FXUS66 KMTR 091223
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
523 AM PDT Sun Aug 9 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1206 AM PDT Sun Aug 9 2026
- Widespread moderate with areas of major HeatRisk for the
interior Sunday
- Near-critical fire weather conditions in the higher terrain
Sunday
- Minor coastal flooding for bayshore locations along the San Francisco
Bay, San Pablo Bay, and Monterey Bay through Thursday morning
&&
.SHORT TERM...
Issued at 1206 AM PDT Sun Aug 9 2026
(Today and tonight)
The 00Z balloon observed a 925 mb temperature of 30.35 degrees
Celsius (third hottest for the date and time), 850 mb temperature of
28.75 degrees Celsius (second hottest for the date and time), and
500 mb height of 5970 meters (record for the date and time).
Satellite shows stratus clearing off the coast due to a
strengthening ACV-SFO surface pressure gradient. Northerly/offshore
flow down the Northern Sacramento Valley Sunday morning will help
push the marine layer back to the coast. Both of these factors
should support Sunday being the hottest day of this event; however,
if the marine layer prevails and northerly/offshore winds don't
develop we could have a repeat of Saturday with our forecast being
too hot for the lower elevations. Nonetheless, temperatures will be
10 to 15 degrees above normal outside of the marine layer influence
with widespread moderate HeatRisk and areas of major HeatRisk in
the higher terrain. Minor coastal flooding is expected during the
highest high tide (evening/overnight) for bayshore locations
along the San Francisco Bay, San Pablo Bay, and Monterey Bay
through Thursday morning. Have opted to leave the Pacific Coast
out as it would be marginal and overnight if it occurred; however,
high tide may cutoff access to some beaches.
&&
.LONG TERM...
Issued at 1206 AM PDT Sun Aug 9 2026
(Monday through Saturday)
Cooling trend kicks off Monday as upper-level shortwave troughing
develops off the West Coast. This will moderate temperatures to near
normal, deepen the marine layer bringing a chance for drizzle, and
increase onshore flow. A reinforcing trough Wednesday will bring
more prominent troughing off the West Coast Thursday and into the
weekend. This along with a resurgence in monsoonal moisture may be
enough to bring elevated rain showers/thunderstorms to the forecast.
While there's no mention in the official forecast now, it may be
warranted as we get closer.
&&
.AVIATION...
(12Z TAFS)
Issued at 523 AM PDT Sun Aug 9 2026
A narrow channel of coastal stratus, fog and mist /MVFR-IFR-LIFR/
have held together and remains trapped beneath a strong lower level
temperature inversion associated with a strong 500 mb high pressure
system. Part of this stratus, fog and mist has reached into the Bay
Area and the north Central Coast. The marine layer depth is approx
1000 feet.
Satellite shows an area of clear sky out over the outer waters and
offshore waters which corresponds to an area of warmer air, primarily
drifting southward. Satellite also shows stratus and fog redeveloping
to our north. The leading edge of this stratus may join the nearly
stationary stratus south of Point Arena during and just after daybreak.
This pattern is characterized by large temperature differences over
short distances, mainly in the vertical and also from the coastline
to inland valleys (sharply warmer aloft, at the August maximum
otherwise not atypical of either July or August). It's noteworthy
that air temperatures in the lower level temperature inversion are
in the 80s and lower 90s. With the wind flow weighted to onshore,
we're essentially in a persistence forecast. Diurnal surface
warming will tap the lower level temperature inversion warmth,
vertical mixing and warming will mix out the stratus, fog and mist
to VFR back to the coastline today. The gradient of surface
pressure remains weighted to some northerly (ACV-SFO) otherwise
onshore per NAM output showing 4 mb to almost 5 mb SFO-SAC during
the afternoon and evening.
Depending how well (or not) the coastal stratus to our north holds
together during mixing today will determine if coastal stratus,
fog and mist return tonight and Monday morning. The NAM has been
verifying a little better than the HRRR model on this forecast.
The NAM leans toward stratus, fog and mist returning tonight and
Monday morning.
Recent GFS/NAM/HRRR output has also showed mid level instability
arriving from southern California approx Monday timeframe, but it
appears to dissipate while running northward into the strong ridge
and mid level dry air just to our north.
Vicinity of SFO...Stratus /IFR/ forecast mix out time is 17z
today. VFR for the remainder of the day then stratus is forecast
to return 10z Monday. West to northwest wind 5 to 15 knots except
15 to 25 knots this afternoon and evening.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Stratus, fog and mist /IFR-LIFR/ mixing
out to MVFR-VFR by late morning. Stratus, fog and mist /IFR-LIFR/
returns tonight and Monday morning. Mainly west to northwest winds
5 to 15 knots.
&&
.MARINE...
(Today through Friday)
Issued at 436 AM PDT Sun Aug 9 2026
Fresh to strong northwest winds persist with moderate seas
building to rough seas up to 12 feet today, lasting through mid
week. Northern coastal waters will be hazardous for small craft
with occasional near gale force gusts possible through early week.
The San Francisco Bay north of the Bay Bridge into the Delta will
have gusty onshore winds through late evening. Long period
southerly swells at around 15 to 17 seconds are still present in
the coastal waters.
&&
.FIRE WEATHER...
Issued at 1206 AM PDT Sun Aug 9 2026
Near-critical fire weather conditions persist today across the
interior due to very hot and dry conditions. Strong thermal belting,
extended burn periods, elevated PIG, and near or record BI/ERC
values are expected. Temperatures will be 10 to 15 degrees above
normal with a compressed marine layer. Poor minimum daytime relative
humidity as low as 10% in the afternoons with little-to-no recovery
overnight is expected with nocturnal drying in the highest terrain.
Even with critically dry conditions, wind is the limiting factor
preventing this from being a Fire Weather Watch/Red Flag Warning
event. Outside of the diurnal sea breeze that will bring breezy
conditions in the afternoons and evenings through gaps and passes,
onshore flow will be light. Additionally, critical conditions are
not expected to meet coverage or duration requirements to warrant a
product. Localized elevated fire weather conditions in the highest
terrain can be expected Monday due to the combination of recent
hot/dry conditions and the increase in onshore flow. Depending how
the pattern and marine layer evolves, a headline may be warranted
and this could last through Wednesday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 2 AM PDT Thursday for CAZ006-506-
508-529-530.
Heat Advisory until 10 PM PDT this evening for CAZ503-504-510-
512>518.
PZ...Small Craft Advisory from 9 AM this morning to 9 PM PDT this
evening for SF Bay N of Bay Bridge.
Small Craft Advisory from 9 AM this morning to 3 AM PDT Monday
for Pt Arena to Pt Reyes 0-10 nm.
Small Craft Advisory until 9 PM PDT Monday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Canepa
MARINE...Canepa
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Prepared by Boulder-Creek.com Weather at: Sun Aug 9 08:30:02 PDT 2026
From the National Weather Service
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