FXUS66 KMTR 051725
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1025 AM PDT Wed Aug 5 2026
...New UPDATE, MARINE, FIRE WEATHER...
.KEY MESSAGES...
Updated at 1148 PM PDT Tue Aug 4 2026
- Elevated fire weather threat across the interior North and East
Bays, and the Diablo and Santa Lucia ranges through midweek.
- Widespread Moderate HeatRisk returns for the end of the work
week.
- Critical fire weather conditions develop Friday and Saturday.
&&
.UPDATE...
Issued at 950 AM PDT Wed Aug 5 2026
Solid marine layer this morning impacting the Bay Area and
Central Coast. As another forecaster in the office pointed out
this morning pretty impressive to see an over performing marine
layer deepen to nearly 1800 feet with anomalously high 500mb
heights. Not as much drizzle being reported this morning, but the
ample moisture in the low levels has led to mist. Above the marine
layer is a completely different story. High elevation sensors
showed another classic nocturnal drying event this morning.
Sensors showed humidity "recovery" to around 30 percent yesterday
evening then humidity values plummeted through sunrise as the
marine layer moved inland. This would definitely be a fire weather
concern, but fortunately winds are relatively light. Other item
of note this morning is some high level smoke being picked up by
satellite lurking over the coastal waters and potentially the
North Bay. Surface sensors are not showing it reaching the ground
as of now therefore it's likely high level drift smoke.
For today - the over performing marine layer is bringing much
needed relief to the interior. No Heat Advisories are needed as
temperatures cool and HeatRisk concerns decreases. The marine
layer will slowly ease back to the coast, but linger for the day.
No update needed this morning.
Will be focusing on another round of heat later in the week and
developing near critical fire weather conditions.
MM
&&
.SHORT TERM...
Issued at 1148 PM PDT Tue Aug 4 2026
(Tonight through Thursday)
The picture from space this evening shows stratus snaking through
the the region. Earlier model guidance struggled with the onset of
its arrival and to some extent the depth as well. It's currently
around 1000ft and has the potential to get to 1200ft, with coastal
drizzle and patchy fog possible tonight and into Wednesday morning.
The good news is that this will cool temperatures overnight and
perhaps take a bit longer to retreat to the coast Wednesday morning.
With ridging not as strong and those factors in place, the threat
for Moderate HeatRisk has been pulled back with only a few pockets
found across the interior Central Coast and interior E and S Bay
regions. Some of those pockets include areas above the marine layer,
where temperature recovery won't be as great. Like the prior shift,
will not be issuing a Heat Advisory for Wednesday.
Wednesday night into Thursday morning, the forecast will be on
repeat. Look for the marine layer and associated stratus to seep
into the region leading to another cloudy night.
&&
.LONG TERM...
Issued at 1148 PM PDT Tue Aug 4 2026
(Thursday night through next Tuesday)
The ridge starts to build back into the region on Thursday, which
will compress the marine layer to perhaps 500ft for the next few
days. At the surface, winds will have shifted from the south to a
more north/northwesterly flow; however, aloft, guidance shows the
southerly flow holding. Deterministic models and ensembles show a
slug on moisture pushing into our region Thurday-Saturday giving us
PWATs ranging from 0.75-1.0, which could lead to things feeling
humid. In terms of the heat portion of the forecast, it looks to be
on track. Data continues to signal 850mb temps in the 28-30C range,
leading to toastier daytime and overnight temperatures. Highs,
especially for interior areas and those not affected by the marine
layer, will be in the 90s to low 100s while low temperatures there
fall into the upper 60s-70s. Widespread Moderate HeatRisk is
expected, with some pockets of Major HeatRisk on Saturday.
Sunday and Monday, the upper level pattern shows the ridge
flattening some. The latest NBM seems to keep temperatures a tad
warmer on Sunday, with the more notable interior cooling trend
occuring on Monday and Tuesday.
&&
.AVIATION...
(12Z TAFS)
Issued at 509 AM PDT Wed Aug 5 2026
Strong 500 mb high pressure remains over the area, however there
is also a stationary and weak surface low pressure system over the
CA coastal waters within the marine layer. Counter-clockwise winds
with the surface low is entraining higher surface dewpoint air (water
vapor) from the west, the water vapor upon closing in on the CA
coastline is crossing a cooler gradient in sea surface temperatures.
This is resulting in stratus and mist/fog /MVFR-IFR/ reaching the
coastline and inland on onshore winds. Even under strong high pressure,
the marine layer has deepened to ~ 1800 feet. It's sharply drier and
warmer aloft at and above the marine layer temperature inversion,
supporting nocturnal radiative cooling to space. Onshore winds are
supported by a 2.4 mb SFO-SAC pressure gradient. It'll take the
morning for stratus, mist/fog to mix out to VFR.
Wildfire smoke from the Pacific Northwest may also get entrained
into the aforementioned low over the coastal waters today based on
recent high resolution near surface smoke forecasts. If smoke is
drawn into the stratus, this may lower surface visibilities in a
mix of smoke, haze and mist today.
Coastal stratus, mist/fog /MVFR-IFR/ return inland on onshore winds
tonight and Thursday morning.
Vicinity of SFO...Stratus /MVFR/ until 20z today then mixing out
to VFR. Stratus returns late tonight-early Thursday by 10z. Light
and variable wind becoming northwest 10 to 15 knots in the afternoon
and evening.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Stratus /IFR/ until late morning then
mixing out to MVFR-VFR. Stratus /IFR/ returns this evening and
overnight. Mainly northwest winds 5 to 15 knots.
&&
.MARINE...
(Today through Monday)
Issued at 950 AM PDT Wed Aug 5 2026
Light to gentle south to southwesterly breezes will persist
through Thursday along with seas between 2-6 feet. Winds will
begin to increase again out of the northwest going into Friday
with building seas and hazardous conditions for small craft
returning for the upcoming weekend.
&&
.FIRE WEATHER...
Issued at 1000 AM PDT Wed Aug 5 2026
Marine layer brought relief to lower elevations this morning, but
above the marine layer another nocturnal drying event bringing
single digit RH. Simply put, little to no recovery last night.
Cooler and slightly more humid conditions are expected across the
interior today and Thursday. Despite the "cooldown" temperatures
across the interior will still be in the 90s to low 100s Wed/Thu
with min RH teens. Still expecting an onshore push each
afternoon/evening with breezy to gusty winds leading to a few
hours of elevated fire weather conditions.
High pressure will build back over the district late Thursday into
Friday. As such, the marine layer will become more compressed and
interior heat increasing and humidity decreasing. Peak times of
greatest concern will be each afternoon/evening Sat/Sun. This will
push Energy Release Component (ERC) back up to critical levels.
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 continue to respond 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.
Lastly, we're tracking some high level moisture streaming in from
the south Friday and over the weekend. As of right now, upper
level instability seems limited so high based thunderstorms are
not likely. If anything, some high cirrus is possible with some
higher dew points.
MM
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...KR
LONG TERM....KR
AVIATION...Canepa
MARINE...RGass
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Prepared by Boulder-Creek.com Weather at: Wed Aug 5 10:30:02 PDT 2026
From the National Weather Service
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