FXUS66 KMTR 071226
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
520 AM PDT Fri Aug 7 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1211 AM PDT Fri Aug 7 2026
- Warming trend through the weekend will result in moderate to
major HeatRisk for the interior
- Near-critical fire weather conditions in the higher terrain
through Sunday
- Minor coastal flooding for bayshore locations along the San
Francisco Bay, San Pablo Bay, and Monterey Bay Friday evening
through Thursday morning
&&
.SHORT TERM...
Issued at 1211 AM PDT Fri Aug 7 2026
(Friday through Sunday)
Upper-level ridging across the Four Corners will expand westward
Friday. Areas of moderate HeatRisk across the interior on Friday
will expand to become widespread on Saturday and Sunday with areas
of major HeatRisk. Saturday and Sunday will be the hottest days of
the week with temperatures 10-15 degrees above normal. Subsidence
aloft will compress the 1,000 foot marine layer, keeping relief
confined to the immediate coast. Near-critical fire weather
conditions persist through the weekend across the interior due to
very hot and dry conditions. 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 Friday evening through Thursday morning. The Pacific Coast may
need to be added Sunday through Tuesday, it will depend on how the
positive anomaly performs leading up to it.
&&
.LONG TERM...
Issued at 1211 AM PDT Fri Aug 7 2026
(Monday through Thursday)
Global ensemble clusters are in agreement that upper-level shortwave
troughing will develop Monday off the West Coast between subtropical
high pressure across the Eastern Pacific Ocean and the Four Corners
high. This will result in temperatures moderating to near normal and
the return of a marine layer around 1,000 feet. The surface pressure
gradient will support northerly winds across the waters that will be
hazardous to small craft.
&&
.AVIATION...
(12Z TAFS)
Issued at 520 AM PDT Fri Aug 7 2026
Mid and high clouds are moving northward over the forecast area
and there may be a few high based light showers/virga based on
KMUX data. Strong 500 mb high pressure continues and the lower
level temperature inversion prevails in the hills and mountains.
At the same time, the marine layer is well established with a cool
air intrusion on northwest winds along the coastline and into the
nearby coastal valleys. The SFO-SAC pressure gradient is 2.9 mb,
1 mb greater than 24 hours ago, a decent onshore gradient for 5
am.
At the moment the overall pattern is stable and not likely to
become disrupted. Even though mid and high clouds are present and
slowing radiative cooling, cool air advection is taking place
within the marine layer, supporting stratus, fog and mist /MVFR-
IFR-LIFR/. It's favorable thermodynamic conditions e.g. at the
air/sea interface for stratus and fog and btw, kudos to the NAM
1000 mb forecasts for helping to quickly identify (and observations
to verify) this shallow cool air intrusion. Cloud ceilings have
lowered to IFR-LIFR during the overnight hours because of favorable
air/sea conditions and a slight decrease in the marine layer depth.
This complex cloud pattern mix will carry through morning, the
question is how long will it take to mix out the stratus, fog and
mist under August sun angle surface heating? Some delay in surface
heating means potential delay for stratus, fog and mist to mix out
to VFR. Interestingly, weak thermal troughing is located at the
500 mb level today and is slow to move northward out of the
forecast area. Timing mix out is a low confidence forecast, with
uncertainty will go with late morning mix out, but it may also take
another couple hours to fully mix out along the coastline. The NAM
forecasts upwards of 4 mb SFO-SAC pressure gradient in the afternoon.
So far the marine layer has essentially been deeper compared to model
forecasts. Satellite shows widespread stratus and fog over the coastal
and offshore waters. With onshore winds dominating, expect stratus,
fog and mist /MVFR-IFR-LIFR/ to return inland tonight and Saturday
morning.
Vicinity of SFO...Stratus /IFR/ until 18z today, but caveat is
the current combination of onshore winds, a cooler air intrusion,
mid and high clouds may end up slowing mix out to VFR by another
hour or two today. Stratus /IFR/ is forecast to return mid evening
and continue tonight and Saturday morning. West to northwest wind
5 to 15 knots.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Stratus /IFR-LIFR/ improving to MVFR by
late morning. The surface cool air intrusion affecting vicinity
KSFO, KOAK, KHAF and areas southward along the coast is still
arriving here at this hour. While mid and high clouds are drifting
overhead, this will at least temporarily slow diurnal warming while
the cooler air undercuts the lower level temperature inversion.
Caveat, here too it may take another couple hours for stratus
/IFR-LIFR/ to lift to MVFR-VFR which may turn out to be by early
afternoon. West to northwest winds 5 to 15 knots.
&&
.MARINE...
(Today through Wednesday)
Issued at 428 AM PDT Fri Aug 7 2026
Northwest winds will gradually increase to a fresh to strong
breeze, particularly across the outer northern waters on Saturday
through the middle of next week. Moderate northwest seas also
build to rough by Sunday morning continuing into the early part of
next week. Winds and seas remain elevated through the middle of
next week. A long period southerly swell of 15 to 17 seconds will
impact the coastal waters beginning Saturday morning.
&&
.FIRE WEATHER...
Issued at 1211 AM PDT Fri Aug 7 2026
Near-critical fire weather conditions persist through the weekend
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.
Sarment
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory from 5 PM this afternoon to 2 AM PDT
Thursday for CAZ006-506-508-529-530.
Heat Advisory from 10 AM Saturday to 10 PM PDT Sunday for CAZ503-
504-510-512>515-517-518.
PZ...Small Craft Advisory from 3 PM to 9 PM PDT Saturday 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: Fri Aug 7 06:30:02 PDT 2026
From the National Weather Service
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