Small Craft Advisory 174, NWS San Francisco Bay Area, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
910 PM PST FRI FEB 13 2009

...WET AND WINDY STORM APPROACHING...BUT TIMING UNCERTAIN...

.DISCUSSION...AS OF 9:10 PM PST FRIDAY...A COLD UPPER TROUGH IS
CURRENTLY CENTERED ALONG THE WEST COAST. A SHORTWAVE DROPPING SE
TOWARDS THE BASE OF THE LONGWAVE TROUGH CONTINUES TO PRODUCE
SCATTERED SHOWERS ACROSS OUR FORECAST AREA THIS EVENING. A LINE OF
HEAVY SHOWERS AND A ISOLATED THUNDERSTORM PRODUCED SMALL HAIL
AROUND 6 PM THIS EVENING AS IT SWEPT ACROSS SANTA CLARA AND SANTA
CRUZ COUNTIES. AN ISOLATED THUNDERSTORM WITH SMALL HAIL WAS ALSO
REPORTED NEAR BERKELEY AT ABOUT 8:30 PM THIS EVENING. SHOWERS AND
ISOLATED THUNDERSTORMS WILL TAPER OFF FROM NW TO SE OVERNIGHT AS THE
UPPER TROUGH AXIS ROTATES INLAND.

THE PRIMARY FORECAST ISSUE CONCERNS THE ANTICIPATED IMPACT ON OUR
AREA FROM THE SYSTEM CURRENTLY DROPPING SE ALONG 140W. THE MODELS
AGREE THAT THIS SYSTEM WILL CONTINUE TO MOVE SOUTHEAST OVER THE NEXT
24 HOURS...MERGING WITH A COUPLE OF UPPER LOWS CURRENTLY ALONG THE
WEST COAST...AND FORMING A DEEP UPPER LOW OFF THE NORTHERN CA COAST
BY THIS TIME TOMORROW EVENING. THE MODELS ALSO AGREE THAT THIS UPPER
LOW WILL THEN BECOME NEARLY STATIONARY OFFSHORE AND MOVE VERY SLOWLY
TO THE EAST FROM SATURDAY NIGHT THROUGH MONDAY NIGHT. THE MODELS DO
NOT AGREE...HOWEVER...ON HOW FAR OFFSHORE THE LOW WILL INITIALLY
DEVELOP AND ALSO ON JUST HOW SLOWLY IT WILL MOVE TO THE EAST. THESE
FACTORS WILL HAVE A MAJOR DETERMINING FACTOR ON WHEN AND WHERE THE
HEAVIEST PRECIP AND STRONGEST WINDS WILL IMPACT PORTIONS OF OUR
FORECAST AREA. THE NAM FORMS THE LOW FARTHEST OFFSHORE COMPARED TO
BOTH THE GFS AND ECMWF AND ALSO MOVES IT TO THE EAST MORE SLOWLY
COMPARED TO THE OTHER MODELS. REGARDLESS OF WHICH MODEL VERIFIES...
PRECIPITATION ON SATURDAY WILL BE LIGHT AND MOSTLY CONFINED TO THE
NORTH BAY WHERE WARM ADVECTION WILL BEGIN TO LIFT MOISTURE MOVING IN
FROM THE SOUTHWEST AHEAD OF THE SYSTEM OFFSHORE. MUCH OF THE SOUTHERN
AND EASTERN PORTIONS OF OUR FORECAST AREA MAY REMAIN DRY THROUGH
SATURDAY. BUT GRADUALLY THE MOISTURE PLUME AND STRONG LIFT WILL SHIFT
TO THE EAST AND SOUTH AND ACROSS OUR REGION. THE NAM KEEPS THE MOST
SIGNIFICANT RAINFALL CONFINED TO NORTHWEST SONOMA COUNTY AND TO AREAS
OFFSHORE ALL THE WAY THROUGH SUNDAY NIGHT...AND THEN THE NAM FINALLY
PUSHES THE HEAVIER RAIN BAND THROUGH ON MONDAY. THE GFS AND ECMWF
BRING THE HEAVIEST RAINFALL INTO THE NORTH BAY AFTER MIDNIGHT
SATURDAY NIGHT...AND ACROSS THE REST OF THE FORECAST AREA ON SUNDAY.
THESE TWO MODELS ALSO AGREE THAT THE FOCUS OF HEAVIEST PRECIP WILL
STALL OUT OVER THE MONTEREY BAY AREA FROM SUNDAY AFTERNOON THROUGH
SUNDAY NIGHT...AN OUTCOME THAT WOULD DEFINITELY BE OF CONCERN IN AND
NEAR THE BURN SCAR AREAS OF MONTEREY AND SANTA CRUZ COUNTIES. GIVEN
THE OVERALL MODEL AGREEMENT ON THE SLOW EASTWARD PROGRESSION OF THIS
SYSTEM...FEEL IT IS LIKELY THAT THERE COULD BE AN EXTENDED PERIOD OF
MODERATE TO HEAVY RAINFALL ACROSS A PORTION OF OUR FORECAST AREA
SOMETIME LATE THIS WEEKEND. GIVEN THE AGREEMENT SHOWN BETWEEN THE 12Z
ECWMF AND 00Z GFS...WILL FAVOR THE FASTER TIMING OF THESE MODELS AS
OPPOSED TO THE NAM SOLUTION. HOWEVER...WILL WAIT TO SEE WHAT THE 00Z
ECWMF LOOKS LIKE BEFORE COMMITTING COMPLETELY TO THE FASTER TIMING.

