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FXUS66 KMTR 241203
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
403 AM PST TUE NOV 24 2009
.DISCUSSION...AS OF 3:00 AM PST TUESDAY...LARGE SCALE SYNOPTIC
PATTERN SHOWS FLAT RIDGE OVER CALIFORNIA WITH WEAK OFFSHORE FLOW AS
HIGH PRESSURE BUILDS INTO THE GREAT BASIN BEHIND AN EXITING TROUGH
THROUGH THE PLAINS. A LARGE UPPER LOW IS COVERING MOST OF THE GULF
OF ALASKA AND AND WILL BE THE NEXT WEATHER MAKER FOR THE PACNW AND
NORTHERN CA.
SKIES ARE MOSTLY CLEAR WITH JUST A FEW PATCHES OF DENSE FOG IN THE
NORTH BAY...MAINLY NEAR SANTA ROSA. A FEW HIGH CLOUDS ARE
SPILLING OVER THE RIDGE INTO NORTHERN CA AND A FEW OF THESE MAY
PASS QUICKLY OVER THE FAR NORTH THIS MORNING. TEMPERATURES ARE
RUNNING IN SOME LOCATIONS 3 TO 5 DEGREES WARMER THIS EVENING AS THE
OFFSHORE GRADIENT OF CLOSE TO 1 MB FROM SAC TO SFO AND OVER 10 MB
FROM WMC TO SFO IS GENERATING SOME NE WINDS IN THE EAST BAY HILLS.
WINDS OF 15 TO 25 KTS HAVE BEEN REPORTED AT ELEVATIONS ABOVE 1000
FT. WHERE THE WINDS ARE BLOWING TEMPERATURES ARE IN THE LOW 50S
AND WHERE THESE WINDS ARE NOT BLOWING TEMPERATURES ARE IN THE MID 30S
TO LOWER 40S.
TODAY SHOULD BE MOSTLY SUNNY WITH A FEW DEGREES OF WARMING OVER
YESTERDAY. EXPECT HIGHS IN THE MID TO UPPER 60S NORTHERN HALF OF
THE DISTRICT WITH UPPER 60S TO MID 70S SOUTHERN HALF. THIS IS SOME
5 TO 10 DEGREES ABOVE NORMAL. EXPECT THIS CLEAR MILD WEATHER TO
CONTINUE WITH EVEN A FEW MORE DEGREES OF WARMING FOR WEDNESDAY AS
HIGH PRESSURE DOMINATES THE AREA. MAY AGAIN SEE SOME PATCHY GROUND
FOG IN PROTECTED LOW LYING AREAS FOR WED MORNING BUT MOSTLY IN THE
NORTH BAY. OFFSHORE WINDS SHOULD HELP KEEP FOG FROM DEVELOPING FOR
MOST OF THE EAST AND SOUTH BAY AND MONTEREY BAY AREA.
THANKSGIVING DAY WILL START OFF WITH JUST SOME PATCHY DENSE GROUND
FOG IN THE NORTH BAY VALLEYS GIVING WAY TO INCREASING CLOUDS FROM
THE NORTH AND WEST AS A WEAK COLD FRONT APPROACHES. EXPECT JUST A
SLIGHT CHANCE OF RAIN ALONG THE SONOMA COAST BY LATE THURSDAY WITH
CHANCES INCREASING AND SPREADING SOUTH TO THE BAY AREA BY THURSDAY
NIGHT. THIS WILL BE A WEAK SYSTEM AND RAIN AMOUNTS WILL BE LESS
THAN A TENTH OF AN INCH IN THE NORTH A MAYBE A FEW HUNDREDTHS AROUND
THE BAY BY SUNRISE FRIDAY. JUST A FEW LOW CLOUDS WILL DRIFT AS FAR
SOUTH AS MONTEREY BAY FRIDAY MORNING WITH CLEARING SKIES IN THE
NORTH.
