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

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000
FXUS66 KMTR 120417
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
917 PM PDT FRI JUL 11 2014

.SYNOPSIS...A WARMING TREND IS EXPECTED ACROSS INLAND AREAS
OVER THE WEEKEND AND INTO EARLY NEXT WEEK AS HIGH PRESSURE BUILDS
OVER CENTRAL CALIFORNIA. NEAR-COASTAL SITES WILL CONTINUE TO
EXPERIENCE ONSHORE FLOW AND NEAR-NORMAL CONDITIONS.

&&

.DISCUSSION...AS OF 9:15 PM PDT FRIDAY...VISIBLE SATELLITE IMAGERY
THIS EVENING SHOWS A LOW LEVEL CYCLONIC CIRCULATION OFF THE
MENDOCINO COUNTY COAST ASSOCIATED WITH A WEAK SURFACE LOW.
SOUTHERLY FLOW TO THE EAST AND SOUTH OF THIS LOW CENTER LIFTED A
DEEP MARINE LAYER UP AND OVER THE COASTAL TERRAIN AND PRODUCED
AREAS OF DRIZZLE OVERNIGHT...MAINLY ALONG SOUTH-FACING COASTAL
SLOPES. MEASURABLE RAINFALL OCCURRED LAST NIGHT ACROSS THE NORTHERN
PERIPHERY OF MONTEREY BAY...AT HALF MOON MAY...AND IN CARMEL. MOST
LOCATIONS PICKED UP AROUND 0.03 TO 0.04.

THE MARINE LAYER DEEPENED SUFFICIENTLY TO MIX OUT ACROSS MUCH OF
OUR AREA LATE IN THE DAY...ESPECIALLY SOUTH OF SAN MATEO COUNTY.
EXPECT LOW CLOUDS TO ONCE AGAIN BECOME WIDESPREAD OVERNIGHT AS THE
MARINE INVERSION BECOMES BETTER ESTABLISHED AND ONSHORE FLOW
PERSISTS. THERE SHOULD BE LESS IN THE WAY OF DRIZZLE OVERNIGHT AS
THE SURFACE LOW LIFTS TO THE NORTH AND LOW LEVEL SOUTHERLY FLOW
WEAKENS...RESULTING IN LESS LOW LEVEL CONVERGENCE AT THE COAST.

THE WARMING TREND FOR INLAND AREAS OVER THE WEEKEND STILL LOOKS
ON TRACK AS THE MODELS CONTINUE TO FORECAST THE UPPER RIDGE OVER
THE SOUTHWESTERN CONUS TO BUILD TO THE WEST AND ACROSS CALIFORNIA.
LOOK FOR THE DEPTH OF THE MARINE LAYER TO DECREASE SIGNIFICANTLY OVER
THE WEEKEND WHILE 850 MB TEMPERATURES WARM ABOUT 5 TO 6 DEGREES C.
INLAND TEMPERATURES...WHICH HAVE BEEN AS MUCH AS TEN DEGREES F
BELOW NORMAL THE PAST FEW DAYS...WILL WARM BACK TO NEAR NORMAL ON
SATURDAY AND THEN AT LEAST A FEW DEGREES ABOVE NORMAL ON SUNDAY. A
SHALLOW...YET PERSISTENT...MARINE LAYER...ALONG WITH CONTINUED
ONSHORE FLOW...WILL MEAN MUCH LESS WARMING FOR COASTAL AREAS THIS
WEEKEND.

