414
FXUS66 KMTR 211814
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1114 AM PDT MON SEP 21 2026
..NEW AVIATION, MARINE
KEY MESSAGES
UPDATED AT 1201 AM PDT MON SEP 21 2026
- COOL CONDITIONS TODAY
- WARMING TREND RESUMES TUESDAY, COOLER AGAIN FRIDAY INTO THE
WEEKEND
SHORT TERM
ISSUED AT 1201 AM PDT MON SEP 21 2026
(TODAY AND TONIGHT)
THE LOCAL WEATHER PATTERN REMAINS DOMINATED BY A CUT-OFF LOW WITH A
BROAD ROTATION CENTERED TO THE WEST OF MONTEREY AND SAN LUIS OBISPO
COUNTIES. THE SATELLITE IMAGERY REVEALS SCATTERED TO BROKEN HIGH
CLOUDS ASSOCIATED WITH THE LOW PRESSURE SYSTEM, SOME LOW CLOUDS
DEVELOPING ACROSS THE INLAND VALLEYS, AND SOME CONVECTIVE ACTIVITY
SOUTH OF THE MONTEREY COUNTY COAST THAT HAS BEEN POTENT ENOUGH TO
SHOW UP ON THE RADAR RETURNS. ONE OF THOSE CELLS EVEN LIT UP
LIGHTNINGCAST (LONG STORY SHORT, A SATELLITE BASED ALGORITHM FOR
LIGHTNING PROBABILITY) WITH A 50% PROBABILITY OF LIGHTNING BUT FELL
APART BEFORE REACHING THE SOUTHERN MONTEREY COUNTY SHORE. HOWEVER,
MOST OF THEM HAVEN'T CORRESPONDED TO ANY RAINFALL AT THE SURFACE, AS
THE 00Z (5 PM PDT) SOUNDING FROM OAKLAND REVEALED THAT THE LOWER
LAYERS REMAIN VERY DRY. BUT COULD WE STILL SEE A HIGH BASED
THUNDERSTORM THROUGH THE NIGHT? IT'S UNLIKELY, EVEN IF THE HRRR
CONTINUES TO HOLD ON TO K-INDEX VALUES UP TO 22C, WHICH WOULD
CORRESPOND TO ISOLATED THUNDERSTORMS, AND SOME LIGHTNING FLASH
SIGNALS NEAR THE SANTA LUCIA RANGE AROUND 4 AM TO 8 AM. CAPE VALUES
ARE STILL THE LIMITING FACTOR; THE 00Z OAK SOUNDING CONTINUED TO
SHOW A CONDITIONALLY UNSTABLE LAYER WITH LAPSE RATES AROUND 8.5
C/KM, BUT THIS LAYER WAS STILL BELOW THE MOIST AIR. THIS IS
SUPPORTED BY THE REFLECTIVITY VALUES AT -10 C AND -20 C, WHICH
REMAIN BELOW THE VALUES THAT WOULD BE NECESSARY TO SUPPORT THE ICE
PARTICLES THAT WOULD BE NECESSARY FOR THUNDERSTORM DEVELOPMENT. WE
WILL CONTINUE TO MONITOR THE CELLS AS THEY DEVELOP, BECAUSE WHILE
THE RISK OF LIGHTNING IS VERY SMALL, THE CHANCE THAT IT WOULD BE
ACCOMPANIED BY WETTING RAINS (AT LEAST 0.1" OF RAIN) ARE ZERO, THUS
POSING AN OBVIOUS CONCERN FOR DRY LIGHTNING-INDUCED FIRE STARTS.
AS THE LOW MOVES ONSHORE LATER THIS MORNING AND PASSES THROUGH TO
OUR SOUTH, COOL CONDITIONS WILL CONTINUE WITH HIGHS RANGING FROM THE
MIDDLE 70S TO LOWER 80S IN THE INLAND VALLEYS, THE MIDDLE 60S TO THE
MIDDLE 70S NEAR THE BAYS, AND THE UPPER 50S TO MIDDLE 60S ALONG THE
PACIFIC COAST. TYPICAL DIURNAL ONSHORE WINDS DEVELOP THIS AFTERNOON
AND EVENING WITH GUSTS REACHING 20 TO 30 MILES PER HOUR THROUGH GAPS
AND PASSES, AND WITHIN THE NORTHERN SALINAS VALLEY.
