FXUS66 KMTR 260609
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
1000 PM PST FRI DEC 25 2009

UPDATED FOR EVENING AVIATION DISCUSSION

.DISCUSSION...AS OF 8:15 PM PST FRIDAY...SKIES ARE MOSTLY CLEAR
ACROSS THE DISTRICT...ASIDE FROM SOME HIGH CLOUDINESS BEGINNING TO
MOVE IN FROM THE WEST. THESE CLOUDS ARE THE HARBINGER OF AN
APPROACHING PACIFIC FRONTAL RAIN-BAND. THIS SYSTEM IS WEAKENING AS IT
NEARS THE COAST...BUT IS NONETHELESS EXPECTED TO SPREAD GENERALLY
LIGHT RAIN EASTWARD ACROSS MUCH OF OUR CWA BEGINNING IN THE SATURDAY
AFTERNOON TO SATURDAY NIGHT TIME FRAME. A CHANCE OF SHOWERS WILL THEN
CONTINUE SUNDAY THROUGH MONDAY...WITH DRY CONDITIONS RETURNING
DISTRICT-WIDE BY MONDAY EVENING.

THE NUMERICAL MODELS HAVE BEEN STRUGGLING WITH THE DETAILS OF THIS
SYSTEM...WITH SIGNIFICANT RUN-TO-RUN AND BETWEEN MODEL VARIATIONS IN
SOLUTION. THERE IS BETTER AGREEMENT ON THE BASIC IDEA HOWEVER...WHICH
IS THAT THE APPROACHING FRONT WILL CONTINUE TO WEAKEN AS IT MOVES
INTO THE COAST...AND THEN DISSIPATE AS IT PROGRESSES INLAND. THE
ASSOCIATED UPPER LEVEL LOW BEHIND IT WILL ALSO WEAKEN AS IT
APPROACHES THE COAST NEAR THE CALIFORNIA/OREGON BORDER...THOUGH
IT WILL LEAVE RESIDUAL WEAK UPPER LEVEL TROUGHING ALONG THE COAST IN
ITS WAKE. AT THE SAME TIME...A NEW UPPER LEVEL DISTURBANCE FARTHER TO
THE SOUTHWEST WILL DEVELOP AND STRENGTHEN WHILE RAPIDLY MOVING
SOUTHEASTWARD TOWARDS BAJA CALIFORNIA...AND THUS SHIFT THE FOCUS OF
THE SOUTHERN BRANCH OF THE UPPER LEVEL FLOW WELL SOUTH OF OUR AREA
BY MONDAY AFTERNOON.

THAT SAID...MODEL OUTPUT PRECIP TIMING...AREAL COVERAGE AND AMOUNT
VARY WIDELY. OF THE FOLLOWING 5 OPERATIONAL MODEL SOLUTIONS...THE
NEW 00Z NAM...18Z GFS...12Z ECMWF...12Z GEM (CANADIAN)...AND 12Z
UKMET...THE NAM IS THE DRIEST AND HAS THE EARLIEST END TO THE
PRECIP...AND THE ECMWF IS THE WETTEST. BEST AGREEMENT BETWEEN THE
MODELS IS FOR THE SATURDAY EVENING TIME FRAME...WHEN ALL MODELS BRING
THE INITIAL FRONTAL BAND THROUGH MUCH OF THE DISTRICT WITH
ACCOMPANYING RAINFALL. AM THEREFORE GOING TO UPDATE THE FORECAST TO
RAISE POPS INTO THE LIKELY RANGE FOR THE SATURDAY NIGHT PERIOD
ACROSS ALL BUT WELL INLAND AND SOUTHERNMOST PORTIONS OF THE CWA.
WILL MAINTAIN CURRENT CHANCE POPS FOR SATURDAY AFTERNOON BUT BUMP
THEM UP A BIT AS ESPECIALLY THE NEW NAM SOLUTION IS TRENDING
EARLIER WITH THE PRECIP START...AND ALSO STAY WITH CHANCE POPS FOR
SUNDAY GIVEN MUCH WIDER RANGES THEN IN MODEL OUTPUT. MODEL CONSENSUS
IN CONSIDERATION OF SMALLER SCALE TOPOGRAPHIC MODULATION WOULD
SUGGEST RAINFALL TOTALS THROUGH MONDAY MORNING RANGING FROM AROUND A
FEW HUNDREDTHS OF AN INCH IN THE SOUTHERN INTERIOR TO AS MUCH AS 3/4
OF AN INCH IN THE WETTEST SPOTS IN THE COASTAL HILLS. TOTALS IN THE
LOWLANDS OF THE SF AND MONTEREY BAY REGIONS PRESENTLY APPEAR LIKELY
TO BE AROUND 1/4 OF AN INCH.

