349
FXUS66 KMTR 020743
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1243 AM PDT WED SEP 2 2026
..NEW SHORT TERM, LONG TERM
KEY MESSAGES
UPDATED AT 1015 PM PDT TUE SEP 1 2026
- COOLER WEATHER CONTINUES THIS WEEK
- RAIN LIKELY WEDNESDAY AND THURSDAY MORNING FOR THE NORTH BAY
SHORT TERM
ISSUED AT 1015 PM PDT TUE SEP 1 2026
(TONIGHT THROUGH THURSDAY)
NOW THAT METEOROLOGICAL SUMMER (JUNE - AUGUST) IS OVER, THE MARINE
LAYER WON'T BE AS PERSISTENT. WE'VE BEEN THROUGH "MAY GREY",
"JUNE GLOOM", "NO SKY JULY", AND "FOGUST", BUT THERE ISN'T A CUTE
NAME FOR SEPTEMBER. THAT'S FOR A GOOD REASON. THE COMBINATION OF
EARLY SEASON COLD FRONTS AND OFFSHORE WIND EVENTS TEND TO DISRUPT
THE MARINE LAYER PERIODICALLY IN THE FALL. AT THE CURRENT MOMENT,
THE MARINE LAYER HAS GOTTEN TOO DEEP FOR ITS OWN GOOD. IT WAS
MEASURED AT 5,200 FEET ON THE 00Z SOUNDING. THAT'S ABOUT AS DEEP
AS WE EVER SEE IT BEFORE IT MIXES OUT. SINCE THE BALLOON LAUNCH,
THE DEPTH HAS ONLY GROWN DUE TO UNSTABLE SURFACE LAYER MIXING FROM
BELOW, AND A 500 MB TROUGH ENCOURAGING EXPANSION ABOVE.
THE AIR MASS IS COLD FOR THIS TIME OF YEAR. THE 00Z WEATHER
BALLOON MEASURED THE 850 MB (~5,000 FT) TEMPERATURE AT 42F. THAT
MAKES THE TOP 10 COLDEST 850 TEMPS WE'VE SEEN IN THE FIRST WEEK OF
SEPTEMBER. THE NORMAL TODAY IS 66F. 42F IS THE NORMAL FOR LATE
FEBRUARY. DESPITE THIS, IF YOU'RE IN A COASTAL CITY IT MAY
ACTUALLY FEEL WARM SINCE THE MARINE LAYER IS TAKING A LITTLE
VACATION. INLAND AREAS ARE REALLY FEELING THE DIFFERENCE THOUGH,
WITH TEMPERATURES 10-15 F COOLER THAN NORMAL. SEPTEMBER IS
TYPICALLY THE WARMEST MONTH FOR MANY OF OUR CITIES, SO BELOW
NORMAL IS A GOOD THING FOR MOST.
THE APPROACHING TROUGH IS SUPPORTING A COLD FRONT LOCATED ABOUT
200 MILES WEST OF THE OR/CA BORDER. THERE IS SOME VERY LIGHT PRE-
FRONTAL RAIN REACHING THE GROUND IN THE NORTH BAY TONIGHT, BUT
WE'VE ONLY SEEN A FEW SCATTERED HUNDREDTHS SO FAR. THIS RAIN WILL
PICK-UP THROUGH THE MORNING. THE BEST WINDOW FOR SANTA ROSA IS
5-10 AM. THE TOTAL ACCUMULATION SHOULD BE BETWEEN A FEW HUNDREDTHS
AND A TENTH OF AN INCH THIS MORNING. THE PRE-FRONTAL RAIN BAND
WILL DRY OUT QUITE A BIT AS IT MOVES SOUTH, WITH MUCH LOWER RAIN
CHANCES SOUTH OF THE GOLDEN GATE. TEMPERATURES TODAY WILL BE VERY
SIMILAR TO YESTERDAY (60S AND 70S), ALTHOUGH THERE WILL BE MORE
CLOUD COVER ACROSS THE BAY AREA, SO IT MAY FEEL A LITTLE COOLER
DESPITE THE 850 TEMPS REBOUNDING A BIT.
THE ACTUAL COLD FRONT ARRIVES TO THE NORTH BAY THURSDAY MORNING,
BRINGING A MORE SIGNIFICANT ROUND OF RAIN. THE LATEST FORECAST
HAS INCREASED THE QPF TO A RESPECTABLE 0.25-0.5" ACROSS SONOMA
AND MARIN COUNTY. THAT'S REAL RAIN. THE NORMAL FOR THE ENTIRE
MONTH IS 0.15" AT SANTA ROSA. LIKE WEDNESDAY, THE RAIN CHANCES
DROP OFF SIGNIFICANTLY SOUTH OF THE GOLDEN GATE, BUT WE DO EXPECT
A FEW HUNDREDTHS ACROSS THE COASTAL AREAS OF THE CWA GRADUALLY
MOVING SOUTH THROUGH THE DAY THURSDAY. TEMPERATURES WILL CONTINUE
TO STAY WELL BELOW NORMAL THROUGH THE DAY.
