285
FXUS66 KMTR 202026
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
126 PM PDT WED AUG 20 2025
..NEW SHORT TERM, LONG TERM
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.
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
(18Z TAFS)
ISSUED AT 1036 AM PDT WED AUG 20 2025
VFR CONDITIONS ARE EXPECTED TO PREVAIL THROUGH THE FORECAST PERIOD
FOR ALL TERMINALS. A COMBINATION OF VERY DRY AIR ABOVE A
COMPRESSED AND DIMINISHED MARINE LAYER WITH WEAK OFFSHORE FLOW AT
TIMES SHOULD KEEP MARINE STRATUS CONFINED TO THE IMMEDIATE COAST,
WHEN PRESENT.
VICINITY OF SFO...EXPECT VFR TO PREVAIL THROUGH THE FORECAST
PERIOD WITH A LOW PROBABILITY CHANCES OF IFR CEILINGS EARLY
THURSDAY MORNING, PRIMARILY FOR KOAK.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...VFR IS EXPECTED THROUGH THE FORECAST
PERIOD WITH LOW PROBABILITY OF IFR STRATUS CEILINGS EARLY THURSDAY
MORNING AT KMRY. THE COMPRESSED MARINE LAYER SHOULD PREVENT
MARINE LAYER FROM ADVANCING INTO THE SALINAS VALLEY.
MARINE
(TODAY THROUGH MONDAY)
ISSUED AT 1036 AM PDT WED AUG 20 2025
BREEZY TO GUSTY NORTHWEST WINDS WILL PREVAIL THROUGH LATE WEEK,
ESPECIALLY ACROSS THE OUTER WATERS AND COASTAL JET REGIONS NEAR
POINT SUR AND POINT REYES, RESULTING IN WIDESPREAD HAZARDOUS
CONDITIONS FOR SMALL CRAFT. WIND AND SEAS WILL EASE FOR THE
WEEKEND.
FIRE WEATHER
ISSUED AT 338 AM PDT WED AUG 20 2025
WILL KEEP FIRE WEATHER HEADLINE GOING. AS NOTED BELOW, HEIGHTENED
FIRE WEATHER CONCERNS WILL DEVELOP ON THURSDAY AND LAST INTO THE
WEEKEND. THE WARM AND DRY CONDITIONS WILL PERSIST BOTH DAY AND
NIGHT, ESPECIALLY INTERIOR LOCATIONS AND HIGHER TERRAIN. DO WANT
TO MENTION A NON-ZERO CHANCE FOR DRY LIGHTNING FRIDAY AND SATURDAY
ACROSS THE CENTRAL COAST.
THE WARMING AND DRYING TREND EXPECTED THROUGH THE LATER PART OF
THE WEEK IS CAUSING ELEVATED FIRE WEATHER THREATS ACROSS THE
REGION. IN PARTICULAR, THE INLAND REGIONS WILL SEE POOR TO VERY
POOR DAYTIME HUMIDITY RETENTION, COMPOUNDED WITH POOR OVERNIGHT
HUMIDITY RECOVERIES IN THE HIGHER ELEVATIONS, LEADING TO
WIDESPREAD CURING OF FUELS. THE LATEST ERC AND BURNING INDEX
FORECASTS ALL SHOW DRYING FUELS ACROSS THE REGION AND THE
POSSIBILITY OF LARGE FIRES TO TAKE HOLD AND SPREAD RAPIDLY.
ALTHOUGH A GRADUAL COOLING TREND WILL BEGIN OVER THE WEEKEND, THE
FIRE WEATHER CONDITIONS WILL BE SLOW TO RECOVER, AND ELEVATED FIRE
WEATHER THREAT CONTINUES THROUGH THE EARLY PART OF NEXT WEEK.
DIALH/MM
MTR WATCHES/WARNINGS/ADVISORIES
CA...HEAT ADVISORY FROM 10 AM THURSDAY TO 10 PM PDT FRIDAY FOR CAZ504-
510-512>518.
PZ...SMALL CRAFT ADVISORY UNTIL 9 AM PDT FRIDAY FOR PIGEON PT TO PT
PINOS 10-60 NM-PT ARENA TO PT REYES 0-10 NM-PT ARENA TO PT
REYES 10-60 NM-PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 3 AM PDT THURSDAY FOR PT REYES TO
PIGEON PT 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 REYES TO PIGEON PT 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 9 PM PDT THIS EVENING FOR PIGEON PT
TO PT PINOS 0-10 NM.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...JM
MARINE...JM
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