Flash Flood Guidance
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897
AWUS01 KWNH 212151
FFGMPD
CAZ000-220930-

MESOSCALE PRECIPITATION DISCUSSION 0034
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
450 PM EST SAT JAN 21 2017

AREAS AFFECTED...N-CNTRL CA COAST

CONCERNING...HEAVY RAINFALL

VALID 212149Z - 220930Z

SUMMARY...THE APPROACH OF A STRONG COLD FRONT IS EXPECTED TO BRING
THE ONSET OF HEAVY RAINFALL RATES BETWEEN 06Z AND 09Z TO THE
N-CNTRL CA COAST.

DISCUSSION...RECENT GOES W VISIBLE SATELLITE AND AMSU 89 GHZ
IMAGERY HELPED IDENTIFY THE POSITION OF AN OCCLUDED CYCLONE
RAPIDLY MOVING EAST THROUGH THE ERN PACIFIC...WITH A TRIPLE POINT
LOW NEAR 37N 133W AT 21Z. SSMIS/AMSU RAIN RATES WERE CONSISTENT
WITH AN AREA OF 0.25+ IN/HR JUST AHEAD OF THE TRIPLE POINT AND
RELATED WARM FRONT...WITH THE 19Z AMSU PASS IDENTIFYING THE
HEAVIEST RAIN RATES NEAR 37N 130W. OFTEN TIMES...DOUBLING THE AMSU
RAIN RATE CAN BE A GOOD ESTIMATE FOR EXPECTED RAIN RATES OVER LAND
ONCE TERRAIN IS ACCOUNTED FOR. THE GOES HD WIND PRODUCT OVERLAPPED
THE 12Z GFS WITH THE CORE OF A STRONG 500 MB JET MAX OF 110-120
KT...SLIGHTLY WEAKER THAN THE GFS...BUT ONLY BY 5-10 KT.

AS THE OCCLUDED SYSTEM ADVANCES EAST OVERNIGHT...THE TRIPLE POINT
IS EXPECTED TO REACH THE COASTAL RANGES JUST NORTH OF SAN
FRANCISCO ALONG WITH 850 MB WINDS OF 60-70 KT. THE LEFT EXIT
REGION OF THE MID-UPPER LEVEL JET STREAK ALONG WITH INCREASING
DIFFLUENCE SHOULD HELP TO ENHANCE LIFT AS STRONGER FORCING NEAR
THE TRIPLE POINT ARRIVES. NORTH OF THE BAY REGION...PRECIPITABLE
WATER VALUES SHOULD REMAIN BELOW 1 INCH WITH RAIN RATES IN EXCESS
OF 0.5 IN/HR BEING RELATIVELY SHORT LIVED.
MEANWHILE...PRECIPITABLE WATER VALUES MEETING OR SLIGHTLY
EXCEEDING 1 INCH ARE FORECAST SOUTH OF THE BAY INTO THE SANTA CRUZ
AND SANTA LUCIA RANGE WHERE THE DURATION OF HEAVIER RAIN RATES MAY
PERSIST FOR SEVERAL HOURS AFTER 09Z GIVEN MORE OF A PERPENDICULAR
ANGLE OF THE LOW LEVEL WINDS TO THE TERRAIN. GIVEN THE ANOMALOUSLY
WET ANTECEDENT CONDITIONS ACROSS THE REGION AND SENSITIVITY TO
HEAVY RAIN ACROSS BURN SCAR LOCATIONS...FLOOD OR FLASH FLOOD
ISSUES MAY ARISE.

OTTO

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

ATTN...WFO...LOX...MTR...

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

LAT...LON   38782283 38432263 37642219 37122182 36442145
            36022113 35652084 35402080 35442111 37062257
            38092323 38572327 38762321



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