High Surf Advisory 6, NWS San Francisco Bay Area, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
932 AM PST MON DEC 8 2014

...A POWERFUL PACIFIC STORM SYSTEM WILL IMPACT THE REGION FROM
LATE WEDNESDAY THROUGH LATE THURSDAY...

.SYNOPSIS...PERIODS OF LIGHT RAIN ARE LIKELY IN THE NORTH BAY
THROUGH MONDAY EVENING. OTHERWISE...MAINLY DRY CONDITIONS WILL
PREVAIL THROUGH WEDNESDAY MORNING. A POWERFUL STORM SYSTEM WILL
BEGIN TO SPREAD RAIN INTO THE NORTH BAY BY WEDNESDAY AFTERNOON.
HEAVY RAIN AND POSSIBLE THUNDERSTORMS ARE THEN FORECAST TO MOVE
FROM NORTH TO SOUTH THROUGH THE REGION FROM LATE WEDNESDAY NIGHT
THROUGH THURSDAY EVENING. IN ADDITION...A PERIOD OF STRONG...AND
POSSIBLY DAMAGING...WINDS IS FORECAST FROM WEDNESDAY NIGHT THROUGH
MIDDAY THURSDAY. SHOWERS WILL LINGER THROUGH FRIDAY BEFORE DRY
WEATHER RETURNS ON SATURDAY.

&&

.DISCUSSION...AS OF 8:40 AM PST MONDAY...KMUX CONTINUES TO SHOW A
FEW SHOWERS THIS MORNING OVER THE NORTH BAY. RAIN SHOULD END BY
THIS EVENING. MAIN ITEM OF CONCERN FOR THE FORECAST REMAINS THE
STRONG STORM EXPECTED TO HIT OUR AREA WEDNESDAY NIGHT THROUGH
THURSDAY NIGHT. LATEST GUIDANCE IN SHOWS LITTLE CHANGE FROM
PREVIOUS RUNS -- LOOKING FOR A GOOD SHOT OF HEAVY RAIN, VERY GUSTY
WINDS AND POSSIBLE THUNDERSTORMS. GFS/ECMWF/NAM ALL SHOW STRONG
WINDS COMING ACROSS WITH 925 MB SPEEDS LOCALLY IN THE 60-70 KT
RANGE. LOCAL WRF THAT GOES OUT THROUGH WEDNESDAY NIGHT HAS SURFACE
GUSTS OF 30-40 KT FOR MOST OF THE SF BAY REGION AT THE SAME TIME.
WILL BE ISSUING A HIGH WIND WATCH FOR OUR ENTIRE AREA FOR THE
STORM.

.PREVIOUS DISCUSSION...MOIST SOUTHWESTERLY FLOW CONTINUES ACROSS
OUR REGION EARLY THIS MORNING. RADAR IS CURRENTLY SHOWING WEAK
RETURNS ACROSS NW SONOMA COUNTY AND A FEW GAUGES IN NORTHERN
SONOMA COUNTY HAVE PICKED UP LIGHT AMOUNTS OF PRECIP OVERNIGHT.
WEAK IMPULSES MOVING THROUGH THE MOIST SWLY FLOW AND INTO NORTHERN
CA ARE EXPECTED TO PRODUCE SCATTERED LIGHT SHOWERS ACROSS THE
NORTH BAY COUNTIES THROUGH THIS EVENING...BUT NOT LIKELY SOUTH OF
THE GOLDEN GATE. OTHERWISE...MOSTLY DRY CONDITIONS ARE EXPECTED
THROUGH WEDNESDAY MORNING.

MODELS CONTINUE TO MAINTAIN THEIR CONSISTENCY IN FORECASTING A
POWERFUL STORM MOVING THROUGH OUR AREA LATER THIS WEEK...EXPECTED
TO BE ONE OF THE STRONGEST STORMS IN TERMS OF WIND AND RAIN
INTENSITY SINCE THE OCTOBER 13 2009 STORM OR THE JANUARY 4 2008
STORM. BECAUSE OF THE HIGH DEGREE OF MODEL CONSISTENCY...WHICH
HAS HELD SINCE FRIDAY...FORECAST CONFIDENCE IS HIGH THAT THIS
STORM WILL PRODUCE SIGNIFICANT WIND AND RAIN...ESPECIALLY ON
THURSDAY. THERE ARE SLIGHT DIFFERENCES BETWEEN THE MODELS IN
REGARD TO TIMING OF PRECIP ONSET...BUT THE GENERAL IDEA IS THAT
RAIN WILL BEGIN IN THE NORTH BAY ON WEDNESDAY AFTERNOON AND THEN
BECOMING HEAVY IN THE NORTH BAY BY LATE WEDNESDAY EVENING. HEAVY
RAINFALL WILL THEN MARCH SLOWLY AND STEADILY SOUTHWARD THROUGH THE
REMAINDER OF THE SF BAY AREA FROM LATE WEDNESDAY NIGHT THROUGH
THURSDAY...AND SOUTH INTO THE MONTEREY BAY AREA AND CENTRAL COAST
REGION FROM MIDDAY THURSDAY THROUGH THURSDAY EVENING. RAIN RATES
WILL BE ESPECIALLY INTENSE IN THE COASTAL RANGES WHERE A STRONG
AND MOIST LOW LEVEL SOUTHERLY FLOW WILL BE LIFTED BY THE TERRAIN.
BOTH THE NAM AND GFS FORECAST 925 MB SOUTHERLY WINDS OF UP TO 70
KNOTS COINCIDENT WITH THE FRONTAL RAIN BAND. THE FRONT WILL MOVE
ENTIRELY THROUGH OUR AREA BY MIDNIGHT THURSDAY NIGHT...BUT SHOWERS
WILL CONTINUE THROUGH FRIDAY AND FRIDAY EVENING. RAINFALL TOTALS
WITH THIS SYSTEM ARE CURRENTLY FORECAST TO RANGE FROM 1 TO 2
INCHES IN THE VALLEYS AND ACROSS THE INTERIOR MOUNTAINS OF
MONTEREY AND SAN BENITO COUNTIES...FROM 2 TO 4 INCHES ACROSS THE
LOWER ELEVATIONS AND URBAN AREAS OF THE SAN FRANCISCO BAY AREA...AND
FROM 5 TO 8 INCHES IN THE COASTAL MOUNTAINS. LOCALLY HIGHER
AMOUNTS ARE POSSIBLE IN THE COASTAL RANGES OF THE NORTH BAY AND IN
THE SANTA CRUZ AND SANTA LUCIA MOUNTAINS.

