Flash Flood Guidance
Issued by NWS

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000
AWUS01 KWNH 220715
FFGMPD
CAZ000-221500-

MESOSCALE PRECIPITATION DISCUSSION 0036
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
214 AM EST SUN JAN 22 2017

AREAS AFFECTED...COASTAL CALIFORNIA

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

VALID 220715Z - 221500Z

SUMMARY...SHORT BUT INTENSE CONVECTIVE SHOWERS TO AFFECT THE
NORTHERN CALIFORNIA COAST IN THE NEXT FEW HOURS...MORE PROLONGED
SWLY ONSHORE MST FLUX FROM SANTA CRUZ MTNS SOUTH TO POINT
CONCEPTION WITH INCREASED FF THREAT THERE THROUGH 15Z.

DISCUSSION...WV/IR LOOP DENOTES RAPIDLY DEEPENING LOW LIFTING
NORTHEAST TOWARD THE CENTRAL OR COAST...WITH A DRAPED COLD
EXTENDING SOUTHWARD.  THE UPPER LEVEL SUBTROPICAL JET SHIELD
OBSCURES THE AREA SW FROM SONOMA COUNTY SOUTH AND SOUTHWEST OUT TO
SEA..WITH A DEEP MOISTURE PLUME DIRECTLY UNDERNEATH WITH LEADING
ISENTROPIC MID-LEVEL SHOWERS WITH TPWS UP TO 1.0" CURRENTLY WITH
1.25" RAPIDLY APPROACHING WITH WARM FRONT THAT REACHES SAN
FRANCISCO BAY AND SOUTHWARD AROUND 10Z-11Z.  HEIGHT FALLS AND
STRONG UNDERCUTTING CAA (SEEN SOUTH OF 40N...WEST OF 130W) WILL
STEER THIS AXIS SOUTH ALONG THE COAST WITH TIME WITH 50-60KTS OF
SSWLY FLOW VEERING TO SWLY WITH THE WARM FRONT ALLOWING FOR DEEPER
ISENTROPIC ASCENT WITH INCREASE ORTHOGONAL FLOW TO THE COASTAL
TERRAIN FROM MARIN AROUND 10Z TO SANTA CRUZ AROUND 10Z TO N SANTA
LUCIA AROUND 11/12Z. DEEPER ISENTROPIC ASCENT AND CORE OF MOISTURE
AXIS...RATES UP TO .5"/HR ARE EXPECTED ACROSS THE SANTA CRUZ AND
SANTA LUCIA RANGES FROM 12-14Z...LEADING TO  2-4" POSSIBLE THROUGH
15Z.  GIVEN THE SOILS ARE EXTREMELY SATURATED (MORE THAN 8" OVER
NORMAL OVER THE LAST 2 WEEKS) ALONG THE LENGTH OF THE
COAST...THERE IS A GREATER THREAT FOR FLASH FLOODING CONDITIONS.

FURTHER NORTH...BHX RADAR ALREADY DETECTING DEEPER CONVECTIVE
CORES WHERE THE SUBTROPICAL JET HAS ALREADY DROPPED SOUTH
INCREASING DIFFLUENCE AND STEEPENING MID-LEVEL LAPSE RATES.
HI-RES CAMS ARE FAIRLY AGREEABLE IN PLACEMENT/TIMING WITH SHORT
BURST RATES OF .5"/HR EXPECTED FROM NW CA TOWARD THE NORTH BAY
COUNTIES.  WITH FAST MOVING FRONT THAT IS MORE PARALLEL TO THE
COAST THE DURATION SHOULD BE SHORTER AND POTENTIALLY LIMIT
FLOODING CONCERNS TO MOST PRONE AREAS NORTH OF SONOMA COUNTY.

MAIN UNCERTAINTY AT THIS TIME IS LOCATION OF SWITCH FROM FRONTAL
ZONE BEING MORE N-S TO SW-NE NEAR THE TRIPLE POINT...USING
IR/SWIR/AMSU 89GHZ TRENDS THIS APPEARS TO BE VERY NEAR
SONOMA/MENDOCINO COUNTY LINE SO SOUTH OF THIS AREA...THREATS ARE
HIGHER BUT ALSO LESS CERTAIN THAN SOUTH OF THE BAY.  STILL THERE
IS A GOOD LIKELIHOOD FOR TOTALS IN EXCESS OF 1.5" FOR NORTH BAY
COUNTIES INCLUDING THE NAPA/PETALUMA VALLEYS.

GALLINA

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

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

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

LAT...LON   40612409 40512386 40302367 39892336 39512313
            39042278 38602245 38352208 37902187 37562181
            37252177 37032158 36712139 36442098 36052075
            35702059 35302052 35022044 34812030 34582021
            34452032 34452050 34502063 34852073 35062073
            35222086 35492112 35912158 36402200 36762222
            37322262 37752278 37902305 38172328 38602357
            39042390 39512400 39932411 40222452 40552429




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