000
FXUS66 KMTR 140344
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
844 PM PDT MON MAY 13 2013

.DISCUSSION...AS OF 8:44 PM PDT MONDAY...CURRENT SATELLITE IMAGERY
DEPICTS MOST OF THE LOW CLOUDS THAT HUNG ON THROUGH THE AFTERNOON
HAVE ALL BUT DISSIPATED AHEAD OF A SHORT WAVE THAT IS FORECAST TO
MOVE INLAND OVERNIGHT. THE MAIN FORECAST CONCERN FOR TONIGHT WILL
BE HOW MUCH THE APPROACHING DISTURBANCE WILL AFFECT THE MARINE
LAYER. LATEST OBSERVATIONS SHOW THE MARINE LAYER CONTINUES TO
DEEPEN...HOWEVER THE QUESTION REMAINS HOW DEEP WILL THE MARINE
LAYER GET...OR WILL IT MIX OUT COMPLETELY AS THE SHORTWAVE TROUGH
MOVES ACROSS THE REGION? CURRENT THINKING IS THAT THERE WILL BE
FAR FEWER LOW CLOUDS TUESDAY MORNING THAN THERE WERE THIS MORNING
THEREFORE AN EVENING UPDATE WAS JUST ISSUED TO SCALE BACK ON THE
EXTENT OF THE LOW CLOUDS AND PATCHY FOG THROUGH THE OVERNIGHT AND
EARLY MORNING HOURS.

A QUICK LOOK AT THE GRADIENTS ACROSS THE REGION SHOWS A SIGNIFICANT
INCREASE IN THE NORTHERLY GRADIENT FROM ACV TO SFO WITH A ONLY A
SLIGHT CHANGE IN THE ONSHORE GRADIENT FROM SFO TO WMC. THIS CHANGE
HOWEVER HAS RESULTED IN STRONGER WINDS ACROSS THE REGION WITH PEAK
SUSTAINED WINDS AROUND 15 MPH WITH GUST TO AROUND 20 MPH IN MOST
LOCATIONS. A FEW LOCATIONS REPORTED SUSTAINED WINDS AROUND 25 MPH
WITH GUSTS TO OVER 30 MPH. THE INCREASE IN THE WINDS AND THE
DEEPENING OF THE MARINE LAYER DEFINITELY CONTRIBUTED TO THE COOLER
TEMPERATURES FELT ACROSS THE DISTRICT TODAY. MOST LOCATIONS
EXPERIENCED A FEW DEGREES OF COOLING BY THIS AFTERNOON WITH
COASTAL SITES ONLY REACHING IN TO THE LOWER 60S TO MID 70S WITH
MID 70S TO LOWER 80S INLAND. WARMEST INLAND AREAS REACHED INTO THE
MID 90S TO NEAR 100.

PREVIOUS DISCUSSION...FOR THE REST OF THE WEEK...500 MB HEIGHTS
WILL SLOWLY LOWER AS THE FLOW SWITCHES FROM ZONAL TO A LONGWAVE
TROF THAT WILL BE NEAR THE WEST COAST ON THURSDAY. THE FLOW AT THE
SURFACE WILL CONTINUE TO BE MODERATE FROM THE WEST WHILE THE
MARINE LAYER DEEPENS EVEN MORE. BY THURSDAY THE RANGE OF HIGHS
WILL BE MINIMAL -- JUST LOWER 60S AT THE COOLEST ON THE COAST TO
MID 70S FOR FAR INLAND SPOTS.

QUIET WEATHER IS EXPECTED INTO THE FIRST HALF OF NEXT WEEK AS THE
FLOW ALOFT REMAINS ZONAL WITH LITTLE DIFFERENCE IN 500 MB HEIGHTS
FROM DAY TO DAY. 6-10 DAY FORECAST CALLS FOR NEAR NORMAL TEMPERATURES
FOR MOST OF OUR AREA.

