703
FXUS66 KMTR 210351
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
851 PM PDT WED AUG 20 2025
..NEW UPDATE, MARINE, FIRE WEATHER
KEY MESSAGES
UPDATED AT 1216 PM PDT WED AUG 20 2025
- A HEAT ADVISORY IS IN EFFECT FROM THURSDAY - FRIDAY FOR MUCH OF
THE INTERIOR BAY AREA AND CENTRAL COAST.
- TEMPERATURES WILL REMAIN ABOVE NORMAL THROUGH THE WEEKEND.
- ELEVATED FIRE WEATHER CONDITIONS ACROSS THE INTERIOR THURSDAY
INTO THE WEEKEND.
UPDATE
ISSUED AT 833 PM PDT WED AUG 20 2025
SATELLITE IMAGERY SHOWS A PATCH OF STRATUS ALONG THE IMMEDIATE
PACIFIC COAST FROM HALF MOON BAY DOWN TO DAVENPORT. A HEAT
ADVISORY WILL GO INTO PLACE FOR THE INTERIOR BAY AREA AND CENTRAL
COAST AT 10 AM THURSDAY. FOR THE NORTH BAY VALLEYS AND THE SAN
FRANCISCO BAYSHORE, THE ADVISORY CONTINUES THROUGH THURSDAY NIGHT,
WHILE ACROSS THE REST OF THE INTERIOR BAY AREA AND CENTRAL COAST,
THE ADVISORY EXTENDS THROUGH FRIDAY NIGHT. ONE OF THE MAIN
MOTIVATIONS FOR EXTENDING THE AREA UNDER HEAT ADVISORY WAS
TEMPERATURES RUNNING FAR ABOVE THE FORECAST TODAY ACROSS THE
BAYSHORE REGIONS. FOR EXAMPLE, OAKLAND AIRPORT AND SAN FRANCISCO
AIRPORT REPORTED HIGHS OF 82, WHILE REDWOOD CITY REPORTED A HIGH
OF 95. THE RESPECTIVE FORECAST HIGHS TODAY WERE 78, 76, AND 87.
EVEN IF THURSDAY'S HIGHS DON'T SIMILARLY OVERSHOOT THE CURRENT
FORECAST, THE FACT THAT THIS IS GOING TO BE THE FIRST WIDESPREAD
WARMING OF THE SUMMER SEASON, RATHER LATER IN THE SUMMER COMPARED
TO THE PATTERNS LAST YEAR, WILL RESULT IN PEOPLE NOT BEING AS
ACCLIMATED TO THE HEAT AS THEY WERE THIS TIME LAST YEAR. OR IN
LAYMAN'S TERMS, PEOPLE HAVEN'T SEEN TEMPERATURES THIS HOT SO FAR
THIS SUMMER. THIS WILL RESULT IN AN INCREASED RISK OF HEAT-
RELATED ILLNESSES, ESPECIALLY FOR VULNERABLE POPULATIONS INCLUDING
CHILDREN, THE ELDERLY, PREGNANT WOMEN, PEOPLE WITH CERTAIN
CHRONIC ILLNESSES, AND ANYONE WORKING OR LIVING OUTDOORS WITHOUT
ADEQUATE COOLING OR HYDRATION.