LONG TERM
ISSUED AT 1201 AM PDT MON SEP 21 2026
(TUESDAY THROUGH SUNDAY)
THE CUT-OFF LOW WILL CONTINUE TO MOVE TO THE EAST AND WILL BE OUT OF
OUR AREA BY TUESDAY MORNING, DEGRADING INTO A SHORTWAVE TROUGH AS IT
MOVES INTO THE GREAT BASIN. THIS WILL ALLOW MORE ZONAL FLOW TO
DEVELOP OVER THE REGION AND START A WARMING TREND THROUGH WEDNESDAY
AND THURSDAY. THE WORD "RIDGE" MIGHT BE A LITTLE STRONG FOR WHAT THE
MODELS ARE EXPECTING, BUT THERE IS DEFINITELY A NORTHWARD BUMP IN
THE FLOW THAT DEVELOPS BY WEDNESDAY. TEMPERATURES ON WEDNESDAY AND
THURSDAY WILL RANGE FROM THE MIDDLE 80S TO MIDDLE 90S IN THE INLAND
VALLEYS TO THE MIDDLE 70S TO MIDDLE 80S NEAR THE BAYS, AND THE
MIDDLE 60S TO LOWER 70S ALONG THE PACIFIC COAST. THERE ARE SOME
POCKETS OF MODERATE HEATRISK (MODERATE RISK FOR HEAT-RELATED ILLNESS
AMONG SENSITIVE POPULATIONS) POSSIBLE ON THOSE TWO DAYS IN FAVORED
INTERIOR LOCATIONS, BUT WIDESPREAD HEAT IMPACTS ARE NOT EXPECTED. AS
THE PREVIOUS FORECASTER NOTED, HEAT ADVISORIES IN LATE SEPTEMBER
TYPICALLY HAPPEN WITH OFFSHORE FLOWS AND LONGER DURATION EVENTS.
BY FRIDAY, ANOTHER TROUGH ARRIVES FROM THE EASTERN PACIFIC AND
REVERSES THE WARMING TREND, WITH TEMPERATURES CLOSER TO SEASONAL
AVERAGES INTO THE WEEKEND. LONGER-RANGE OUTLOOKS SHOW A SLIGHT LEAN
TOWARDS TEMPERATURES WARMER THAN SEASONAL AVERAGES AND PRECIPITATION
TOTALS ABOVE SEASONAL AVERAGES FOR THE LAST FEW DAYS OF SEPTEMBER
INTO THE FIRST FEW DAYS OF OCTOBER.
AVIATION
(18Z TAFS)
ISSUED AT 1113 AM PDT MON SEP 21 2026
STARTING THE DAY OFF WITH WIDESPREAD MVFR CEILINGS WITH IFR CEILINGS
IN THE NORTH BAY. CONDITIONS GRADUALLY IMPROVE TO VFR BY THE EARLY
AFTERNOON AT MOST SITES WITH LOWER CONFIDENCE FOR CLEARING AT HAF
AND MRY. THIS CLEARING WINDOW WILL BE SHORT-LIVED AS AN EARLY ONSET
OF MVFR TO IFR STRATUS IS EXPECTED TO ROLL IN AROUND/AFTER SUNSET.
OVERNIGHT, THERE IS A LOW CHANCE (20%) FOR DRIZZLE, MAINLY AT HAF,
MRY, SNS, APC, AND OAK. CEILINGS ARE EXPECTED TO HANG ON THROUGH
LATE TUESDAY MORNING AT THE EARLIEST. MODERATE ONSHORE WINDS
INCREASE TO 10-15 KT THROUGH THE AFTERNOON AND EASE BELOW 10 KT
OVERNIGHT AS THEY SHIFT SOUTH TO SOUTHWEST, DEPENDING ON THE LOCAL
TERRAIN.
VICINITY OF SFO...MVFR CEILINGS PERSIST WITH MODERATE CONFIDENCE ON
A BRIEF SCATTERING PERIOD THIS AFTERNOON. SOUTHWEST WINDS GRADUALLY
BECOME WESTERLY AND INCREASE TO MODERATE THROUGH THE DAY BEFORE
SWITCHING BACK TO A LIGHT SW FLOW OVERNIGHT. BORDERLINE MVFR-IFR
CEILINGS RETURN EARLY THIS EVENING BETWEEN 5-8 PM PDT WITH MEDIUM TO
HIGH CONFIDENCE ON THE TIMING AND HEIGHTS. THESE CONDITIONS WILL
PERSIST THROUGH EARLY TUESDAY AFTERNOON AS THOSE WESTERLY WINDS
INCREASE. CURRENT MODEL GUIDANCE IS HINTING AT A LOW, NON-ZERO
CHANCE FOR DRIZZLE AT THE TERMINALS EARLY TUESDAY MORNING, BUT
CONFIDENCE IS TOO LOW TO INCLUDE IN THE TAF.
