONSHORE COMPONENT. THE GFS FORECASTS FORTY TO FIFTY
KNOTS OF ONSHORE FLOW IN THE 850-700 MB LAYER WITH STRONG MOISTURE
FLUX VALUES IN THAT LAYER.

GIVEN THE WET WINTER...MANY STATIONS HAVING RECORDED A TOP FIVE
WETTEST THREE MONTH STRETCH SINCE NOVEMBER 17 PER THE REGIONAL
CLIMATE CENTER PERSPECTIVES TOOL...BOTH FLOODING AND FLASH
FLOODING WILL BE POSSIBLE.

BURKE

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

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

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

LAT...LON   40492445 40302387 39412300 38792209 38662151
            39182134 40092196 40262155 39202060 38412020
            37942014 37942093 37632109 36672105 35302058
            34992095 36322219 37482304 38662389 40182478



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