FXUS66 KMTR 271819
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
1015 AM PST FRI NOV 27 2009

UPDATED FOR AVIATION/MARINE DISCUSSIONS...

.DISCUSSION...AS OF 9:00 AM FRIDAY BACK EDGE OF FRONT NOW PASSING
THE SOUTH BAY WITH JUST MINOR ECHOS SEEN ON MT VACA RADAR.
SOME WEAK CONVECTION BEHIND FRONT PASSING OVER SAN PABLO BAY AND
WESTERN MARIN COUNTY. MAIN UPPER LOW NOW SHOWING SIGNS OF CLOSING
OFF IN WV IMAGERY WITH GOOD DEFORMATION ZONE NW OF DRY SLOT
APPARENT.  UNDER THIS DEFORMATION GOOD SHOWERS SEEN ON EKA RADAR
MOVING SOUTH.

SCENARIO FOR TODAY LOOKS TO BE CLEARING SKIES BEHIND THE FRONT WITH
SCATTERED SHOWERS DEVELOPING FIRST OVER THE NORTH BAY THEN MOVING
SOUTH DURING THE DAY AS THE COLD UPPER LOW DROPS OVER THE AREA.
HAVE UPDATED THE ZONES THIS AM TO ADD SLIGHT CHANCE OF THUNDER FROM
THE BAY AREA SOUTH FOR THIS EVENING AS NAM SHOWS GOOD INSTABILITY
AND GOOD UVV IN DEFORMATION ZONE...COMMA HEAD...OF THE UPPER CLOSED
LOW. EXPECT MAIN UPPER LOW TO DROP DOWN THE CENTRAL VALLEY WITH
WRAP AROUND MOISTURE AND GOOD VERTICAL MOTION IN THE AFOREMENTIONED
COMMA HEAD TO SUPPORT SOME THUNDERSTORMS AND SHOWERS BEGINNING THIS
AFTERNOON OVER THE BAY AREA AND MOVING SOUTH DURING THE EVENING
HOURS. HAVE SEEN THIS PATTERN DEVELOP ORGANIZED CONVECTIVE SYSTEM
THAT MOVES DIRECTLY SOUTH THROUGH THE SANTA CLARA VALLEY AND
CONTINUE OVER THE GABILAN RANGE.  COULD BE SOME SMALL HAIL
EMBEDDED IN THESE CELLS GIVEN COLD NATURE OF UPPER LOW BUT HAVE NOT
EMPHASIZED THAT IN ZONES.

HIGH SURF WILL NOW CONTINUE THROUGH SATURDAY DAYTIME HOURS AS LARGE
OCEAN SWELLS COMBINED WITH LONG NW FETCH WILL KEEP HIGH WAVES UP
THROUGH THE DAY TOMORROW.  BEACH-GOERS SHOULD TAKE EXTRA PRECAUTIONS
WHEN VENTURING TO THE COAST TODAY AND SATURDAY.

ONCE THE UPPER LOW DROPS SOUTH RAPID CLEARING WILL TAKE PLACE
OVERNIGHT FROM NORTH TO SOUTH.  THE NORTHWEST FLOW WILL MAKE IT
SLOW TO CLEAR IN THE MONTEREY BAY AND NORTHERN SALINAS VALLEY AREAS
BUT THEY TOO WILL CLEAR BY SATURDAY AFTERNOON.  THE OTHER CHALLENGE
IN THE FORECAST IS THE NE WINDS BEHIND THE UPPER LOW.  STRONG
SUBSIDENCE AND WINDS TO NEAR 50 KTS COULD PRODUCE WINDS TO
ADVISORY LEVEL BY SUNRISE TOMORROW.  WILL TAKE A LOOK AT OUR LOCAL
WRF THAT RUNS LATER THIS AM TO DETERMINE IF ADVISORY IS WARRANTED.
LOOKS LIKE BEST LOCATIONS WOULD BE NORTH AND EAST BAY HILLS.