EXTENDED...ONCE THE WEAK COLD FRONT PASSES SKIES SHOULD CLEAR BY
LATE FRIDAY MOST AREAS WITH HIGHS NEAR NORMAL. A WARMING TREND
WILL BEGIN FOR THE WEEKEND INTO EARLY NEXT WEEK AS MODELS NOW IN
AGREEMENT THAT A RIDGE OF HIGH PRESSURE WILL DOMINATE THE WEST
COAST THROUGH THE EXTENDED PERIOD. THE LATEST ECMWF DOES SHOW A
SECONDARY TROUGH DIGGING DOWN INTO THE GREAT BASIN ON SATURDAY THAT
WOULD BRING BREEZY NORTH WINDS TO THE AREA SATURDAY WITH JUST
SEASONAL TEMPERATURES WHILE THE GFS WOULD HAVE LESS WINDS AND
MILDER TEMPERATURES FROM SATURDAY AS IT HAS THIS TROUGH FARTHER
EAST. EITHER WAY THE WEEKEND WILL BE SUNNY AND MILD AND THIS DRY
WEATHER PATTERN WILL CONTINUE INTO EARLY NEXT WEEK.
WEEK TWO AND BEYOND...THERE ARE TWO INTERESTING FEATURES THAT MAY
BEGIN TO SHIFT THE LARGE SCALE WEATHER PATTERNS BY EARLY DECEMBER.
A WEAK BUT PROGRESSIVE MJO IS NOW OUT IN THE WESTERN PACIFIC AND IS
EXPECTED TO MAINTAIN SOME STRENGTH AND CONTINUE WEST OVER THE NEXT
TEN DAYS. IN ADDITION TROPICAL STORM NIDA IS EXPECTED TO BECOME A
SUPER TYPHOON AND TURN NORTH OUT NEAR 135 E AND POSSIBLY GET
ENTRAINED INTO THE WESTERLIES CLOSE TO DEC 1. THE MODELS HAVE BEEN
HINTING AT A POSSIBLE BREAK THROUGH JET UNDER WHAT WILL BECOME A
HIGH AMPLITUDE RIDGE OVER THE WEST COAST THE NEXT 5 TO 7 DAYS. THE
GFS HAS BEEN MORE ROBUST WITH THIS BREAKTHROUGH THAN HAS THE
ECMWF. AS WE HAVE A MODERATE EL NINO PATTERN EXISTING THE FORECAST
ISSUE IS WHEN AND IF THIS EL NINO WILL MANIFEST ITSELF WITH A MORE
SOUTHERLY JET INTO CA AND INCREASING WET WEATHER. EARLY DECEMBER
IS ABOUT THE TIME WHEN A MODERATE EL NINO MIGHT BEGIN TO EXERT
ENOUGH FORCING ON THE EAST ASIAN JET TO ALLOW IT TO EXTEND TO THE
WEST COAST AT THE LATITUDE OF CENTRAL CALIFORNIA. THE KICKER MAY
JUST BE WHAT HAPPENS TO TYPHOON NIDA AND IF THE MJO CAN SUSTAIN
ITSELF LONG ENOUGH TO EXTEND THE EAST ASIAN JET UNDER THE HIGH
AMPLITUDE RIDGE. IF THIS WERE TO HAPPEN IT APPEARS THAT THE
EARLIEST THIS WOULD BE THE SECOND WEEK OF DECEMBER. WITH THE
MODELS AND THERE ENSEMBLES CONTINUING TO PUSH THIS BACK CONFIDENCE
IS NOT HIGH. BUT THE MANY FACTORS NOTED ABOVE ARE ALL MAJOR PLAYERS
THAT WOULD LEAD TO A CHANGE TO WET WEATHER BY LATE WEEK TWO OR
EARLY WEEK THREE.
&&
.AVIATION...AS OF 4 AM PST TUESDAY...DRY AIRMASS REMAINS IN PLACE
WITH MOSTLY CLEAR SKIES. ONLY RESTRICTION IS PATCHY FOG IN THE N
BAY NEAR KSTS. THIS FOG WILL LINGER THROUGH MID MORNING.
ALL TERMINALS(EXCEPT KSTS)...VFR CONDITIONS EXPECTED WITH LIGHT
WINDS.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.TDY...NONE.
PUBLIC FORECAST: REYNOLDS
AVIATION/MARINE: MEHLE
NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO
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