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

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000
FXUS66 KMTR 171806
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
1106 AM PDT WED SEP 17 2014

.SYNOPSIS...A COOLING TREND IS FORECAST THROUGH TOMORROW AS AN
UPPER LEVEL LOW PRESSURE TROUGH OVER THE EASTERN PACIFIC
APPROACHES THE CALIFORNIA COAST. SHOWERS AND ISOLATED
THUNDERSTORMS WILL BE POSSIBLE THIS EVENING THROUGH THURSDAY
AFTERNOON AS THE SYSTEM GRADUALLY MOVES INLAND.

&&

.DISCUSSION...AS OF 09:15 AM PDT WEDNESDAY...THE CURRENT FORECAST
APPEARS TO BE ON TRACK THIS MORNING WITH COASTAL CLOUDS EXPECTED
TO BURN-OFF OVER INLAND AREAS BY LATE MORNING. CLOUD COVER WILL BE
ON THE INCREASE FROM THE NORTHWEST AS THE MID/UPPER LEVEL TROUGH
APPROACHES. CHANCES FOR PRECIPITATION WILL INCREASE OVER THE NORTH
BAY BY THIS EVENING AND WILL SPREAD SOUTHWARD TONIGHT THROUGH
THURSDAY MORNING. NO UPDATES ARE NEEDED AT THIS TIME AND WILL BE
HONING IN ON THE DETAILS AS THE 12Z MODEL DATA COMES IN LATER THIS
MORNING.

&&

.PREV DISCUSSION...AS OF 3:00 AM PDT WEDNESDAY...EVERYTHING STILL
APPEARS ON TRACK FOR POSSIBLE RAIN FOR TONIGHT INTO THURSDAY AS A
STORM SYSTEM CURRENTLY AROUND 132W CONTINUES TO TRACK TOWARD
PACNW/NORCAL REGION. BEFORE ANY RAIN ARRIVES, WE SHOULD SEE
GENERALLY SUNNY CONDITIONS WITH A FEW PATCHES OF FOG MOSTLY
CONFINED TO THE COAST DURING THE MORNING HOURS. AS THE DAY
PROGRESSES, HIGH LEVEL CLOUDS WILL BEGIN TO FILTER IN FIRST
ACROSS THE NORTH BAY AND THEN MORE TO THE SOUTH AS THE EVENING
APPROACHES. HIGHS SHOULD BE COOLER TODAY IN ALMOST ALL LOCATIONS
AS THE SYSTEM/LONGWAVE TROF APPROACH. LOOK FOR MOSTLY 60S AND 70S
AT THE COAST WITH 70S AND 80S FOR INLAND SPOTS. IN SOME AREAS IT
WILL BE A DROP OF UP TO 15 DEGREES COMPARED TO TUESDAY.

ATTENTION WILL THEN SWITCH TO RAIN CHANCES AS THE SYSTEM AND
ASSOCIATED COLD FRONT NEAR OUR CWA. LATEST MODEL GUIDANCE IS IN
FAIRLY GOOD AGREEMENT AND BRINGS THE FIRST CHANCE OF RAIN TO THE
NORTH BAY STARTING IN THE EVENING HOURS AND THEN DOWN TO AS FAR
AS THE MONTEREY/SLO BORDER DURING THE OVERNIGHT HOURS. OVERALL
GUIDANCE HAS CHANGED LITTLE FROM 24 HOURS AGO SO STILL APPEARS
THAT THIS "EVENT" WILL FEATURE VERY LIGHT RAINFALL TOTALS --
MOSTLY UNDER 1/10" -- FOR MOST OF THE CWA. IF WE DO GET MORE THAN
A TENTH ANYWHERE, THE FAVORED SPOTS WOULD BE THE USUAL SPOTS SUCH
AS THE NORTHERN COASTAL RANGES. LOOKING BACK AT THE RECORDS, IT`S
BEEN NEARLY 45 DAYS SINCE SAN FRANCISCO HAD MEASURABLE RAINFALL
AND AROUND 145 SINCE MORE THAN AN TENTH WAS RECORDED SO EVEN A
SMALL AMOUNT WILL BE WELCOME. BIGGEST HAZARD IS EXPECTED TO BE
VERY SLICK ROADS DUE TO OILS THAT HAVE BUILT UP ON THEM.

