FXUS66 KMTR 060454
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
954 PM PDT Wed Aug 5 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 2 PM PDT Wed Aug 5 2026
- Elevated fire weather threat across the interior North and East
Bays, and the Diablo and Santa Lucia ranges through midweek.
- Widespread Moderate and areas of Major HeatRisk returns for
the end of the work week.
- Critical fire weather conditions develop Friday and Saturday.
&&
.SHORT TERM...
Issued at 200 PM PDT Wed Aug 5 2026
(This evening through Thursday night)
While not as extreme as yesterday, today's 24 hour trend still
indicates a downward trend of an additional 2 to 10 degrees.
Persistent onshore flow and lurking marine layer led to additional
cooling observed today. A welcomed relief for the interior to get
a break before the next round of heat.
Tonight through Thursday night:
Despite high pressure overhead the marine layer will continue to
be the main driver of overall weather in the Bay Area and Central
Coast broadly speaking. Clouds will move inland at night and clear
back to the coast in the afternoon. Greatest impact of the marine
layer will be near the coast/bays and lower elevations. Thermal
belts remain active with the hills remains mild and dry through
the short term. Thursday will once again feature a broad range of
temps from the coast to the interior.
Onshore flow will also bring some breezy winds each
afternoon/evening. Additionally, some high level smoke may lead to
some haze.
&&
.LONG TERM...
Issued at 230 PM PDT Wed Aug 5 2026
(Friday through next Tuesday)
No significant changes for Friday through the weekend from
previous forecast. The upper level ridge will flex again over the
region bringing higher 500mb heights and even warmer 850mb temps.
As a result of the large synoptic set up we'll see the following
in response: compression of the marine layer, increase of
temperatures through Saturday and then moderating on Sunday,
increasing HeatRisk values to include Major, decreasing RH, and
enhancement of the thermal belts. Less confident for a Heat
Advisory across the interior on Friday, but very confident we'll
need a Heat Advisory on Saturday. It's also possible that we may
need one on Sunday as well since heat impacts are cumulative and
we'll be day three of the warm up. Starting to sound like a broken
record at this point, but the onshore flow/marine layer will keep
the natural AC at the coast/bays. We'll see above normal temps,
just not to critical levels. The upper level ridge eases its grip
by Monday/Tuesday with subtle cooling as the heat shifts back
toward the Central Valley.
One other item of note for the long term will be the influx of
upper level moisture. Guidance still shows a solid push of high
PWAT air from SoCal. The leading edge will nose in as early as
Thursday, but the bulk of it will arrive Thursday night through
through early Saturday. We do have some steep lapse rates aloft,
but thankfully instability is somewhat lacking. Having the ridge
in place helps. Really need to have a trigger for convection to be
a concern, like being on the edge of the ridge with a shortwave
trough or vort max. Therefore, expecting mainly cirrus at this
point. Will continue to monitor regardless.
&&
.AVIATION...
(06Z TAFS)
Issued at 953 PM PDT Wed Aug 5 2026
VFR prevails early tonight with only KHAF and KSNS with MVFR
ceilings thus far. The stratus push inland isn't as dramatic as
thought just three hours ago, but confidence remains high that it
will eventually spread over most of our area below 1500ft by sunrise
Thursday. With high pressure continuing to build, expect VFR
conditions by late morning for most terminals, possibly early
afternoon for the Monterey Bay sites as a compressed marine layer
retreats back to the coast.
Vicinity of SFO...Stratus lingering in vicinity of the terminals but
currently slow to make much more progress. The inland push isn't
taking hold as quickly as though just a few hours ago, but
confidence is high that IFR/LIFR ceilings will develop after
midnight and persist through late morning. VFR through the
remainder of the forecast period once stratus retreats back to the
coast.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...MVFR ceilings will become IFR/LIFR after
midnight and continue through late morning at KSNS, possibly
early afternoon for KMRY. Moderate to high confidence that
IFR/LIFR ceilings return by early evening before sunset with VFR
most of the afternoon on Thursday.
&&
.MARINE...
(Tonight through next Tuesday)
Issued at 953 PM PDT Wed Aug 5 2026
Light to gentle breezes will continue through Thursday with seas
between 2-6ft. Northwest winds begin to increase Friday into the
weekend with building seas and hazardous conditions for small
craft. A long period southwesterly swell of 15-17sec begins on
Saturday, peaks on Sunday then begins to subside by the beginning
of next week.
&&
.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...MM
LONG TERM....MM
AVIATION...JM
MARINE...JM
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Prepared by Boulder-Creek.com Weather at: Thu Aug 6 00:30:02 PDT 2026
From the National Weather Service
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