LONG TERM
ISSUED AT 1015 PM PDT TUE SEP 1 2026
(THURSDAY NIGHT THROUGH NEXT TUESDAY)
AFTER THE FRONT MOVES THROUGH THURSDAY NIGHT THE MARINE LAYER
WILL GRADUALLY START TO REORGANIZE ON FRIDAY, BUT IT WILL TAKE
SOME TIME. THE 500 MB TROUGH WILL SPAWN A CUT-OFF LOW THAT WILL
LINGER SOMEWHERE OVER THE PACIFIC NORTHWEST. MEANWHILE HIGH
PRESSURE OVER THE SW US WILL START TO BUILD TOWARDS CALIFORNIA.
THE GRADIENT BETWEEN THESE TWO FEATURES WILL KEEP PERSISTENT W-SW
FLOW AT 500 MB WELL INTO NEXT WEEK. AS THE LOW STARTS TO WEAKEN,
THE HIGH WILL EXERT MORE INFLUENCE, ALLOWING THE TEMPERATURES TO
GRADUALLY RISE THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK.
THE EXACT LOCATION OF THE CUT-OFF LOW IS THE WILDCARD IN THE
FORECAST. WITHOUT THE STEERING INFLUENCE OF THE JET STREAM, IT'S
VERY TRICKY TO PIN DOWN WHERE THIS THING WILL MEANDER. IF IT
HAPPENS TO TAKE A DIVE SOUTH TO CALIFORNIA, IT COULD OPEN THE DOOR
FOR TROPICAL MOISTURE ASSOCIATED WITH TROPICAL STORM MARIE TO
REACH SOUTHERN CALIFORNIA. DIRECT IMPACTS FROM THIS TROPICAL
CYCLONE ARE LOOKING MUCH LESS LIKELY NOW. THE STORM SPEED HAS
SLOWED AND IT WILL MOST LIKELY CONTINUE TO MARCH NW INTO THE OPEN
OCEAN. THE ASSOCIATED MOISTURE MAY STILL REACH SOUTHERN
CALIFORNIA, BUT EVEN IN THAT SCENARIO, IT'S UNLIKELY ANY
SIGNIFICANT RAIN MAKES IT FURTHER NORTH THAN THE CENTRAL COAST.
AVIATION
(06Z TAFS)
ISSUED AT 944 PM PDT TUE SEP 1 2026
IT'S CURRENTLY VFR AT THE AIRPORT TERMINALS EXCEPT KHAF REPORTS
LOW CLOUDS /IFR/. MID TO UPPER LEVEL TROUGHING WILL PASS ACROSS
THE FORECAST AREA RESULTING IN VARIABLE CEILINGS /VFR-MVFR-IFR/
TONIGHT AND WEDNESDAY MORNING WITH GENERALLY IMPROVING CONDITIONS
TO MVFR-VFR LATE WEDNESDAY MORNING AND AFTERNOON.
VICINITY OF SFO...VFR-MVFR WITH LOW CLOUDS FORMING OVERNIGHT AND
WEDNESDAY MORNING. CONDITIONS VFR WEDNESDAY AFTERNOON AND EARLY
EVENING, MVFR WITH LOW CLOUDS RETURNING MID WEDNESDAY EVENING.
SOUTHWEST TO WEST WIND 5 TO 15 KNOTS.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...VFR-MVFR WITH A FEW PATCHES OF LOW CLOUDS
FORMING OVERNIGHT AND WEDNESDAY MORNING. CONDITIONS VFR WEDNESDAY
AFTERNOON AND EARLY EVENING THEN LOW CLOUDS /MVFR/ RETURNS MID
WEDNESDAY EVENING. VARIABLE WINDS 5 KNOTS OR LESS BECOMING WEST TO
NORTHWEST 5 TO 15 KNOTS WEDNESDAY AFTERNOON AND EVENING.
MARINE
(TONIGHT THROUGH NEXT MONDAY)
ISSUED AT 920 PM PDT TUE SEP 1 2026
LIGHT NORTHWESTERLY BREEZES WITH OCCASIONAL MODERATE BREEZES WILL
PERSIST THROUGH THE WEEK WITH SLIGHT TO MODERATE WIND DRIVEN
SEAS. TROPICAL CYCLONE ACTIVITY WILL PRODUCE LONG PERIOD SOUTHERLY
SWELL FOR THE END OF THE WEEK INTO THE WEEKEND.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...NONE.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...CANEPA
MARINE...CANEPA
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