028
FXUS66 KMTR 211123
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
423 AM PDT MON JUL 21 2025
..NEW AVIATION, MARINE
KEY MESSAGES
UPDATED AT 1221 AM PDT MON JUL 21 2025
- BELOW NORMAL TEMPERATURES CONTINUE THROUGH THE WEEK.
- LOCALIZED ELEVATED FIRE WEATHER THREAT FOR THE EAST BAY HILLS
MONDAY DUE TO GUSTY ONSHORE WINDS.
- GUSTY WINDS RETURN LATE TONIGHT AND CONTINUE THROUGH MONDAY
NIGHT ALONG THE COAST, RIDGETOPS, AND MOUNTAIN GAPS/PASSES.
SHORT TERM
ISSUED AT 1221 AM PDT MON JUL 21 2025
(TODAY AND TONIGHT)
THE MARINE LAYER IS DEEPENING AS STRONG ONSHORE WINDS FUNNEL
THROUGH INLAND GAPS AND PASSES. ALTAMONT PASS IS REPORTING WSW
WINDS GUSTING TO NEAR 50 MPH. FORTUNATELY THE RELATIVE HUMIDITY
IS QUITE HIGH TONIGHT, KEEPING FIRE DANGER SUPPRESSED. THE SFO-
SAC GRADIENT WAS +3.4 MB AT MIDNIGHT, ON A DECREASING TREND. THAT
SHOULD ALLOW THE WINDS TO EASE THROUGH THE MORNING HOURS BEFORE
THE TYPICAL DIURNAL GRADIENT REBUILDS THIS AFTERNOON. THE WRF
ENSEMBLE MEAN BUILDS THE GRADIENT BACK UP TO +4.3 MB LATE
AFTERNOON/EARLY EVENING WHICH SHOULD BRING SIMILAR STRENGTH WIND
GUSTS THROUGH EARLY TUESDAY MORNING. THE DEEP MARINE LAYER, HIGH
RELATIVE HUMIDITY AND STRONG ONSHORE WINDS WILL ENHANCE OROGRAPHIC
LIFT ALONG THE COASTAL MOUNTAINS, AND THERE'S A GOOD CHANCE FOR
DRIZZLE DURING THE MORNING COMMUTE ALONG HWY-1 AND THE NORTHERN
REACHES OF I-280/HWY-101. OTHERWISE IT WILL BE COOLER THAN
YESTERDAY, WITH MEAGER CLEARING IN THE AFTERNOON AND MAX
TEMPERATURES 5-15 DEGREES BELOW NORMAL.
LONG TERM
ISSUED AT 1221 AM PDT MON JUL 21 2025
(TUESDAY THROUGH SUNDAY)
IN SHORT, THE COOL TROUGHING PATTERN IS HERE TO STAY. AT 500 MB, A
POSITIVELY TILTED TROUGH WILL LIKELY SPAWN A CUT-OFF LOW
WEDNESDAY WHICH WILL THEN LINGER A FEW DAYS BEFORE BEING PICKED UP
BY A MORE ROBUST TROUGHING PATTERN THIS WEEKEND. IN THE MEDIUM
TERM, THIS WILL LEAD TO A RELAXED PRESSURE GRADIENT AND PRETTY
BENIGN CONDITIONS THROUGH THE WORK WEEK. THE 850 MB TEMPERATURE
WAS LAST MEASURED AT NEAR THE 75TH PERCENTILE FOR THIS TIME OF
YEAR (21.8 C). AS THE TROUGH DEVELOPS THIS BENCHMARK WILL FALL,
AND THERE IS A 90% CHANCE THAT IT STAYS BELOW THE 25TH PERCENTILE
(16.5 C) FROM TUESDAY - THURSDAY. THAT MEANS COOL, CLOUDY WEATHER
FOR THOSE IN THE MARINE LAYER AND CONTINUED COMFORTABLE CONDITIONS
FOR THOSE FURTHER INLAND. LATER IN THE WEEK, A HIGH PRESSURE
RIDGE WILL GRADUALLY BUILD OVER THE DESERT SW. THIS WILL BRING A
HEAT WAVE TO THE OUR NEIGHBORING STATES, AND NWS TUSCON IS
ADVERTISING A 90% CHANCE OF TEMPERATURES HITTING 105 DEGREES IN
THEIR HOME CITY NEXT SUNDAY (7/27). THERE IS NO INDICATION THAT
THIS RIDGE WILL MOVE ANYWHERE NEAR THE BAY AREA, BUT THE GRADIENT
WILL INCREASE SUBSTANTIALLY BETWEEN OUR DEEPENING TROUGH OFFSHORE
AND THE BUILDING RIDGE TO OUR SE. THIS PATTERN WILL SUPPORT 500 MB
WINDS UP TO 50 KTS FROM THE SW, AND SHOULD BRING ANOTHER ROUND OF
BREEZY SURFACE WINDS NEXT WEEKEND. THIS PROLONGED COOL PERIOD IS
HELPING SUPPRESS WILDFIRE POTENTIAL AND THE ENERGY RELEASE
COMPONENT IS EITHER BELOW AVERAGE OR FORECAST TO FALL BELOW
AVERAGE THIS WEEK FOR ALL REGIONS ACROSS THE BAY AREA AND CENTRAL
COAST.
