000
FXUS66 KMTR 201201
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
501 AM PDT MON MAY 20 2013

.DISCUSSION...AS OF 3:30 AM PDT MONDAY...CLEAR SKIES ACROSS THE
ENTIRE AREA THIS MORNING. WINDS ARE GENERALLY LIGHT EXCEPT FOR
MODERATE NORTHERLY WINDS OVER THE MAYACAMAS MOUNTAINS IN THE NORTH
BAY AND NORTHERN DIABLO RANGE IN THE EAST. THESE WINDS HAVE PICKED
UP CONSIDERABLY SINCE MIDNIGHT WITH HAWKEYE RAWS SHOWING GUSTS
UPWARDS OF 38 MPH IN THE LAST HOUR. THESE WINDS WILL DIMINISH
FAIRLY QUICKLY LATER THIS MORNING.

ANOTHER DAY OF WARM TEMPERATURES IS IN STORE TODAY WITH MOST AFTERNOON
HIGHS WARMING ANOTHER 5 TO 10 DEGREES OVER YESTERDAY. HIGHS AROUND SF BAY
WILL BE IN THE UPPER 70S TO MID 80S...AND UP TO THE MID 90S
INLAND. WITH LIGHT OFFSHORE FLOW EVEN THE COAST WILL SEE
TEMPERATURES INTO THE LOW 70S BEFORE A SEABREEZE DEVELOPS IN THE
AFTERNOON. INCREASED ONSHORE FLOW LATE IN THE DAY LOOKS TO
DEVELOP A MARINE LAYER OPENING THE POSSIBILITY OF LOW CLOUDS
TONIGHT.

AN UPPER LOW ORIGINATING FROM THE SUB ARCTIC BRINGS A MUCH COOLER
AIRMASS INTO THE REGION TOMORROW. 850 MB TEMPERATURES PLUNGE FROM
NEAR 20 C TODAY TO AROUND 2 C LATE TUESDAY WHILE THICKNESS VALUES
PLUMMET FROM 570 DM TO 550 DM OVER THE SAME TIME SPAN. THIS WILL
RESULT IN SFC HIGH TEMPERATURES COOLING BY AROUND 15 DEGREES OVER
MONDAY. AS THE COOL AIR MOVES NORTH TO SOUTH ONLY THE SOUTHERN
REACHES OF THE CWA WILL MANAGE TO REMAIN WARM WHERE THE SOUTHERNMOST
END OF THE SALINAS VALLEY COULD STILL REACH 90.

ONCE THE DRY COLD FRONT MOVES THROUGH DURING THE DAY TUESDAY NORTHWEST
WINDS PICK UP DRAMATICALLY. THE STRONGEST WINDS WILL BE NEAR THE
COAST TUESDAY AFTERNOON INTO EVENING. LATEST NAM AND GFS HAVE
BACKED OFF SLIGHTLY ON THE WIND SPEED HOWEVER THE TAIL END OF THE
LOCAL HIGH RES WRF SUGGESTS STRONG WINDS PUSHING INTO SF BAY.
WINDS DECREASE ALONG THE COAST OVERNIGHT BUT REMAIN GUSTY OVER THE
HILLS INTO EARLY WEDNESDAY MORNING AND REMAIN MODERATE OVER THE
WATERS FOR MUCH OF THE REST OF THE WEEK.

TEMPERATURES REMAIN BELOW NORMAL THROUGH THE WORK WEEK AS THE
TROUGH SETTLES IN AND ONSHORE FLOW REMAINS AT THE COAST. ALL
PRECIP FROM THIS SYSTEM IS SLATED TO STAY NORTH OF THE AREA. THE
DEEP UPPER LOW LIFTS NORTH BY THE START OF THE WEEKEND HOWEVER
MODELS CONTINUE TO SHOW A WEAK TROUGH REMAINING. THIS WILL LIKELY
KEEP TEMPERATURES COOL THROUGH AT LEAST THE FIRST HALF OF THE
WEEKEND. THE GEFS SHOWS INCREASING UNCERTAINTY BY SUNDAY WITH
HALF OF THE ENSEMBLE MEMBERS TRENDING TOWARD ZONAL FLOW. ECMWF
DETERMINISTIC AND ENSEMBLE MEANS KEEP SOME TROUGHING INTACT
INTO THE START OF NEXT WEEK.

&&

.AVIATION...AS OF 4:59 AM PDT MONDAY...THE ACV-SFO GRADIENT IS
MODERATELY STRONG AT 6 MB AND IS FORECAST TO HOLD BETWEEN 4-6 MB
THROUGH EARLY TUE MORNING. THE WMC-SFO GRADIENT HAS INCREASED TO 7
MB OVERNIGHT BUT IS FORECAST TO DIMINISH TO 1-3 MB BY THIS
AFTERNOON. THE UKI-STS GRADIENT IS STRONG AT 2.7 MB WITH LITTLE
CHANGE IN STRENGTH EXPECTED THROUGH TONIGHT.

MODERATE TO HIGH CONFIDENCE VFR PREVAILS THROUGH EARLY TUE MORNING.
N-NW WINDS OVER THE COASTAL WATERS TUESDAY WILL HELP DRIVE IN SOME
STRATUS PROBABLY RIGHT TO THE IMMEDIATE COASTLINE. LOW CONFIDENCE
AS TO WHETHER OR NOT STRATUS WILL BE USHERED INLAND THOUGH AM
LEANING TOWARD A YES VOTE ON THAT ONE MOSTLY DUE TO A STRONG 5 MB
SFO-SAC GRADIENT PREDICTED TUE AFTERNOON PER THE LATEST NAM MODEL.
APPRECIABLE LOW-MID LEVEL COOLING (~ 80 METERS COOLING AT 500 MB
HEIGHT LEVEL FROM 21Z TODAY TO 21Z TUESDAY) WILL LIKELY RAISE
MARINE LAYER INVERSIONS OPENING THE WINDOW FOR A STRATUS INTRUSION.

VICINITY OF KSFO...VFR. SFO-SAC GRADIENT SHOULD REMAIN RATHER SUBDUED
TODAY DUE TO LARGER SCALE OFFSHORE GRADIENT WMC-SFO AT 7 MB. THE WMC-SFO
GRADIENT GRADUALLY TRENDS WEAKER THIS AFTERNOON. BY 23Z WEST WINDS
SHOULD BE A LITTLE STRONGER BUT WIND GUSTS SHOULD HOLD JUST BELOW
20 KNOTS.

KSFO BRIDGE APPROACH...VFR.

MONTEREY BAY AREA TERMINALS...NOCTURNAL RADIATIONAL COOLING PROCESSES
ARE NARROWING TEMPERATURE-DEWPOINT DEPRESSIONS BUT SO FAR SKIES ARE
REMAINING CLEAR. THESE SITUATIONS CAN REALLY GO EITHER WAY I.E.
GOING EITHER BRIEF CLOUDS OR STEADY CLEAR SKIES THROUGH DAYBREAK.
DIFFICULT TO SAY BUT WILL LEAN TOWARD CLEAR SKIES HOLDING ITS OWN
THROUGH DAYBREAK.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TDAY...SCA...PT ARENA TO PT REYES 0-10 NM
             SCA...PT ARENA TO PIGEON PT 10-60 NM

&&

$$

PUBLIC FORECAST: AC
AVIATION/MARINE: CANEPA

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