THIS SYSTEM DOES NOT CURRENTLY HAVE A GREAT DEAL OF MOISTURE
ASSOCIATED WITH IT (PWS OF LESS THAN ON INCH)...BUT STRONG
DYNAMICS AND A STALLING FRONT WOULD STILL CAUSE SIGNIFICANT RAIN
ACCUMULATION EVEN WITH ONLY MODERATELY HIGH PWS. PRECIP TOTALS IN
PORTIONS OF THE COASTAL RANGES COULD EASILY REACH 2-4 INCHES BY
MONDAY AFTERNOON...AND POSSIBLY HIGHER. WITH THE CURRENT DEGREE OF
UNCERTAINTY DUE TO MODEL DIFFERENCES...WILL WAIT UNTIL SATURDAY
BEFORE MAKING DECISIONS ON ANY POSSIBLE HYDRO PRODUCTS.

WINDS WILL INCREASE LATE TOMORROW AS A RELATIVELY DEEP SURFACE LOW
FORMS NEAR 40N/130W. THE STRONGEST GRADIENTS WILL REMAIN OFFSHORE
THROUGH TOMORROW SO IT'S LIKELY THAT SOUTHEAST WINDS WON'T REALLY GET
CRANKING OVER LAND UNTIL LATE SATURDAY EVENING...AND CONTINUING INTO
SUNDAY. A HIGH WIND WATCH IS CURRENTLY IN EFFECT FROM SATURDAY
EVENING THROUGH SUNDAY AFTERNOON FOR OUR ENTIRE FORECAST AREA. AS WE
GET CLOSER TO THE EVENT AND TIMING BECOMES MORE CERTAIN...THE MOST
LIKELY SCENARIO WILL BE TO TRANSITION THE HIGH WIND WATCH INTO A HIGH
WIND WARNING FOR COASTAL AND MOUNTAIN ZONES AND INTO A WIND
ADVISORY FOR THE VALLEY ZONES. WINDS SHOULD EASE BY SUNDAY EVENING AS
THE GRADIENTS BEGIN TO RELAX. BUT THE SURFACE LOW WILL REMAIN OFF THE
NORTHERN CA COAST AND SO WE CAN EXPECT BLUSTERY SOUTHERLY WINDS TO
CONTINUE RIGHT INTO MONDAY. ONE ITEM OF INTEREST FROM THE 00Z NAM...
THAT MODEL DEVELOPS A MESO SURFACE LOW JUST OFF THE NORTHERN MONTEREY
COUNTY COAST ON MONDAY MORNING AND FORECASTS 925 MB WIND SPEEDS OF 70
KNOTS NEAR POINT SUR AT 18Z MONDAY. SINCE THE NAM MODEL IS CURRENTLY
BEING VIEWED WITH SOME SKEPTICISM...WILL NOT BUY INTO THIS
POTENTIALLY DAMAGING WIND SCENARIO...BUT IT IS SOMETHING TO KEEP AN
EYE ON.

NEXT WEEK'S WEATHER CONTINUES TO LOOK UNSETTLED...WITH THE
POSSIBILITY OF ANOTHER DEEP UPPER LOW FORMING OFFSHORE AROUND
MIDWEEK. HOWEVER...THE MEDIUM RANGE MODELS ARE BEGINNING TO TREND
TOWARD A SOLUTION THAT KEEPS THE LOW CENTERED TOO FAR OFFSHORE TO
HAVE A MAJOR IMPACT ON OUR AREA. IN ANY CASE...RAIN CHANCES WILL
CONTINUE RIGHT ON THROUGH NEXT WEEK AND NO CHANGES WILL BE MADE TO
THE EXTENDED FORECAST UNTIL THE FULL SUITE OF 00Z MODEL DATA ARE
AVAILABLE.

&&

.AVIATION...AS OF 10:30 PM PST FRIDAY...LAST LINE OF SHOWERS MOVING
THROUGH THE SFO BAY AREA OVER THE NEXT FEW HOURS. CEILINGS
EXPECTED TO REMAIN ABOVE 3500 FEET. WINDS WILL BE LIGHT SOUTHWEST
EXCEPT SOME VALLEY WINDS WILL KEEP SJC AND POSSIBLY SNS LIGHT
SOUTHEAST THROUGH THE NIGHT.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TNGT...SCA...ALL COASTAL WATERS.
            .SCA...FOR ROUGH BAR CONDITIONS.
     .SAT EVENING THROUGH SUNDAY AFTERNOON....HIGH WIND WATCH...ALL
             PUBLIC FORECAST ZONES

&&

PUBLIC FORECAST: DYKEMA
AVIATION/MARINE: W PI

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO

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