Small Craft Advisory 279, NWS San Francisco Bay Area, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
900 PM PDT WED JUN 22 2011

.DISCUSSION...AS OF 9:00 PM PDT WEDNESDAY...TEMPERATURES COOLED
ANYWHERE FROM 5 TO ABOUT 20 DEGREES THIS AFTERNOON COMPARED TO
TUESDAY AFTERNOON AS THE MARINE LAYER RETURNED TO THE COAST. THE
MOST DRAMATIC COOLING OCCURRED IN LOCATIONS NEAR THE BAY WHILE THE
FAR INLAND VALLEYS COOLED ABOUT 5-8 DEGREES. FOR EXAMPLE PLACES LIKE
CONCORD AND LIVERMORE THAT TOPPED 100 ON TUESDAY "COOLED" INTO THE
LOWER 90S. FOR THURSDAY THE MORE NOTED COOLING WILL TAKE PLACE
ACROSS THE FAR INLAND VALLEYS WITH TEMPS IN THE 90S BECOMING LESS
COMMON EXCEPT INTERIOR MONTEREY/SAN BENITO COUNTIES.

AS THE SUN IS SETTING THE MARINE LAYER IS NOW SHOWING SIGNS OF
DEEPENING AND UP TO 1500 FOOT AROUND MONTEREY BAY WITH LOW CLOUDS
OOZING INLAND. THE CLOUD DECK HAS PUNCHED THROUGH THE GOLDEN GATE
WITH A FINGER EXTENDING INTO THE BERKELEY HILLS. THE ONSHORE SFO TO
SAC GRADIENT IS A VERY IMPRESSIVE 4.8 MB WHICH WAS PROGGED PROPERLY
YESTERDAY BY THE NAM AND LOCAL WRF MODELS. THE DEEPENING MARINE
LAYER IN RESPONSE TO 500 MB TROUGH OFF THE NORCAL COAST AND
INCREASING ONSHORE GRADIENTS ALL SUGGEST THAT WE SHOULD SEE A
PRETTY IMPRESSIVE MARINE PUSH OVERNIGHT INTO THURSDAY WITH CONTINUED
MODEST COOLING FOR ALL LOCATIONS ALONG WITH LOCALLY BREEZY WINDS
THIS EVENING AND AGAIN THURSDAY AFTERNOON.

THE SYNOPTIC PATTERN BECOMES PRETTY PERSISTENT FROM FRIDAY AND
THROUGH THE WEEKEND FEATURING CYCLONIC FLOW ALOFT. IT OFTEN GETS
HARD TO BEAT A COMBINATION OF CLIMATOLOGY AND PERSISTENCE TYPE
FORECASTING THIS TIME OF YEAR. SO A FORECAST OF WIDESPREAD NIGHT
AND MORNING LOW CLOUDS AND FOG FOLLOWED BY AFTERNOON SUNSHINE LOOKS
PRETTY GOOD FROM FRIDAY AND THROUGH THE WEEKEND WITH TEMPERATURES
AVERAGING NEAR OR SLIGHTLY BELOW NORMAL GIVEN THE CYCLONIC FLOW AND
MODERATE TO STRONG ONSHORE PRESSURE GRADIENTS.

THE LONG RANGE FORECAST HAS ONE INTERESTING FEATURE AND THAT IS AN
ABNORMALLY STRONG TROUGH FORECAST TO REACH FAR NORTHERN CALIFORNIA
AND OREGON BY ABOUT 12Z NEXT WEDS. ACTUALLY THE ECMWF RUN FROM 24
HOURS AGO SHOWED RAIN OVER THE BAY AREA...BUT HAS SINCE BACKED OFF.
LATEST GFS AND ECMWF RUNS SUGGESTING RAIN CHANCES WILL STAY TO OUR
NORTH BUT OF COURSE WILL BE WATCHED. AT THIS TIME THE LIKELY
SCENARIO WILL KEEP THAT LARGE TROUGH WELL TO OUR NORTH HOWEVER
DURING THESE TYPE OF PATTERNS WE NEED TO WATCH TO OUR SW FOR ANY
SUB-TROPICAL MID AND HIGH LEVEL MOISTURE THAT CAN BE ADVECTED INTO
THE MEAN FLOW. THE MODELS DONT EXPLICITLY SHOW THIS BUT THIS IS THE
TYPE OF PATTERN WHEN THIS HAS OCCURRED IN THE PAST.

&&

.AVIATION...AS OF 10:30 PM PDT WEDNESDAY...STRATUS BANKED UP
AGAINST THE COASTAL HILLS BUT SHALLOW MARINE LAYER WILL KEEP IT
FROM SPREADING SFO UNTIL AFTER LATE DESPITE THE STRONG NORTHWEST
GRADIENT.

VICINITY OF KSFO...VFR CONDITIONS THROUGH THE AIRPORT EVENING
RUSH HOUR. EXPECT STRATUS TO SPREAD THROUGH THE GOLDEN GATE TO THE
EAST BAY FIRST BEFORE REACHING THE AIRPORT BY 12Z.

KSFO BRIDGE APPROACH...VFR CONDITIONS WILL PREVAIL THROUGH 06Z.
EXPECT STRATUS TO SPREAD THROUGH THE GOLDEN GATE TO THE EAST
BAY FIRST BEFORE REACHING THE APPROACH ZONE AFTER MIDNIGHT.

MONTEREY BAY AREA TERMINALS...IFR CIGS SPREADING INTO MRY AND SNS
EARLY. LIFR CONDITIONS WITH VISIBILITY 1 MILE OR LESS AND CIGS NR
100 FEET POSSIBLE AFTER 08Z.

&&

.MTR WATCHES/WARNINGS...
    .TNGT...SCA...PT ARENA TO POINT PIEDRAS BLANCAS 0-60 NM

&&

PUBLIC FORECAST: RWW
AVIATION/MARINE: RILEY/W PI