INSTABILITY WILL BE ON THE INCREASE DURING THE OVERNIGHT HOURS AS
THE UPPER LEVEL LOWS GETS INTO NORTHERN CALIFORNIA. MOSGUIDE
THUNDERSTORM NUMBERS SHOW UP TO A 25% CHANCE OF THUNDER OVER THE
NORTH BAY FOR THE OVERNIGHT AND MORNING HOURS WHICH SEEMS
REASONABLE GIVEN THE PROXIMITY OF THE LOW. THUNDERSTORM COVERAGE
WAS SLIGHTLY EXPANDED FOR THE FORECAST ALTHOUGH IT WAS LEFT OUT
OF ALL AREAS SOUTH OF THE NORTH BAY.

RAIN CHANCES ARE EXPECTED COME TO AN END BY THURSDAY EVENING AS
THE UPPER LEVEL LOW DIVES TO THE SOUTHEAST NEAR OUR SOUTHERN
COASTAL WATERS. THIS SHOULD END UP BEING THE COOLEST DAY OF THE
WEEK.

TEMPS WILL START TO WARM BACK UP FOR FRIDAY INTO NEXT WEEK AS A
RIDGE OF HIGH PRESSURE BUILDS BACK ACROSS OUR REGION WHILE 850 MB
TEMPS INCREASE. BY SUNDAY MOST LOCATIONS WILL BE NEAR NORMAL TO A
BIT ABOVE NORMAL WITH PLENTY OF SUNSHINE.

LONGER RANGE GUIDANCE IS NOW IN LESS AGREEMENT WITH SOME OF THE
MODELS BUILDING A VERY STRONG RIDGE OF HIGH PRESSURE ACROSS OUR
AREA ALONG WITH OFFSHORE WINDS AND VERY WARM 850 MB TEMPS. OTHER
GUIDANCE (INCLUDING THE OPERATIONAL GEM AND ECMWF SOLUTIONS)
OFFER A DIFFERENT SOLUTION AND HAVE MILDER CONDITIONS WITH A
LONGWAVE TROF OFF THE COAST AND ANY RIDGING MUCH LESS PRONOUNCED.
FOR NOW WILL JUST ADVERTISE WARM BUT NOT HOT CONDITIONS FOR THE
SECOND HALF OF NEXT WEEK.

&&

.AVIATION...AS OF 10:30 AM PDT WEDNESDAY...LOW CLOUDS HAVE CLEARED
FROM THE TERMINALS BUT CONTINUE ALONG THE COAST AND MRY BAY.
THE APPROACHING UPPER TROUGH WILL BRING DEEPER LOW LEVEL MOISTURE
TO THE SFO BAY AREA TONIGHT. MODELS BRING LOWER CIGS INTO SFO
AFTER 06Z WITH POSSIBLE LIGHT RAIN AFTER 12Z. WINDS WILL SWITCH TO
THE SOUTH TO SOUTHEAST LATE TONIGHT BUT REMAIN 10 KT OR LESS.

VICINITY OF KSFO...VFR THROUGH EARLY EVENING. MVFR CIGS AFTER 06Z
WITH POSSIBLE LIGHT RAIN. SOUTHWEST WINDS THROUGH THE DAY UNDER
15 KT. WINDS BECOMING SOUTH TO SOUTHEAST AFTER 07Z.

KSFO BRIDGE APPROACH...SIMILAR TO KSFO.

MONTEREY BAY AREA TERMINALS...BECOMING VFR. LOW CLOUDS ARE
EXPECTED TO LINGER OVER MRY BAY SO THERE WILL BE AN EARLY RETURN
INTO MRY AND SNS. IFR CIGS EXPECTED AT MRY AFTER 02Z AND SNS AFTER 03Z.

&&

.MARINE...AS OF 9:15 AM PDT WEDNESDAY...AS OF 10 AM BUOY REPORTS
INDICATE 3-FOOT SOUTHWEST SWELLS CONTINUING WITH 18 SECOND PERIODS.
SOUTHWEST SWELLS WILL INCREASE SLIGHTLY AND SHIFT MORE WEATERLY
AS THE OFFSHORE UPPER TROUGH GENERATES ADDITIONAL WAVES. THE WAVES
WILL BE A MIX OF 18 SECOND PERIOD AND 14-15 SECOND PERIOD WAVES
THROUGH TONIGHT. THEREFORE THE BEACH HAZARDS STATEMENT WILL BE
EXTENDED THROUGH EARLY THURSDAY MORNING. SHORTER PERIOD WAVES WILL
BECOME DOMINANT ON THURSDAY.

&&

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

&&

$$

PUBLIC FORECAST: RGASS
AVIATION/MARINE: W PI

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