Flash Flood Guidance
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000
AWUS01 KWNH 200951
FFGMPD
CAZ000-201800-

MESOSCALE PRECIPITATION DISCUSSION 0060
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
450 AM EST MON FEB 20 2017

AREAS AFFECTED...CA COAST...SACRAMENTO VALLEY...SIERRA-NEVADA
FOOTHILLS

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

VALID 200947Z - 201800Z

SUMMARY...ATMOSPHERIC-RIVER DRIVEN HEAVY RAINS ARRIVING EARLY THIS
MORNING WILL INCREASE IN INTENSITY BETWEEN 12Z AND 18Z. RUNOFF AND
ISOLATED FLASH FLOODING CONCERNS ARE EXPECTED.

DISCUSSION...BLENDED-TPW DATA ALONG WITH GOES-W IR/WV IMAGERY
SHOWS THE NEXT ATMOSPHERIC RIVER SURGE AIMING FOR CENTRAL CA TODAY
AS AN AREA OF LOW PRESSURE OFFSHORE NORTHERN CA LIFTS NORTHEAST
TOWARD SOUTHWEST OR. THE MODEL GUIDANCE INDICATES A STRENGTHENING
OF THE LOW LEVEL JET THIS MORNING...REACHING 40 TO 50 KTS BY
1200Z..AND CONTINUING THROUGH 1800Z AS THE CYCLONE APPROACHES.
FACILITATING THIS WILL ALSO BE THE PROXIMITY AND ARRIVAL OF A
STRONG UPPER LEVEL JET STREAK THAT IS CURRENTLY OVER 150 KTS AT
250 MB SOUTHWEST OF THE DEEPENING LOW CENTER.

THE LATEST BLENDED-TPW SHOWS A SURGE OF 1.2 TO 1.4 INCH PWATS
POOLED ALONG AN APPROACHING WARM FRONT AND ALSO DOWN TO THE
SOUTHWEST ALONG A WELL-DEFINED COLD FRONT. OFFSHORE RAINFALL RATES
FROM A 0524Z AMSU PASS WERE GENERALLY BELOW A 0.25 INCH/HR...WITH
THE EXCEPTION OF A FEW 0.30 TO 0.35 INCH/HR RATES ALONG THE
NORTHERN END OF THE WARM FRONT. SINCE THEN IR SATELLITE IMAGERY
HAS BEEN DEPICTING GRADUALLY COOLING CLOUD TOPS JUST OFFSHORE AND
ESPECIALLY UPWIND ALONG THE COLD FRONT...SUGGESTIVE OF INCREASING
RAINFALL RATES SINCE THE LAST AMSU PASS.

RECENT RUNS OF THE HRRR ALONG WITH THE 00Z ARW AND 00Z NSSL-WRF
ARE IN STRONG AGREEMENT IN BRINGING HEAVY RAINS TO THE CENTRAL CA
COASTAL RANGES BY 1200Z...WITH HEAVIER RAINS THEREAFTER EXPECTED
TO SET UP INLAND ACROSS THE SACRAMENTO VALLEY AND THE
SIERRA-NEVADA FOOTHILLS THROUGH 1800Z.

ON AVERAGE...RAINFALL RATES OF AROUND A 0.25 TO PERHAPS
OCCASIONALLY CLOSE TO A 0.50 INCH/HR CAN BE EXPECTED TO DEVELOP IN
THE SACRAMENTO VALLEY. HOWEVER...MUCH HEAVIER RATES ARE EXPECTED
FOR THE CENTRAL CA COASTAL RANGES IN VICINITY OF SAN MATEO AND
ESPECIALLY SANTA CRUZ COUNTIES. HERE...GIVEN THE ORTHOGONAL
ORIENTATION OF THE LOW LEVEL JET RELATIVE TO THE TERRAIN...THE AID
OF STRONG OROGRAPHICS IS EXPECTED TO GENERATE 0.75+ INCH/HR
RAINFALL RATES. FARTHER INLAND INTO THE FOOTHILLS OF THE
SIERRA-NEVADA BELOW SNOW LEVEL...THERE WILL 0.50 TO 0.75 INCH/HR
RAINFALL RATES DEVELOPING.

THROUGH 1800Z...EXPECT ADDITIONAL RAINFALL AMOUNTS OF AS MUCH AS 1
TO 2 INCHES IN THE SACRAMENTO VALLEY...2 TO 4 INCHES IN THE
SIERRA-NEVADA FOOTHILLS...AND 3 TO 6 INCHES FOR THE COASTAL RANGES
WITH THE HEAVIEST TOTALS EXPECTED FOR SANTA CRUZ COUNTY.

THESE RAINS ARE EXPECTED TO RESULT IN ADDITIONAL CONCERNS FOR
RUNOFF PROBLEMS AND AT LEAST ISOLATED FLASH FLOODING GIVEN THE
SATURATED SOIL CONDITIONS AND LOCALIZED BURN SCARS. DEBRIS FLOWS
AND MUDSLIDES WILL BE POSSIBLE. ADDITIONAL HEAVY RAINS WILL OCCUR
AFTER 1800Z...SO STAY TUNED FOR MORE UPDATES.

ORRISON

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

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

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

LAT...LON   39992149 39772092 39212035 38281992 37211901
            36611863 36601907 36981946 37091972 37312024
            37202085 36872145 36532163 36252148 35582082
            35462125 36112192 36762217 37522269 37872302
            38322310 38842267 38762193 39082140 39462154




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