DIALH
SHORT TERM
ISSUED AT 1216 PM PDT WED AUG 20 2025
(THIS EVENING THROUGH THURSDAY)
TEMPERATURES ARE CURRENTLY AROUND 5 DEGREES WARMER THAN THIS TIME
YESTERDAY WITH A FEW MORE HOURS OF HEATING IN STORE THIS
AFTERNOON. EVEN IN THE SHORT TERM, THE TEMPERATURE FORECAST HAS
BEEN A BIT OF A CHALLENGE. THE COOLER THAN NORMAL SUMMER SEEMS TO
HAVE MADE A NEGATIVE IMPACT ON THE NBM BIAS CORRECTION. OUT OF
ROUGHLY 1000 OBSERVATIONS YESTERDAY, 444 WERE AT OR ABOVE THE NBM
90TH PERCENTILE FOR MAX TEMPERATURE. THAT FACT HAD ME ALL GEARED
UP TO USE THE NBM 90TH PERCENTILE AS THE BASELINE FOR THE
FORECAST, UNTIL I SAW THE OUTPUT. IT WAS JUST TOO HOT TO PASS THE
SANITY CHECK. 117 F AT TASSAJARA CAN CERTAINLY HAPPEN, BUT
PROBABLY NOT WITH A 25C 850 TEMPERATURE. FORTUNATELY THE
DETERMINISTIC NBM HAS INCREASED A BIT FROM YESTERDAY, LIKELY AS A
RESULT OF THE COOL BIAS WE'VE SEEN RECENTLY. THERE IS ALSO A
COMPLICATING FACTOR OF HIGH CLOUDS ROLLING IN TOMORROW. THE FIRST
SIGN OF THE INCOMING MONSOON MOISTURE WILL BE IN THE UPPER
LEVELS. THE 300 MB RH WAS LAST MEASURED AT 4% AND IS EXPECTED TO
BE OVER 95% IN THE NEXT 24 HOURS. THIS WILL SURELY BRING SOME
AMOUNT OF UPPER LEVEL CLOUDS, WHICH HAVE A SLIGHT COOLING EFFECT
DURING THE DAY. THE COVERAGE, TIMING AND OPACITY OF THESE CLOUDS
ARE HARD TO PIN DOWN, HOWEVER. AFTER MUCH ANALYSIS, I DECIDED TO
INCREASE THE DETERMINISTIC NBM MAX TEMPERATURES BY ONE DEGREE
FAHRENHEIT FOR ALL LAND AREAS FROM THURSDAY-SATURDAY.
LONG TERM
ISSUED AT 1216 PM PDT WED AUG 20 2025
(THURSDAY NIGHT THROUGH NEXT TUESDAY)
THE HIGH CLOUDS THURSDAY ACTUALLY HAVE A WARMING EFFECT AT NIGHT
BY TRAPPING SOME IR RADIATION. THIS EFFECT, COMBINED WITH
CONTINUED BUILDING OF THE RIDGE AND 850 TEMPERATURES WILL CAUSE
OVERNIGHT LOWS TO STAY IN THE UPPER 50S TO LOW 60S FOR MOST, AND
EVEN MID 70S AT HIGHER ELEVATION STARTING THURSDAY NIGHT. THAT'S
ABOUT 5 DEGREES WARMER THAN NORMAL FOR THE LOW-LANDS, AND UP TO
15 DEGREES WARMER IN THE MOUNTAINS. TYPICALLY, EVEN UNDER RIDGING
CONDITIONS, CLEAR SKIES BRING A STRONG NOCTURNAL INVERSION AND
COOL MORNINGS AT THE SURFACE, BUT THE EXTRA HUMIDITY AND CLOUDS
WILL LIMIT THAT EFFECT THIS TIME. THE MAX TEMPERATURES AND
HEATRISK HAVE ALSO TRENDED UP FRI-SAT, AND IT'S LIKELY WE WILL
HAVE TO EXTEND SOME OR ALL OF THE HEAT ADVISORY INTO SATURDAY.
THIS MONSOON MOISTURE, COMBINED WITH CONDITIONALLY UNSTABLE LAPSE
RATES ABOVE 700 MB, BRING A SMALL CHANCE FOR ELEVATED CONVECTION
STARTING FRIDAY. THERE IS NO CLEAR TRIGGER FOR THIS INSTABILITY TO
BE REALIZED, AND THUNDERSTORMS WILL LIKELY BE CONFINED TO HIGHER
ELEVATIONS IN THE SIERRA. THE INSTABILITY AND MOISTURE LOOK TO
WANE BY SUNDAY, BUT THE NBM PROBABILITY OF THUNDER STICKS AROUND
(ALBEIT LESS THAN 5%) THROUGH THE FORECAST PERIOD.
THE HEAT WILL GRADUALLY IMPROVE THROUGH EARLY NEXT WEEK WITH
TEMPERATURES EVENTUALLY SETTLING NEAR NORMAL. EVEN THIS WILL FEEL
WARM FOR MANY AS WE APPROACH THE WARMEST TIME OF THE YEAR AFTER A
RELATIVELY COOL SUMMER.
AVIATION
(00Z TAFS)
ISSUED AT 452 PM PDT WED AUG 20 2025
VFR THROUGH THE TAF PERIOD. WINDS REDUCE INTO THE EVENING AND BECOME
MOSTLY LIGHT ACROSS THE REGION INTO THE NIGHT. SOME SPOTTY POCKETS
OF LOW CLOUDS AND FOG LOOK TO AFFECT PORTIONS OF THE COAST OVERNIGHT
AND INTO THURSDAY MORNING, BUT DON'T LOOK TO FILL OVER ANY OF THE
TAF SITES. WINDS BUILD AGAIN INTO THURSDAY AFTERNOON BECOMING
MODERATE FOR MOST AREAS AND BREEZY AT SFO. THESE WINDS LOOK TO
REDUCE AGAIN THAT EVENING.
