Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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000
FXUS66 KMTR 281828
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
1128 AM PDT FRI AUG 28 2015

.SYNOPSIS...SLIGHT COOLING IS EXPECTED TODAY AS THE UPPER RIDGE
OVER CALIFORNIA WEAKENS. A PACIFIC WEATHER SYSTEM WILL BRING A
CHANCE OF LIGHT RAIN TO THE NORTHERN PORTION OF OUR DISTRICT LATE
TONIGHT AND SATURDAY MORNING. MUCH COOLER WEATHER WILL DEVELOP BY
THE WEEKEND. DRY AND SEASONABLY COOL WEATHER IS EXPECTED NEXT
WEEK.

&&

.DISCUSSION...AS OF 8:35 AM PDT FRIDAY...WARM START TO THE DAY
TODAY WITH MANY LOCATIONS ALREADY IN THE UPPER 60S TO UPPER 70S
THANKS IN LARGE PART TO A SURFACE FLOW FROM THE NNW. FORECAST
STILL CALLING FOR A RETURN TO MORE ONSHORE FLOW LATER TODAY IN PART
DUE TO AN APPROACHING TROF/COLD FRONT. THIS WILL DROP HIGHS AS
MUCH AS 20 DEGREES OVER THE WEEKEND COMPARED TO VALUES WE HIT
YESTERDAY. LATEST GUIDANCE IN CONTINUES TO SHOW LIGHT RAIN MAINLY
FOR THE NORTH BAY TONIGHT INTO SATURDAY.

CURRENT FORECAST LOOKS TO BE ON TRACK. NO MAJOR CHANGES EXPECTED.

.PREVIOUS DISCUSSION...WARM CONDITIONS PERSIST ACROSS THE
DISTRICT THIS MORNING WITH 2 AM TEMPERATURES AS MANY AS 5 DEGREES
WARMER THAN 24 HOURS AGO...RANGING FROM THE MID 60S TO THE MID
70S. DEW POINT TEMPERATURES ARE RUNNING A LITTLE WARMER AS WELL
AND ARE GENERALLY IN THE 50S. SATELLITE IMAGERY SHOWS NO STRATUS
OR FOG AROUND THE AREA...ONLY SOME HIGH CLOUDS SPREADING ACROSS
THE NORTHERN PART OF THE STATE. SLIGHTLY COOLER WEATHER IS
ANTICIPATED TODAY AS THE UPPER LEVEL RIDGE IS FORECAST TO WEAKEN
AS AN UPPER TROUGH APPROACHES THE WEST COAST. HIGHS TODAY ARE
EXPECTED TO RANGE FROM THE MID 70S AT THE OCEAN TO THE 80S AND 90S
INLAND...WITH WARMEST INLAND AREAS AGAIN TOPPING THE CENTURY MARK.

THE APPROACHING UPPER LEVEL TROUGH...AND ITS ASSOCIATED COLD
FRONT...ARE PROGGED TO SLOWLY APPROACH THE COAST FROM THE WEST.
PRECIPITATION FROM THIS SYSTEM IS FORECAST TO REMAIN WELL NORTH OF
OUR AREA...HOWEVER MODELS DO CONTINUE TO BRING LIGHT RAIN CHANCES
AS FAR SOUTH AS THE SAN FRANCISCO BAY AREA LATE FRIDAY NIGHT
THROUGH SATURDAY MORNING...ALTHOUGH THE LATEST RUN OF THE NAM12
DOES BRING VERY LIGHT PRECIP TO THE MONTEREY BAY. THE SYSTEM IS
FORECAST TO MOVE THROUGH QUICKLY WITH RAIN CHANCES...SUCH AS THEY
ARE...ENDING BY AFTERNOON. COOLER WEATHER WILL BE THE MAIN BENEFIT
FROM THIS SYSTEM FOR OUR AREA AS THE MARINE LAYER DEEPENS AND
ONSHORE FLOW INCREASES. COOLER CONDITIONS WILL PERSIST THROUGH
THE WEEKEND AND RIGHT ON THROUGH NEXT WEEK AS A LONG-WAVE UPPER
TROUGH REMAINS ANCHORED NEAR THE WEST COAST.

