FXUS66 KMTR 081139
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
439 AM PDT Tue Sep 8 2026

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 1130 PM PDT Mon Sep 7 2026

  - Warming and drying trend will bring widespread moderate to 
    major HeatRisk through Thursday

&&

.SHORT TERM...
Issued at 1151 PM PDT Mon Sep 7 2026
(Tonight through Wednesday)

24 hour trends are warmer and drier. Already a warming trend is 
underway with increasing thermal and height ridging aloft. Any
marine layer influence is confined to within a few hundred feet 
of sea level with only a few patches of fog along the Big Sur 
coast at the moment.

The large scale pattern shift to warmer/hotter temperatures 
involves both the retrogression (westward motion) of the 
subtropical ridge over the central U.S. and the eventual merging 
of tropical energy and moisture from steadily decaying TS Marie 
and a cut-off low far to our west (also forecast to retrogress). 
Strengthening 500 mb heights, large scale compression and the Sep 
sun angle will raise temperatures up further Tuesday, peaking well
above normal both Wednesday and Thursday. A little bit of a 
wildcard to this week's pattern is the northward intrusion of 
tropical moisture including some warm air advection (far from 
decaying TS Marie) which will be west of the 500 mb ridge axis. 
Higher to much higher precipitable water will arrive here, model 
forecasts are still showing near 1.40" in the Bay Area mid-week, 
near the max moving average for Oakland upper air early-mid Sep 
climatology. Mid and high clouds will increase, but we don't 
really know quite yet with high confidence how well the models may
or may not be handling mid-level convective potential (mucape) 
with disagreement among various output from e.g. from the HRRR and
GFS. Good to see the GFS shows very minimal to no mucape anyhow. 
Also, with hotter surface temperatures (greater internal energy) 
air parcels will stay capped as long as the atmosphere is stable.

Hot temperatures for this week are not quite in reach of long term
record highs at this point, except Half Moon Bay may get close if
there's no shallow sea breeze mid-week as an offset to the 
temperature. Will continue to monitor.

A Heat Advisory is in effect for the entire forecast area from 10
AM Wednesday to 10 PM Thursday. We've had a nice 10 to 11 day run
of below normal temperatures (due to a large scale trough), 
especially inland, so the sudden reversal in temperatures will be 
noticeable if not already. With widespread moderate to major 
HeatRisk, this affects most individuals sensitive to heat, 
especially those without effective cooling and/or adequate 
hydration. Please stay out of direct sunlight, and stay properly
hydrated. Hot conditions can cause heat illnesses.

Also, minor coastal flooding is likely within a small window of 
time of about 1 hour nearest high tide Tuesday (possibly Wednesday
as well) for bayshore locations of Marin County. Impacts will 
include minor flooding of the lowest lying areas directly along 
the shoreline. At the San Francisco tidal gauge, high tide is 
expected to be 1 ft above normal at 9:59 PM 9/8. At this time we 
are messaging it here and will include messaging in social media 
vs issuing a coastal flood advisory for Tuesday.

The Beach Hazards Statement was allowed to expire.

&&

.LONG TERM...
Issued at 1151 PM PDT Mon Sep 7 2026
(Wednesday night through next Monday)

Well above normal hot weather continues Thursday. Thereafter
northwest winds and a gradual weakening of the 500 mb ridge will
bring a cooling trend for late in the week and weekend. Daytime
highs will cool back to near normal Friday, then possibly a few
degrees below normal for the upcoming weekend. Late week weather
will be dry with the mid-latitude pattern settling back in.

&&

.AVIATION...
(12Z TAFS)
Issued at 410 AM PDT Tue Sep 8 2026

With light offshore flow the marine layer is struggling to form, 
and VFR conditions are expected through the day. There is a 
slight chance of low ceilings at the coastal terminals this 
morning, but any stratus that manages to form will be battling 
dry air and should clear by the early afternoon. Winds will be 
lighter than normal, with a moderate onshore breeze in the 
afternoon. High clouds will be nearly overcast at times this 
afternoon and evening.

Vicinity of SFO...High confidence in VMC through the TAF period.
GFLAMP shows less than 10% chance of ceilings below 3,000' this
morning. The wind direction is a little more uncertain for the
first portion of the TAF. Light WNW winds are currently being 
reported, but the typical morning NE breeze is likely to become 
established for a few hours before the sea breeze takes over in 
the afternoon. High cloud coverage will increase through the day.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...The VFR forecast is a little more
vulnerable here, with a 20% chance of low ceilings before 18Z. A
cyclonic eddy has been moving north along the Central Coast
overnight, with a bank of coastal stratus forming in the onshore
wind along its southern flank. As this feature continues north, 
it could impact Monterey, but the SW direction and shallow surface
inversion makes it tricky to actually reach the terminal. For
wildfire aviation purposes, the localized marine layer south of
the eddy is 500 ft deep. With SW flow, the stratus that has 
formed along the Big Sur Coast may not clear until well into the 
afternoon. On the other hand, there is very high confidence in 
VFR conditions today at the inland aviation bases.

&&

.MARINE...
(Today through Sunday)
Issued at 410 AM PDT Tue Sep 8 2026

Tuesday will feature a gentle NW breeze with a moderate-rough SW
swell. The swell will subside over the next couple of days, while
the NW winds increase to a moderate breeze.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory from 10 AM Wednesday to 10 PM PDT Thursday for 
     CAZ006-502>506-508>510-512>518-528>530.

PZ...None.
&&

$$

SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...Flynn
MARINE...Flynn

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Prepared by Boulder-Creek.com Weather at: Tue Sep 8 06:30:03 PDT 2026