VICINITY OF SFO...VFR THROUGH THE TAF PERIOD. WINDS REDUCE AND GUSTS
CUT OFF INTO THE NIGHT, WITH ONLY MODERATE WEST WINDS EXPECTED
THROUGH THE NIGHT AND MOST OF THE MORNING. WEST WINDS REBUILD INTO
THE MID TO LATE MORNING ON THURSDAY WITH GUSTS PEAKING AROUND 22
KTS. WINDS AND GUSTS REDUCE AGAIN GOING INTO THURSDAY NIGHT.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...VFR THROUGH THE TAF PERIOD. WINDS BECOME
LIGHT IN THE LATE EVENING AND STAY LIGHT THROUGH THURSDAY AFTERNOON.
MODERATE WEST AND NORTHWEST WINDS BUILD FOR THURSDAY AFTERNOON WITH
SOME INFREQUENT GUSTS. THESE WINDS BECOME LIGHT AGAIN THAT NIGHT.
MARINE
(TONIGHT THROUGH NEXT TUESDAY)
ISSUED AT 833 PM PDT WED AUG 20 2025
EXPECT ELEVATED SEAS TO CONTINUE ALONG WITH BREEZY TO GUSTY
NORTHWEST WINDS WILL PREVAIL THROUGH FRIDAY MORNING, ESPECIALLY
ACROSS THE OUTER WATERS AND COASTAL JET REGIONS NEAR POINT SUR AND
POINT REYES. THIS WILL RESULT IN WIDESPREAD HAZARDOUS CONDITIONS
FOR SMALL CRAFT. WIND AND SEAS WILL BEGIN TO GRADUALLY EASE FRIDAY
AFTERNOON AND EVENING, LEADING TO CALMER CONDITIONS OVER THE
WEEKEND AND WELL INTO THE NEXT WORK WEEK.
FIRE WEATHER
ISSUED AT 833 PM PDT WED AUG 20 2025
ELEVATED FIRE WEATHER THREATS ARE EXPECTED TO BEGIN ON THURSDAY,
WITH HOT AND DRY CONDITIONS RESULTING IN CURING OF FUELS,
ESPECIALLY ACROSS THE INTERIOR WHERE THE COMBINATION OF POOR
OVERNIGHT HUMIDITY RECOVERIES, VERY POOR DAYTIME HUMIDITY
RETENTION, AND WARM OVERNIGHT LOWS IN THE HIGHER ELEVATIONS WILL
BRING LITTLE RELIEF FROM THE DRYING TREND. ERC AND BURNING INDEX
FORECASTS CONTINUE TO SHOW DRYING FUELS AND THE POSSIBILITY OF
LARGE FIRES TO TAKE HOLD AND SPREAD RAPIDLY. IN ADDITION, WE ARE
MONITORING A SURGE OF MONSOONAL MOISTURE SET TO ARRIVE ON FRIDAY
THROUGH THE WEEKEND, WHICH WILL BRING A VERY LOW, BUT NON-ZERO,
CHANCE FOR ELEVATED CONVECTION ACROSS THE CENTRAL COAST. FIRE
WEATHER CONDITIONS IMPROVE IN THE EARLY PART OF NEXT WEEK AS A
GRADUAL COOLING TREND TAKES HOLD.
DIALH
MTR WATCHES/WARNINGS/ADVISORIES
CA...HEAT ADVISORY FROM 10 AM THURSDAY TO 10 PM PDT FRIDAY FOR CAZ504-
510-512>518.
HEAT ADVISORY FROM 10 AM TO 10 PM PDT THURSDAY FOR CAZ506-508.
PZ...SMALL CRAFT ADVISORY UNTIL 3 AM PDT THURSDAY FOR PT ARENA TO PT
REYES 0-10 NM-PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM.
SMALL CRAFT ADVISORY FROM 3 PM THURSDAY TO 9 AM PDT FRIDAY FOR
PIGEON PT TO PT PINOS 0-10 NM-PT ARENA TO PT REYES 0-10 NM-
PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 9 AM PDT FRIDAY FOR PIGEON PT TO PT
PINOS 10-60 NM-PT ARENA TO PT REYES 10-60 NM.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...MURDOCK
MARINE...MURDOCK
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