&&

.AVIATION...AS OF 5:20 PM PDT MONDAY...VISIBLE IMAGERY THIS
AFTERNOON SHOWS STRATUS CLOUDS DISSIPATING/MIXING OUT NORTH OF THE
GOLDEN GATE AS THE ACV-SFO GRADIENT CONTINUES TO RAMP UP PRESENTLY
UP TO 7 MB. ALTHOUGH THE SFO-SAC IS 3.5 MB AND THE HAF-SFO IS STRONG
AT 2 MB AND THE MARINE LAYER IS ROUGHLY 1100-1500 FEET DEEP...THE
STRONG ACV-SFO GRADIENT AND MIXING OUT OF STRATUS ON THE COAST
SHOULD BE ENOUGH TO HELP PREVENT AN EARLY STRATUS INTRUSION THIS
EVENING AT KSFO. WHAT COMPLICATES THINGS LATER IN THE EVENING FOR
THE ENTIRE AREA IS THE ARRIVAL OF A STRONG BUT DRY UPPER LEVEL
LOW. IF THE MARINE LAYER CLOUDS RETURN TO THE IMMEDIATE COAST PRIOR
TO ITS ARRIVAL THEN THIS WILL UNDOUBTEDLY PUSH STRATUS OR HIGHER
BASED LOW CLOUDS INLAND WITH EASE. IF THE STRATUS DOES NOT REDEVELOP
SUFFICIENTLY PRIOR TO ITS ARRIVAL THEN THERE MAY NOT REALLY BE ANY
SIGNIFICANT WIDESPREAD CLOUD COVERAGE TO SPEAK OF.

VICINITY OF KSFO...MARINE LAYER DEPTH...ONSHORE GRADIENT STRENGTH
FROM SFO-SAC AND HAF-SFO SUPPORTS WEST WIND GUSTS TO 30 KNOTS TIL
04Z. INHERITED FORECAST LOOKS OK JUST MINOR TWEAKS MADE TO DURATION
OF CLOUD COVER TONIGHT. MUCH APPRECIATE CWSU INPUT AND AGREE IT`S
A DIFFICULT FORECAST TONIGHT. FORECASTER MAKES GOOD POINT THAT WINDS
JUST ABOVE BOUNDARY LAYER MAY BECOME A BIT MORE NORTHERLY (DRY) VERY
LATE WHICH SHOULD HELP LIMIT CLOUD DURATION AT KSFO TUE MORNING.
MODEL OUTPUT DEPICTS A STRONG TROUGH/VORT CENTER PASSING OVER THE
AREA WHICH IMPLIES MORE MIXING AND POSSIBLE DISRUPTION OF THE MARINE
LAYER INVERSION AND STRATUS CLOUDS.

KSFO BRIDGE APPROACH...HIGH CONFIDENCE VFR CONTINUES THIS EVENING
BUT CONFIDENCE FALLS QUICKLY OVERNIGHT BASED ON ABOVE DISCUSSION.
THE MARINE LAYER SHOULD LIFT AND WEAKEN ENOUGH TO ALLOW LOW CLOUDS
IN THE APPROACH LATE TONIGHT/TUE MORNING. CLEARING SHOULD BE FAIRLY
STEADY TUE MORNING. VFR FORECAST MID-LATE TUE MORNING AND AFTERNOON.

MONTEREY BAY AREA TERMINALS...STRATUS PILING UP ON THE COAST THIS
AFTERNOON WILL LEAD TO IFR THIS EVE. DRIER NORTHERLY FLOW COULD
TEMPORARILY DISRUPT MARINE LAYER ENOUGH TO ALLOW BRIEF CLEARING
LATE TONIGHT...BUT PLENTY OF HUMIDITY WITHIN THE MARINE LAYER
FAVORS A REDEVELOPMENT OF STRATUS LATE TONIGHT. LOW CONFIDENCE.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TNGT...SCA...SF BAY UNTIL 9 PM
             SCA...PIGEON PT TO PT PINOS 0-10 NM UNTIL 9 PM
             SCA...PT ARENA TO PT REYES 0-10 NM
             SCA...PT REYES TO PIGEON PT 0-10 NM
             SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM
             SCA...PT ARENA TO PIGEON PT 10-60 NM
             SCA...PIGEON PT TO PT PIEDRAS BLANCAS 10-60 NM

&&

$$

PUBLIC FORECAST: CW
AVIATION/MARINE: CANEPA

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