WILL BE BREEZY AND COOL SATURDAY ON BACKSIDE OF THE UPPER LOW.
WINDS WILL BE GUSTY ALONG THE COAST AND COASTAL GAPS AS WELL AS IN
THE HILLS.  UPPER LOW EJECTS OUT RATHER QUICKLY SO SUNDAY SHOULD BE
A WARMER AND LESS WINDY DAY WITH OFFSHORE BREEZES ALLOWING TEMPS TO
WARM UNDER SUNNY SKIES.  TEMPS MAY PUSH THE 70 MARK IN THE NORTH
BAY WITH THE NORTH TO NORTH EASY WINDS.  OTHERWISE MOST AREAS WILL
SEE MID TO UPPER 60S. BEST MODEL GUIDANCE SEEMS TO KEEP ENOUGH WINDS
GOING SATURDAY NIGHT THAT SUN MORNING LOWS SHOULD REMAIN IN THE
UPPER 30S TO LOW 40S EXCEPT SOUTHERN MONTEREY COUNTY WHERE LOWS
WILL BE IN THE UPPER 20S TO LOW 30S.  LOWS MAY DROP DOWN A FEW
DEGREES FOR MONDAY MORNING AS WINDS DECREASE SO COULD SEE SOME
SCATTERED FROST IN LOW LYING AREAS OF THE NORTH BAY AND SOUTH BAY
VALLEYS.  MONDAY SHOULD SEE ABOUT THE SAME TEMPS AS SUNDAY AS HIGH
PRESSURE BUILDS OVER THE AREA BUT THERE IS WEAKER OFFSHORE WINDS SO
THE TWO TOGETHER WILL KEEPS TEMPERATURES NEAR SEASONAL NORMS.

EXTENDED...WILL DELAY ANY EXTENDED FORECAST DISCUSSION UNTIL THE
AFTERNOON PACKAGE AS THE MODELS HAVING VERY DIFFICULT TIME DEALING
WITH THE ASSIMILATION OF SUPER TYPHOON NIDA AND THE INCREASING MJO
IN THE WESTERN PACIFIC.  WILL TRY TO LOOK AT MOST RECENT ECMWF AND
GFS ENSEMBLES ALONG WITH LATEST MJO FORECASTS...OF WHICH ALL MODELS
HAVE BEEN DOING A TERRIBLE JOB...TO SEE IF WE CAN CLARIFY THE 4 TO
7 DAY AND WEEK TWO FORECAST A LITTLE BETTER. MODELS HANDLING OF THE
WEAK BREAKTHROUGH ON TUESDAY DOES NOT GIVE ONE MUCH CONFIDENCE AND
AFTER THAT MODEL DISPERSION IS VERY HIGH.  THIS IN ITSELF IS A
SIGNAL THAT THE ENERGY BEING ABSORBED FROM NIDA IS GENERATING THIS
NOISE ALONG WITH THE MODELS WANTING TO COLLAPSE THE MJO CONVECTIVE
SIGNAL BACK TO THE WEST WHEN IN REALITY IT IS MOVING EAST AND
STRENGTHENING.  MORE THIS AFTERNOON.

&&

.AVIATION...AS OF 10:15 AM PST FRIDAY...FRONTAL PASSAGE OCCURRING
THROUGH THE S.F.BAY AREA AT POST TIME WITH AREAS NORTH OF S.F.
COMING INTO THE POST-FRONT NORTHWEST FLOW WITH CIG/VSBY BECOMING
VFR AND SOUTH BAY STILL IN PRE-FRONT MVFR LOWER CIGS/VSBYS. EXPECT
POST-FRONTAL SCT-BKN020 BKN-OVC030-040 WITH SCATTERED LIGHT RAIN
SHOWERS THROUGH 03Z. COLD POOL OF AIR ALOFT SLIPPING SOUTHEAST DOWN
THE COAST DURING THE DAY COULD DEVELOP SOME CONVECTIVE ACTIVITY 00-
04Z TIME PERIOD LATE THIS AFTERNOON AND EARLY EVENING. STILL EXPECT
CIGS/VSBYS TO REMAIN VFR. BY 06Z LOWER AIR MASS BEGINNING TO DRY AS
WINDS ALOFT VEER TO THE N AND NNE BY DAWN SATURDAY. WHILE SKIES
WILL CLEAR...AFTER 09Z SOME THREAT OF LOW-LEVEL WIND SHEAR DUE TO NNE
WINDS 30-40KTS AT 850MB/5000-FT LEVEL.