ONE RECENT MODEL DEVELOPMENT THAT BEARS WATCHING LATE IN THE
WEEKEND AND INTO THE EARLY PART OF NEXT WEEK IS THE INCREASE IN
MID/UPPER LEVEL MOISTURE AND INSTABILITY. ONCE THE UPPER RIDGE
EXPANDS WESTWARD INTO CA...IT IS FORECAST TO LIFT A BIT TO THE
NORTH...RESULTING IN AN INFLUX OF MID/UPPER LEVEL MONSOON MOISTURE
ACROSS SOUTHERN CA BY LATE SUNDAY AS THE FLOW ALOFT BECOMES
SOUTHEAST. THAT MOISTURE IS THEN FORECAST TO SPREAD FROM SOUTH TO
NORTH ACROSS OUR FORECAST AREA FROM LATE SUNDAY NIGHT THROUGH
MONDAY. COINCIDENT WITH THIS MOISTURE IS SOME MID LEVEL
INSTABILITY AS A WEAK UPPER LEVEL DISTURBANCE MOVES NORTHWESTWARD
OUT OF WESTERN MEXICO. THE 00Z NAM FORECASTS MODIFIED TT VALUES AS
HIGH AS 31 C OVER OUR AREA MONDAY INTO MONDAY NIGHT.
HOWEVER...MU CAPE VALUES ARE GENERALLY FORECAST TO REMAIN AT LESS
THAN 100 J/KG. SO AS OF NOW IT LOOKS LIKE MID LEVEL INSTABILITY
NEEDED FOR ELEVATED CONVECTION WILL BE MARGINAL AT BEST. AND...THE
GFS AND ECMWF TEND TO KEEP THE MOST SIGNIFICANT MOISTURE AND
INSTABILITY OFFSHORE. WILL CONTINUE TO MONITOR AND ADD SLIGHT
THUNDERSTORM CHANCES IF AND WHEN FORECAST CONFIDENCE INCREASES
SUFFICIENTLY.

&&

.AVIATION...AS OF 5:55 PM PDT FRIDAY...SKIES HAVE CLEARED ACROSS THE
DISTRICT ASIDE FROM SOME AREAS OF STRATUS ALONG THE COAST. THE
MARINE LAYER IS DEEP...WITH THE 00Z KOAK SOUNDING PUTTING THE
INVERSION BASE AT ABOUT 2800 FT...RECENT MDCRS ASCENT SOUNDINGS
FROM KSFO AT ABOUT 2400 FT AND THE FT ORD PROFILER ABOVE 3300 FT.
FLOW WITHIN THE BOUNDARY LAYER IS PREDOMINANTLY SOUTHWESTERLY...AND
THEN MORE SOUTHERLY ABOVE. LATEST NAM AND HIGH RESOLUTION LOCAL
WRF MODEL OUTPUT ARE CONSISTENT IN INDICATING THE STRATUS DECK
WILL FILL BACK IN AND EXTEND WELL INLAND DURING THE EVENING AND
OVERNIGHT HOURS. MEDIUM-TO-HIGH CONFIDENCE IN FORECAST OF CIG
DEVELOPMENT AT ALL TERMINALS EXCEPT KSJC BEFORE MIDNIGHT.

VICINITY OF KSFO...CONCERN OVER POSSIBILITY OF AN EARLY RETURN HAS
DIMINISHED OVER THE PAST HOUR WITH CLEARING OF STRATUS THAT HAD
PUSHED INTO THE CENTRAL SF BAY AREA DURING THE MIDAFTERNOON HOURS.
CONFIDENCE IS NOW PRETTY HIGH THAT VFR CONDITIONS WILL CONTINUE
THROUGH THE EVENING RUSH. MVFR CIG IS HOWEVER STILL EXPECTED TO
DEVELOP DURING THE LATE EVENING HOURS. WEST WINDS OF 15 TO 20 KT
WILL GRADUALLY DIMINISH AS THE EVENING PROGRESSES.

KSFO BRIDGE APPROACH...SIMILAR TO KSFO.

MONTEREY BAY AREA TERMINALS...MEDIUM-TO-HIGH CONFIDENCE MVFR CIGS
RETURN AROUND MID-EVENING.

&&

.MARINE...AS OF 9:00 PM PDT FRIDAY...FLOW WILL BE LIGHT OUT OF THE
WEST OVERNIGHT AS A LOW PRESSURE SYSTEM MOVES INLAND. NORTHWEST
FLOW INCREASES OVER THE WEEKEND AS HIGH PRESSURE BUILDS OVER THE
EASTERN PACIFIC.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TNGT...SCA...SF BAY NORTH OF THE BAY BRIDGE UNTIL 9 PM

&&

$$

PUBLIC FORECAST: DYKEMA
AVIATION: BLIER
MARINE: BLIER

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