RECENT HEAVY RAINFALL SATURATED THE SOILS...ESPECIALLY IN THE
COASTAL RANGES...AND HEAVY RAINFALL AT THIS POINT WILL RESULT IN
RAPID RISES ON SMALL RIVERS AND STREAMS. IN ADDITION...FLOODING OF
LOW LYING OR POORLY DRAINED URBAN AREAS IS LIKELY...AS ARE MUDSLIDES
IN AREAS OF STEEP TERRAIN. THE MODELS GENERALLY MOVE THE BAND OF
HEAVY RAIN THROUGH AT A STEADY PACE. IF A STRONGER IMPULSE
MOVING ALONG THE FRONTAL BOUNDARY STALLS THE HEAVY RAIN BAND FOR
AN EXTENDED PERIOD OF TIME...MORE SIGNIFICANT FLOODING ISSUES WILL
LIKELY OCCUR. THE PERIOD OF MOST CONCERN FOR FLOODING IS FROM
THURSDAY MORNING THROUGH THURSDAY EVENING.

THE MODELS HAVE RECENTLY BACKED OFF SLIGHTLY ON THE DEPTH OF THE
SURFACE LOW THAT WILL TRACK TO THE NE JUST OFF THE NORTHERN CA
COAST LATE ON WEDNESDAY NIGHT. THE GFS AND ECMWF NOW FORECAST A
MINIMUM PRESSURE OF ABOUT 990 MB (THESE SAME MODELS WERE
FORECASTING A MINIMUM PRESSURE OF ABOUT 980 MB YESTERDAY
MORNING)...WHILE THE NAM IS SLIGHTLY DEEPER AT 988 MB. IN ANY
CASE...STEEP PRESSURE GRADIENTS WILL DEVELOP FROM SOUTH TO NORTH
ON WEDNESDAY NIGHT AND A PERIOD OF STRONG AND POSSIBLY
DAMAGING WINDS IS EXPECTED FROM LATE WEDNESDAY NIGHT THROUGH
MIDDAY THURSDAY. WIDESPREAD WINDS GUSTS OF BETWEEN 40 AND 50 MPH
ARE LIKELY WITH LOCAL GUSTS OF BETWEEN 60 AND 70 MPH EXPECTED NEAR
THE COAST AND IN THE HILLS. HIGH WIND WARNINGS AND/OR WIND
ADVISORIES WILL LIKELY BE POSTED.

DRY WEATHER IS EXPECTED ON SATURDAY...AND PERHAPS INTO SUNDAY...AS
A SHORTWAVE RIDGE BUILDS TEMPORARILY OVER CA. HOWEVER...THE
MEDIUM RANGE MODELS INDICATE THE UNSETTLED WEATHER PATTERN WILL
CONTINUE AND PREDICT MORE RAINFALL AS EARLY AS SUNDAY EVENING.

&&

AVIATION...AS OF 9:30 AM PST MONDAY...SPLIT FORECAST FROM N TO S.
NORTH BAY WILL HOLD ONTO LOWERED CIGS WITH A CHC FOR A PASSING
SHOWER TODAY. FOR SF BAY...CIGS WILL BE MAINLY IN THE LOW END VFR
RANGE...BUT A PASSING CIG BELOW 3K FEET WILL BE POSSIBLE THROUGH
THIS AFTERNOON. KSJC AND MONTEREY PENINSULA WILL BE MOSTLY VFR
THIS AFTERNOON AND EARLY TONIGHT. CIGS RETURN LATE TONIGHT WITH
VERY MOIST LOW LEVELS. PATCHY FOG WILL ALSO BE LATE TONIGHT.
OVERALL CONF IS MEDIUM.

VICINITY OF KSFO...BORDERLINE MVFR/VFR CONDITIONS POSSIBLE AT
KSFO THROUGH 19Z...POSS 21Z. KOAK WILL REMAIN VFR. CIGS RETURN
LATE TONIGHT. WINDS MOSTLY LIGHT TO POSS MODERATE.

KSFO BRIDGE APPROACH...SIMILAR TO KSFO.

MONTEREY BAY AREA TERMINALS...VFR CONDITIONS TODAY WITH MVFR
CONDITIONS RETURNING TONIGHT.

&&

.MARINE...AS OF 09:27 AM PST MONDAY...AN APPROACHING COLD FRONT
FRONT WILL MAINTAIN LIGHT TO MODERATE SOUTHERLY WINDS ALONG THE
CENTRAL COAST TODAY AND TUESDAY. A POWER STORM SYSTEM WILL
APPROACH THE REGION BRINGING HAZARDOUS CONDITIONS TO THE COASTAL
WATERS INCLUDING...LARGE SWELL AND VERY STRONG WINDS.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TDAY...NONE.

&&

PUBLIC FORECAST: BELL/DYKEMA
AVIATION/MARINE: MM

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