VICINITY OF KSFO...MULTIPLE LAYERS AT OR BELOW 4000 FT MSL AT POST
TIME BUT EXPECTED SCT-BKN020 BKN-OVC030 TIL 20Z...THEN SCT-BKN030
WITH SHOWERS IN THE VICINITY TO 00Z. ALSO...WINDS INCREASING AFTER
20Z NW 15G25KT WITH A FEW GUSTS TO 30KT. AFTER 01Z THREAT OF
CONVECTION -TSRA VICINITY AS NW WINDS BEGIN TO DIMINISH. AFTER 04Z
IMPROVING TO SCT030 AND WINDS DIMINISH TO 10-15KT. AFT 09Z...WINDS
BECOMING LIGHT AND VEERING TO NE WITH THREAT OF LOW-LEVEL WIND
SHEAR DUE TO GUSTY WINDS 30-40KT AT 5000 FEET BY DAWN.

KSFO BRIDGE APPROACH...SAME AS KSFO.

MRY BAY AREA TERMINALS...MULTIPLE LAYERS OF CLOUDS WITH MVFR CIGS
AND VFR VSBYS SCT-BKN020 AND BKN-OVC AT OR ABOVE 3000 FT UNTIL 20Z.
WEAK FRONTAL PASSAGE AROUND 20Z WITH WINDS INCREASING FROM THE
NORTHWEST AND SKY COVER BREAKING TO VFR SCT-BKN020-025 AND BKN OCNL
OVC030-035 TIL 04Z. -SHRA VICINITY 20-04Z WITH SLIGHT THREAT OF -TSRA
VICINITY

&&

.MARINE...AS OF 10:15 AM FRIDAY...MODELS CONTINUE TO SHOW SEAS
INCREASING TO 15-18 FEET LATER TODAY SO A HIGH SURF ADVISORY UPDATED
FOR VALID TIME BY THIS AFTERNOON THROUGH 4 PM SATURDAY. SWELL PERIODS
SHOULD BE IN THE 14-15 SECONDS RANGE SO SOME IMPRESSIVE SWELL SETS
ARRIVING AS HOLIDAY WEEKEND CONTINUING. CARE SHOULD BE TAKEN IF
VENTURING OUT TO THE BEACHES AS UNEXPECTED LARGE SWELL SETS/RIP
CURRENTS COULD PRESENT PROBLEMS ESPECIALLY FOR VISITORS NOT FAMILIAR
WITH WEST COAST SWELL BEHAVIOR. ADDITIONALLY...GALE FORCE WINDS
ESPECIALLY OVER THE OUTER CENTRAL COAST WATERS IN POST-FRONTAL
ENVIRONMENT.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
    .TDA...HIGH SURF ADVISORY ON THE COAST 10 AM THROUGH 4 PM
           SATURDAY.
        ...GALE WARNING...PT ARENA TO PIGEON PT 0-10 NM
                          PT ARENA TO PT PIEDRAS BLANCAS 10-60 NM
        ...SCA...PIGEON PT TO PT PIEDRAS BLANCAS 0-10 NM
                 SFO AND MRY BAYS
        ...SCA FOR ROUGH BAR...SF BAR

$$

PUBLIC FORECAST: REYNOLDS
AVIATION/MARINE: GUDGEL

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO