078 
FXUS66 KMTR 052339
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
439 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...
(00Z TAFS)
Issued at 424 PM PDT Wed Aug 5 2026

VFR at all terminals late this afternoon as the marine layer
remains staged at the coastline awaiting a return as we head
toward sunset. KOAK balloon measured ~1500ft marine layer. An
offshore meso-low will help channel stratus through the Bay Area
with widespread coverage expected by early morning Thursday across
most of our area below 1500ft. However, despite the widespread
coverage, compression back to the coastline should be complete by
early afternoon.

Vicinity of SFO...VFR. Expect IFR/LIFR ceilings to develop shortly
after sunset and persist through the late morning. The offshore
Otter Eddy mentioned above will accelerate stratus through the 
Golden Gate and the Bay Area all the way into the Central Valley 
overnight with relatively light onshore flow at the surface but 
moderate lower boundary level flow (~975mb) driving the push 
inland. Expect VFR by late morning through the remainder of the
forecast period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...More typical diurnal pattern this far
south with onset of IFR/LIFR before sunset and persisting through
late morning to early afternoon, with VFR expected thereafter
through the remainder of the forecast period. 

&&

.MARINE...
(Tonight through next Monday)
Issued at 424 PM PDT Wed Aug 5 2026

Light to gentle 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 morning 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...MM
LONG TERM....MM
AVIATION...JM
MARINE...RGass

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