FXUS66 KMTR 042058
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
158 PM PDT Tue Aug 4 2026
...New KEY MESSAGES, SHORT TERM, LONG TERM, FIRE WEATHER...
.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.
&&
.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...
(18Z TAFS)
Issued at 1036 AM PDT Tue Aug 4 2026
MVFR-IFR stratus continues to clear out of the SF Bay and Monterey
Bay regions, and is expected to linger at the immediate coastal
regions. Gentle to moderate onshore winds are expected to develop
in the afternoon and evening with speeds of around 10 to 15 kt.
Stratus will return to the valleys tonight, with some more inland
expansion expected. Moderate confidence in stratus impacts at SJC
overnight into Wednesday morning. Smoke transport from fires
across the Pacific Northwest may result in impacts to slant
visibility due to hazy conditions.
Vicinity of SFO... Stratus has cleared out somewhat early as light
southerly flow impacted the terminal, with the winds turning to the
northwest this afternoon and evening, as speeds reach 10 to 15 kt.
MVFR-IFR stratus will return to the terminal region tonight, and
persist into Wednesday morning. Winds will turn more to the west and
the southwest at around 10 kt for Wednesday afternoon.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... SNS has scattered out to VFR, with VFR
conditions continuing through the afternoon. At MRY, IFR-MVFR
stratus continues through the morning with moderate to high
confidence that a brief period of VFR skies develops in the
afternoon and early evening hours. Moderate northwest winds will
develop in the afternoon and evening hours, becoming light
overnight as LIFR-IFR stratus returns to the region. Clearing of
the stratus is not expected until after the end of the TAF period.
&&
.MARINE...
(Today through Sunday)
Issued at 1000 AM PDT Tue Aug 4 2026
Light to gentle breezes will persist across the majority of the
coastal waters through late week. A localized gentle to moderate
breeze continues across the inner waters. Seas continue to abate
with wave heights between 4-8 feet expected through the later part
of the week. The Pacific High strengthens and shifts eastward
late this week into the weekend, resulting in strengthening winds
and building seas with a return of hazardous conditions for small
craft.
&&
.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...Heat Advisory until 10 PM PDT this evening for CAZ510-513>515.
PZ...None.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...DialH
MARINE...DialH
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Prepared by Boulder-Creek.com Weather at: Tue Aug 4 14:30:02 PDT 2026
From the National Weather Service
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