SFO BRIDGE APPROACH...HIGH CHANCE FOR THE BRIDGE TO CLEAR BEFORE
THE TERMINAL AND IS EXPECTED TO DEVELOP CEILINGS SHORTLY AFTER SFO.
MONTEREY BAY TERMINALS...MVFR STRATUS HANGS ON THROUGH THIS
AFTERNOON. LOW CONFIDENCE ON WHETHER MRY WILL FULLY SCATTER OUT DUE
TO THE PERSISTENT ONSHORE FLOW AND EXPANSIVE STRATUS FEED OVER THE
OCEAN. OVERALL, BORDERLINE MVFR-IFR CEILINGS DEVELOP AT BOTH
TERMINALS EARLY THIS EVENING RIGHT BEFORE SUNSET AND DROP TO IFR
OVERNIGHT THROUGH TUESDAY MORNING. MEDIUM CONFIDENCE ON EXACTLY WHEN
IFR CEILINGS DEVELOP. AFTER MIDNIGHT, THERE IS A 20-30% CHANCE FOR
DRIZZLE TO IMPACT THE TERMINALS, THOUGH CONFIDENCE IS TOO LOW TO
INCLUDE IN THE TAF.
MARINE
(TODAY THROUGH SATURDAY)
ISSUED AT 1113 AM PDT MON SEP 21 2026
THE GRADIENT BETWEEN HIGH PRESSURE OVER THE PACIFIC AND LOWER
PRESSURE OVER CALIFORNIA IS SUPPORTING A MODERATE TO STRONG NW
BREEZE ACROSS THE COASTAL WATERS. WINDS ARE STRONGEST NORTH OF PT.
REYES. THE STRONGER WINDS WILL MOVE SOUTH TO IMPACT THE MAJORITY
OF EXPOSED WATERS OVER THE NEXT 36 HOURS. THESE WINDS WILL BUILD
ROUGH SEAS, WITH SIGNIFICANT WAVE HEIGHT PEAKING AT 10-12 FEET
TUESDAY NIGHT. CONDITIONS WILL GRADUALLY IMPROVE WEDNESDAY THROUGH
THURSDAY.
FIRE WEATHER
ISSUED AT 423 AM PDT MON SEP 21 2026
AN UPPER-LEVEL LOW WILL SUPPORT NEAR TO SLIGHTLY BELOW NORMAL
TEMPERATURES, INCREASED DIURNAL WINDS, A DEEP MARINE LAYER, AND A
CHANCE FOR DRIZZLE TODAY. IT IS ALSO BRINGING ELEVATED RAIN
SHOWERS, FORTUNATELY THERE IS A LOW PROBABILITY FOR THUNDERSTORMS
AS LITTLE TO NO RAINFALL IS EXPECTED TO REACH THE GROUND. WARMING
AND DRYING TREND KICKS OFF TUESDAY WITH TEMPERATURES UP TO 10
DEGREES ABOVE NORMAL AND AREAS OF MODERATE HEATRISK WEDNESDAY AND
THURSDAY. THE MARINE LAYER WILL COMPRESS BELOW 1,000 FEET DURING
THIS WARM-UP WITH NOCTURNAL DRYING EXPECTED ABOVE IT.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...SMALL CRAFT ADVISORY FROM 3 PM THIS AFTERNOON TO 9 PM PDT
TUESDAY FOR PT ARENA TO PT REYES 0-10 NM.
SMALL CRAFT ADVISORY FROM 3 PM TO 9 PM PDT TUESDAY FOR PIGEON PT
TO PT PINOS 0-10 NM-PIGEON PT TO PT PINOS 10-60 NM-PT PINOS
TO PT PIEDRAS BLANCAS 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 9 PM PDT TUESDAY FOR PT ARENA TO PT
REYES 10-60 NM.
SHORT TERM...DIALH
LONG TERM....DIALH
AVIATION...NAVARRETE
MARINE...FLYNN
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