BY MONDAY AFTERNOON...IT APPEARS WE'LL BE IN THE MIDDLE OF A NORTH-
SOUTH SPLIT IN THE LARGE SCALE UPPER LEVEL FLOW...WITH ONE BRANCH
WELL TO OUR NORTH AND THE OTHER WELL TO THE SOUTH...AND A COOL AIR
MASS OVER US BUT WITH VERY LIGHT FLOW ALOFT AND THUS MINIMAL DYNAMIC
FORCING. THE NEXT CHANCE FOR RAIN PRESENTLY APPEARS TO BE IN
THE TUESDAY NIGHT TO WEDNESDAY TIME FRAME...WITH LONGER RANGE MODELS
IN GOOD AGREEMENT IN THEIR PROJECTIONS THAT ANOTHER WEAK PACIFIC
DISTURBANCE WILL THEN MOVE IN FROM THE NORTHWEST. FOR THE LATTER PART
OF NEXT WEEK...BOTH THESE LONGER-RANGE OPERATIONAL MODEL SOLUTIONS
AND THE GFS ENSEMBLE MEMBERS SHOW GOOD AGREEMENT IN KEEPING PRIMARY
UPPER LEVEL FLOW AND PRECIP FOCUSED NORTH OF OUR DISTRICT...THOUGH
WITH RAIN CHANCES CONTINUING IN OUR MORE NORTHERN ZONES.

&&

.AVIATION...AS OF 10:00 PM FRIDAY...CHRISTMAS EVENING...HIGH
PRESSURE ALOFT AND AT THE SURFACE SLIPPING EASTWARD FROM OVER THE
GREAT BASIN AHEAD OF A COLDFRONT APPROACHING THE COAST TONIGHT.
LIGHT AND VARIABLE WINDS WILL BECOME SOUTHEAST AND INCREASE AFTER
14Z OVER THE FORECAST AREA AS THE FRONT APPROACHES. VFR CONDITIONS
BELOW OVERCAST AT OR ABOVE 15 THSD FT MSL TO PREVAIL WITH EXCEPTION
OF PATCHY LATE NIGHT/EARLY MORNING FOG VICINITY OF KSTS. NORTH BAY
VALLEYS AND S.F.BAY AREA TERMINALS CAN EXPECT DEVELOPING -SHRA BY 20Z
SATURDAY DUE TO THE APPROACHING COLD-FRONT. CEILINGS TO LOWER TO MVFR
BY 21Z SATURDAY AFTERNOON WITH MVFR VISIBILITIES IN -RA BY 00Z.

VICINITY OF KSFO...VFR CONDITIONS BELOW OVERCAST AT OR ABOVE 15 THSD
FT MSL THROUGH 14Z SATURDAY MORNING. THEN CEILING LOWERING TO MID-
LEVEL BKN100-130 BY 17Z AND LOWERING BKN-OVC050 BY 19Z. BECOMING
MVFR CEILING SCT-BKN020-030 WITH OCCASIONAL MVFR VISIBILITY IN -SHRA
BY 21Z. GENERALLY WINDS SOUTHEAST 5 KTS THROUGH 14Z THEN INCREASING
TO 5-10KT ON THROUGH SATURDAY.

KSFO BRIDGE APPROACH...SAME AS KSFO.

MRY BAY AREA TERMINALS...VFR BELOW HIGH CLOUDS THROUGH 14Z THEN
CEILING LOWERING TO BKN100-120 BY 20Z SATURDAY. VCSH BY 22Z WITH
CEILINGS LOWERING TO OCCASIONAL MVFR OVC040-050. MVFR CEILINGS
BY 00Z IN -SHRA. WINDS LIGHT SOUTHEAST 3-8KTS OVERNIGHT. INCREASING
SOUTHEAST WINDS 7-12KTS AFTER 14Z SATURDAY MORNING AND THROUGH DAY
WITH THE APPROACHING COLD-FRONT.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
   .TNGT...NONE.

$$

PUBLIC FORECAST: MHS/BLIER
AVIATION/MARINE: GUDGEL

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO