FXUS66 KMTR 050514
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1014 PM PDT Tue Aug 4 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 109 PM PDT Tue Aug 4 2026
- Moderate HeatRisk in the interior today decreasing midweek.
- Elevated fire weather threat across the interior North and East
Bays, and the Diablo and Santa Lucia ranges through the week.
- Widespread Moderate HeatRisk returns for the end of the work
week.
- Critical fire weather conditions develop Friday and Saturday.
&&
.UPDATE...
Issued at 911 PM PDT Tue Aug 4 2026
The Heat Advisory for the East and South Bays was just cancelled
due to temperatures cooling and the marine layer seeping into the
the region. Expect the marine layer to get to around 1000ft
tonight, perhaps even closer to 1200ft given the expansion we've
already seen. This should help bring low temperatures into the 50s
for much of the region, with those above the marine layer and the
thermal belts remaining dry and warm. The forecast for tomorrow
looks to be on track.
&&
.SHORT TERM...
Issued at 109 PM PDT Tue Aug 4 2026
(This evening through Wednesday night)
Rest of today: Marine layer rolled back to the coast, but is
hanging pretty solid along the coastline. Lingering marine layer
plus a slightly stronger onshore flow equals an impressive 24 hour
trend. Temperatures as of 1 pm are currently running a few
degrees cooler to as much a 20 degrees than Monday. Warmest
locations in the forecast area are across the interior with
temperatures still in the mid 90s to low 100s even with the
cooling trend. Will keep Heat Advisory as is for now through 10
PM, but it does appear some spots may fall short of their forecast
highs.
Additionally, visible satellite imagery also shows smoke lurking
over the coastal waters. This smoke is actually from the PacNW
fires. Latest smoke model brings some high level smoke over the
Bay Area later today, which may contribute to some haze or just a
pretty sunset.
Tonight through Wednesday night:
Marine layer along the coast will march inland again tonight. Have
added another round of coastal drizzle. Depth of the marine layer
will remain around the 1k ft mark. Above the marine layer will
remain mild and dry with active thermal belts.
The cooldown experience today will continue on Wednesday thanks
to persistent onshore flow, marine layer influence, a weak
disturbance overhead riding the periphery of the upper ridge.
Event with a "cooldown" far interior locations will be well into
the 90s with a few making a run at 100 degrees. High probability
of exceeding 100 degree will be interior Monterey/San Benito
counties and far interior East Bay. Given the slightly cooler
temperatures HeatRisk values decrease. As such, will not be
issuing a Heat Advisory at this time for Wednesday.
Wednesday night will feature another solid push from the marine
layer, but maybe a tad more compressed given building H5.
&&
.LONG TERM...
Issued at 148 PM PDT Tue Aug 4 2026
(Thursday through next Monday)
Thursday will be a transition day as marine layer compresses and
the upper level ridge makes its presence known again. Temperatures
will be moderating with a large difference with cool temps at
coast and much hotter temperatures inland. Basically, in the big
picture nothing major from what we've been seeing. Just that temps
will be cool enough to keep HeatRisk values in the Minor to
Moderate categories.
Friday through Saturday...the upper level ridge begins to re-assert
its dominance with building 500mb heights and warming 850mb temps
back to critical levels. The building H5 will further compress the
marine layer. The marine layer will remain however. Really need
offshore flow to get rid of the marine layer at this point. 850mb
temps soar back into the 28-30C range, yielding inland max temps
in the low 100s and overnight lows in the 70s. That combo will
increase HeatRisk back into the upper end of Moderate to lower
Major. Therefore, we'll likely need a mix of Heat Adv for interior
locations Friday/Saturday. Still looking above normal at the
coast/bays, but natural AC will keep the real heat inland.
Interesting longwave pattern in the extended as the upper level
ridge looks to become more "split" with a system sweeping through
PacNW. The easing upper level ridge will allow for subtle cooling
across the interior for Sunday and Monday. We'll need to take a
closer look if any Heat Adv will be needed on Sunday since heat
impacts are cumulative.
&&
.AVIATION...
(06Z TAFS)
Issued at 1012 PM PDT Tue Aug 4 2026
Stratus has begun a somewhat triumphant return inland after sunset
tonight. Monterey Bay will soon be IFR/LIFR for the remainder of the
night into Wednesday morning. North Bay, Bay Area and South Bay
terminals are VFR for the moment but should follow suit by midnight
except for South Bay sites, which will take longer, closer to
sunrise. A more organized marine layer around 1000ft maybe as high
as 1500ft for some spots, is spilling over the terrain into the
North Bay attm. A surface trough enhanced by thermal troughing this
afternoon and evening continues to pull stratus farther inland
through the Golden Gate into the bays, with more of the same farther
south into Monterey Bay.
Vicinity of SFO...Similar to SFO.
SFO Bridge Approach...Currently VFR, and while the stratus continues
to fill in through the Golden Gate, it will take more time to
fill in around KSFO, considering it has already begun surrounding
KOAK. An offshore meso- low (Otter Eddy) will linger up and down
the coast north of the Golden Gate tonight and tomorrow, which
will result in light and variable winds at times, occasionally
more established from the south and west. A decent thermal
inversion (H85 25-27C through the morning) and gradual increase
surface pressure under the inversion may trap the stratus a little
longer than guidance is suggesting.
Monterey Bay Terminals...Expecting IFR/LIFR through the remainder
of the night with a more normal diurnal lifting to VFR than
farther north, by early afternoon. The window for VFR may only be
4-5hrs long with stratus expected to return well before sunset
early Wednesday evening.
&&
.MARINE...
(Tonight through next Monday)
Issued at 1012 PM PDT Tue Aug 4 2026
Light and gentle breezes will continue over the outer and coastal
waters through the work week along with seas between 4-8ft. Winds
will begin to increase again out of the northwest going into the
weekend with building seas and hazardous conditions for small
craft returning.
&&
.FIRE WEATHER...
Issued at 155 PM PDT Tue Aug 4 2026
After coordinating with neighboring offices and fire partners
fire weather concerns over the next 5 to 7 days remain with
elevated to near critical conditions.
Temperatures remain around 10-15 degrees above normal across the
interior with the exception of Wednesday and Thursday where
temperatures are more seasonal. High temperatures largely remain
in the upper 80s to 90s with the warmest locations reaching the
low 100s. Daytime humidity retention remains poor across the
interior (15-30%) with humidity values dropping into the 10s
across the interior Central Coast, East Bay Hills/Valleys, and the
higher elevations. Portions of the interior Central Coast may see
daytime humidity values drop into the single digits Tuesday, then
again towards the end of the week into the weekend. Overnight RH
recovery remains poor at locations above 2,000 feet with values
between 25 to 40% expected with some favored locations struggling
to reach even 25%. This will be particularly true across the
Mayacamas, highest peaks of the East Bay Hills, the Diablo Range,
the Santa Cruz Mountains, the Santa Lucia and Gabilan Ranges.
Nocturnal drying (rapid decrease in RH through the pre- dawn
hours) may also transpire which will increase the burn period or
time in which fire behavior may remain active. Energy Release
Component (ERC) values have climbed above the 97th percentile
within the Interior Central Coast and Diablo-Santa Cruz (Diablo
Mountains and Santa Cruz Mountains) zones with the Mid Coast to
Mendocino Zone (North Bay) to reach the 97th percentile this week.
Large fires become more frequent after ERC values cross the 90th
percentile with extreme burning conditions expected with ERC
values above the 97th percentile. 100 hr fuels are also responding
to the hot and dry conditions with values reaching critical
levels across the region. Latest intel also shows Live Fuel
samplings are also heading toward critical levels. The one saving
grace is that we're not predicting any large wind events or
offshore flow wind events during this time period. That being
said, diurnally breezy onshore winds are expected along the coast,
mountain gaps and passes, and along the ridgelines each
afternoon/evening. Given the elevated fire weather concerns and
very dry fuels, individual across the highest risk areas (interior
North and East Bays, the Diablo Range, the Santa Lucia Range, and
interior Central Coast) are advised to avoid activities that may
spark a fire. This includes campfires/barbecues and making sure
chains are secured to avoid sparks if towing anything. Fire crews
are urged to exercise extra caution when executing any prescribed
burns, especially where fuels, topography, and weather align.
MM/KR
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...JM
MARINE...JM
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Prepared by Boulder-Creek.com Weather at: Tue Aug 4 22:30:02 PDT 2026
From the National Weather Service
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