047
FXUS66 KMTR 021201
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
501 AM PDT WED SEP 2 2026
..NEW AVIATION, MARINE
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
(12Z TAFS)
ISSUED AT 427 AM PDT WED SEP 2 2026
LOW LEVEL CLOUDS REMAIN PATCHY THIS MORNING WITH HIGH LEVEL CLOUDS
FILTERING IN ACROSS THE REGION. THE MAIN SHORT TERM ADJUSTMENT IN
THE 12Z TAFS WAS TO REMOVE MVFR CEILINGS FROM MOST AIRPORTS.
CONFIDENCE THAT LOW LEVEL CLOUDS WILL DEVELOP IS DECREASING GIVEN
THE INCOMING SHOWERS AND HIGH CLOUDS DISRUPTING STRATUS FORMATION
ALONG THE COAST. THE MARINE LAYER ALSO REMAINS MIXED OUT GIVEN THE
DEEP INCOMING TROUGH TO THE WEST COAST. SHOWERS ARE STARTING TO
MOVE IN ON RADAR WITH STS REPORTING LIGHT RAIN THIS MORNING.
HIGHEST CONFIDENCE IN LIGHT RAIN REACHING STS/APC BUT CANNOT RULE
OUT A STRAY SHOWER MAKING IT TO SFO/OAK/HAF THIS MORNING. SHOWER
CHANCES DECREASE BY LATE THIS MORNING BEFORE MORE WIDESPREAD
SHOWER CHANCES RETURN TONIGHT. THE SFO-WMC GRADIENT PEAKS AROUND
+10 MB THIS AFTERNOON WHICH SUPPORTS ANOTHER DAY OF BREEZY ONSHORE
WINDS DURING THE AFTERNOON/EVENING ACROSS THE REGION. FOR THE
LONGER TERM, MAIN ADJUSTMENTS WERE TO PUSH BACK THE ARRIVAL OF
LOWER CLOUDS TONIGHT AND TWEAK THE TIMING OF MORE WIDESPREAD
SHOWERS ARRIVING. GUIDANCE IS LEANING TOWARDS LOW CLOUDS COMING IN
CLOSER TO 09-12Z TONIGHT WITH THE WEAK COLD FRONT ARRIVING IN
THAT SAME TIMEFRAME. RAIN CHANCES INCREASE EARLIER ACROSS THE
NORTH BAY BEFORE INCREASING ACROSS THE REST OF THE REGION CLOSER
TO 12Z/SHORTLY AFTER THE END OF THIS TAF PERIOD.
VICINITY OF SFO...PATCHY MVFR CIGS ARE POSSIBLE THIS MORNING BUT
CONFIDENCE IS DECREASING THAT THEY WILL DEVELOP. CLOUD COVER HAS
BEEN COMING IN HIGHER (BASES AROUND 5000 FT) AND IS EXPECTED TO
REMAIN ELEVATED THROUGH THIS EVENING. SHOWERS ARE EXPECTED TO
REMAIN NORTH OF SFO THIS MORNING BUT CANNOT RULE OUT A STRAY
SHOWER BRINGING A TRACE OF RAIN TO SFO THIS MORNING. CEILINGS
LOWER OVERNIGHT TONIGHT AND THE POTENTIAL FOR LIGHT RAIN INCREASES
AS A WEAK COLD FRONT APPROACHES THE REGION. MVFR CIGS COULD
RETURN AS EARLY AS 12Z BUT HAVE PUSHED IT BACK TO 15Z TO BETTER
ALIGN WITH LAMP AND HRRR GUIDANCE. RAIN POTENTIAL INCREASES AROUND
15Z AS SHOWERS PROGRESS NORTH TO SOUTH TONIGHT.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...HIGH CLOUDS STARTING TO DRIFT IN OVER THE
MONTEREY BAY REGION. THE MARINE LAYER REMAINS MIXED OUT THIS
MORNING WHICH IS RESULTING IN MUCH LESS WIDESPREAD STRATUS THIS
MORNING. THE MARINE LAYER IS FORECAST TO REMAIN MIXED OUT THROUGH
THURSDAY AS A DEEP UPPER LEVEL TROUGH MOVES INTO THE WEST COAST
AND BRINGS RAIN POTENTIAL. SHOWERS ARE EXPECTED TO STAY NORTH OF
MRY/SNS THIS MORNING BUT SHOWER CHANCES INCREASE SLIGHTLY TONIGHT
INTO TOMORROW. MVFR-IFR CIGS ARE EXPECTED TO ARRIVE BETWEEN
03-06Z WITH SLIGHTLY HIGHER CONFIDENCE IN AN ARRIVAL CLOSER TO 06Z
AHEAD OF AN INCOMING WEAK COLD FRONT.
MARINE
(TODAY THROUGH MONDAY)
ISSUED AT 427 AM PDT WED SEP 2 2026
LIGHT NORTHWESTERLY BREEZES WITH OCCASIONAL MODERATE GUSTS
PERSIST THROUGH THE WEEK WITH SLIGHT TO MODERATE WIND DRIVEN SEAS.
WINDS INCREASE, BECOMING MORE MODERATE, LATE THIS WEEK WITH
DIURNAL, FRESH GUSTS EXPECTED IN THE VICINITY OF THE MONTEREY BAY.
LONG PERIOD SOUTHERLY SWELL ASSOCIATED WITH TROPICAL CYCLONE
ACTIVITY ARRIVES FRIDAY INTO THE WEEKEND.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...NONE.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...KENNEDY
MARINE...KENNEDY
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