&&

.AVIATION...AS OF 11:15 PM PDT FRIDAY.... VFR CIGS WITH BKN TO OVC
HIGH LEVEL CLOUDS MOVING OVER THE AREA AHEAD OF AN APPROACHING
BAGGY TROUGH. MODEL GUIDANCE UNRELIABLE FOR TONIGHT... WITH WIDE
VARIANCES IN DURATION...CIGS...AND ONSET/MIX OUT TIMES... THUS
VERY LOW CONFIDENCE ON CIGS TONIGHT. THE MODEL NUDGING TOWARDS LOW
CIGS/LONG DURATION IS THE NAM...WHICH HAS CONSISTENTLY
UNDERPERFORMED COMPARED TO THE RUC13...WHICH HAS HIGHER CIGS/SHORTER
DURATION. FORT ORD PROFILER SHOWS EXTREMELY COMPRESSED MARINE LAYER
THAT WOULD STRUGGLE TO PUSH INLAND... ONSHORE FLOW WILL WEAKEN INTO
TOMORROW... AND HIGH CLOUDS WILL INHIBIT OVERNIGHT COOLING... THUS
HAVE LEANED CLOSER TO MODELS...SPECIFICALLY THE RUC...WITH HIGHER
OVERNIGHT CIGS WITH SHORT DURATION. THERE IS A POSSIBILITY OF LOW
CIGS ALONG THE COASTLINE... WHICH ARE NOT ACCOUNTED FOR IN THE TAFS.

VICINITY OF KSFO...VFR. LIGHT TO MODERATE WINDS. STRONGEST WIND
EXPECTED DURING THE AFTERNOON. MVFR CIGS POSBL AFTER 06-8Z... THEN
MIXING OUT BY SUNRISE...HOWEVER READ DISCUSSION...VERY LOW
CONFIDENCE FOR TONIGHT.

KSFO BRIDGE APPROACH...SIMILAR TO KSFO.

MONTEREY BAY AREA TERMINALS...VFR TODAY. LIGHT WINDS. POSSIBLE
MVFR TO IFR CIGS AFTER 08-10Z TONIGHT...READ DISCUSSION...VERY
LOW CONFIDENCE.

&&

.MARINE...AS OF 11:10 AM PDT FRIDAY...PREDOMINATELY LIGHT WESTERLY
WINDS AND SEAS WILL CONTINUE ACROSS THE COASTAL WATERS. A LONG
PERIOD SWELL HAS BEGUN TO MOVE INTO THE AREA. A MORE ENERGETIC
WESTERLY SWELL WILL IMPACT THE REGION THIS WEEKEND.

&&

.BEACHES...AS OF 11:30 AM PDT FRIDAY...A LONG PERIOD WEST SWELL
HAS ARRIVED AND WILL CONTINUE TO ADVANCE ON WEST FACING BEACHES
THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK. THESE LONG PERIOD
SWELLS CAN PRODUCE HAZARDOUS RIP CURRENTS THAT CAN QUICKLY PULL
SWIMMERS OUT TO SEA. THE HIGHEST RISK OF HAZARDOUS CONDITIONS WILL
BE THIS WEEKEND WHEN ONSHORE WINDS WEAKEN AND THE PERIOD
INCREASES FROM 15 TO 18 SECONDS. SWIMMERS CAUGHT IN A RIP CURRENT
SHOULD SWIM PARALLEL TO THE COAST TO ESCAPE THE RIP CURRENTS.


&&

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

$$

PUBLIC FORECAST: BELL/SIMS
AVIATION: DRP
MARINE: DRP


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