751
FXUS66 KMTR 210701
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1201 AM PDT MON SEP 21 2026
..NEW SHORT TERM, LONG TERM
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
(06Z TAFS)
ISSUED AT 934 PM PDT SUN SEP 20 2026
SATELLITE SHOWS MID TO HIGH LEVEL CLOUDS AND KMUX SHOWS A FEW
HIGH BASED SHOWERS, OTHERWISE VFR PREVAILS EXCEPT FOR AREAS OF
COASTAL STRATUS /MVFR-IFR/. ONSHORE WINDS WILL BRING STRATUS AND
PATCHY DRIZZLE /MVFR-IFR-LIFR/ INLAND TONIGHT AND MONDAY MORNING.
CONDITIONS IMPROVE TO MVFR-VFR NEAR THE COAST MONDAY AFTERNOON,
VFR PREVAILS INLAND. STRATUS AND A FEW PATCHES OF LIGHT DRIZZLE
REDEVELOP MONDAY NIGHT AND EARLY TUESDAY MORNING.
VICINITY OF SFO...STRATUS /MVFR/ AND A WEST WIND NEAR 10 KNOTS
TONIGHT AND MONDAY MORNING. MVFR STRATUS CEILING FORECAST UNTIL
19Z MONDAY. STRATUS /MVFR/ REDEVELOPS MONDAY EVENING AND NIGHT.
WEST WIND 15 KNOTS MONDAY AFTERNOON TO MID EVENING.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...STRATUS /MVFR-IFR-LIFR/ INCLUDING PATCHY
LIGHT DRIZZLE TONIGHT AND MONDAY MORNING. CONDITIONS IMPROVE TO
MVFR-VFR BY LATE MONDAY MORNING AND AFTERNOON. STRATUS /MVFR-IFR/
REDEVELOPS MONDAY NIGHT. ONSHORE WINDS 5 TO 15 KNOTS.
MARINE
(TONIGHT THROUGH NEXT SATURDAY)
ISSUED AT 901 PM PDT SUN SEP 20 2026
AN UPPER LEVEL AREA OF LOW PRESSURE WILL KEEP PRESSURE GRADIENTS
GENERALLY LIGHT ACROSS THE SOUTHERN WATERS TONIGHT INTO MONDAY. AS
THE LOW MOVES INLAND SURFACE PRESSURE GRADIENTS WILL INCREASE OVER
THE NORTHERN OUTER WATERS WITH INCREASING NORTHWEST WINDS FOR THE
OFFSHORE WATERS FROM POINT ARENA SOUTHWARD TOWARDS POINT REYES
RESULTING IN BUILDING SHORT PERIOD SEAS. LIGHTER WIND AND SEAS
CONTINUE ACROSS THE SOUTHERN WATERS THROUGH MIDWEEK AS HIGH
PRESSURE REBUILDS.
FIRE WEATHER
ISSUED AT 416 AM PDT SUN SEP 20 2026
NEAR TO SLIGHTLY BELOW NORMAL TEMPERATURES, DIURNAL WINDS, A DEEP
MARINE LAYER, AND A CHANCE FOR DRIZZLE ARE ON TAP FOR SUNDAY AND
MONDAY. WARMING AND DRYING TREND KICKS OFF TUESDAY WITH TEMPERATURES
UP TO 15 DEGREES ABOVE NORMAL BY THURSDAY. AREAS OF MODERATE
HEATRISK ARE EXPECTED AS A RESULT WEDNESDAY AND THURSDAY. THE MARINE
LAYER WILL COMPRESS BELOW 1,000 FEET DURING THIS WARM-UP WITH
NOCTURNAL DRYING EXPECTED ABOVE IT. DIURNAL WINDS WILL PREVAIL;
HOWEVER, THEY WILL BE WEAKER THAN THE PRECEDING DAYS.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 3 AM PDT EARLY THIS MORNING FOR
PIGEON PT TO PT PINOS 10-60 NM-PT PINOS TO PT PIEDRAS
BLANCAS 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 3 PM PDT TUESDAY FOR PT ARENA TO PT
REYES 10-60 NM.
SHORT TERM...DIALH
LONG TERM....DIALH
AVIATION...CANEPA
MARINE...CANEPA
VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO
FOLLOW US ON FACEBOOK, X, AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSBAYAREA
X.COM/NWSBAYAREA
WWW.YOUTUBE.COM/NWSBAYAREA
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page