AVIATION
(12Z TAFS)
ISSUED AT 423 AM PDT MON JUL 21 2025
STRATUS /MVFR-IFR/ MOVING INLAND ON ONSHORE WINDS THROUGH MORNING,
MIXING OUT THIS AFTERNOON TO VFR-MVFR EXCEPT STRATUS IS LIKELY TO
PERSIST ALONG THE IMMEDIATE COASTLINE TODAY. STRATUS /MVFR-IFR/
MOVES INLAND TONIGHT AND TUESDAY MORNING.
VICINITY OF SFO...STRATUS /MVFR/ SCATTERING OUT BY 21Z TODAY, STRATUS
RETURNING BY 03Z THIS EVENING. WEST WIND 10 TO 15 KNOTS EXCEPT 15 TO
25 KNOTS IN THE AFTERNOON AND EARLY EVENING.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...STRATUS /MVFR-IFR/ FOR MUCH IF NOT ALL
TODAY NEAREST THE BAY, KSNS HAS THE BEST CHANCE OF MIXING OUT TO
VFR BY 19Z TODAY. STRATUS /MVFR/ MOVING INLAND TONIGHT AND TUESDAY
MORNING. ONSHORE WINDS 5 TO 15 KNOTS.
MARINE
(TODAY THROUGH SATURDAY)
ISSUED AT 417 AM PDT MON JUL 21 2025
MODERATE BREEZES ARE EXPECTED TODAY, RESULTING IN HAZARDOUS
BOATING CONDITIONS FOR SMALLER VESSELS IN THE NORTHERN SAN
FRANCISCO AND SAN PABLO BAYS AND ACROSS THE NORTHERN OUTER
WATERS. SLIGHT TO MODERATE SEAS PERSIST THROUGH THE REMAINDER OF
THE FORECAST PERIOD. WINDS EASE TUESDAY THROUGH LATE WEEK.
CLIMATE
ISSUED AT 1221 AM PDT MON JUL 21 2025
THIS SUMMER HAS BEEN COOLER THAN AVERAGE AND THAT TREND WILL
CONTINUE FOR AT LEAST ANOTHER WEEK. I DECIDED IT WAS A GOOD TIME
TO ADD A CLIMATE SECTION HERE TO COMPARE JUST HOW DIFFERENT THIS
SUMMER HAS BEEN THAN LAST YEAR. I CREATED A SIMPLE HEAD-TO-HEAD
SCORING SYSTEM TO ADD DAYS ABOVE/BELOW NORMAL WEIGHTED BY HOW FAR
ABOVE/BELOW NORMAL THEY WERE. FOR EXAMPLE, IF THE MAXIMUM
TEMPERATURE WAS 2 DEGREES BELOW NORMAL ON SATURDAY AND 3 DEGREES
ABOVE NORMAL ON SUNDAY, THE SCORE WOULD BE 2-3. I FOCUSED ON MAX
TEMPERATURE AT SAN JOSE FROM JUNE 1 - JULY 20. METEOROLOGICAL
SUMMER STARTS JUNE 1ST AND SAN JOSE IS A GOOD MIXTURE OF SOME
MARINE LAYER INFLUENCE AND INLAND HEAT DEPENDING ON THE DAY.
AGAIN, I JUST MADE THIS SCORING SYSTEM UP TONIGHT, SO HOPEFULLY IT
MAKES SENSE. THE RESULTS ARE PRETTY INTERESTING, PARTICULARLY
WHEN COMPARED WITH LAST YEAR.
2025: COLD: 168, WARM: 36
2024: COLD: 30, WARM: 302
MTR WATCHES/WARNINGS/ADVISORIES
CA...COASTAL FLOOD ADVISORY FROM 7 PM THIS EVENING TO MIDNIGHT PDT
TONIGHT FOR CAZ006-506-508.
PZ...SMALL CRAFT ADVISORY UNTIL 9 PM PDT THIS EVENING FOR SF BAY N OF
BAY BRIDGE.
SMALL CRAFT ADVISORY FROM 3 PM THIS AFTERNOON TO 9 PM PDT THIS
EVENING FOR MRY BAY-PIGEON PT TO PT PINOS 0-10 NM-PT PINOS
TO PT PIEDRAS BLANCAS 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 9 AM PDT THIS MORNING FOR PT ARENA TO
PT REYES 0-10 NM-PT ARENA TO PT REYES 10-60 NM.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...CANEPA
MARINE...CANEPA
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