FXUS66 KMTR 041707
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1007 AM PDT Tue Aug 4 2026

...New UPDATE, MARINE...

.KEY MESSAGES...
Updated at 148 AM PDT Tue Aug 4 2026

 - Moderate HeatRisk in the interior today, becoming patchy 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 looks to return for the end of the 
   work week. 

&&

.UPDATE...
Issued at 1000 AM PDT Tue Aug 4 2026

Two updates to the forecast this morning:

1)Solid marine layer push this morning with ample coverage along
the coast. Despite the coverage it's still rather compressed.
Compressed nature and topographic enhanced has yielded drizzle 
here at the office in Monterey. The chance of drizzle will 
decrease through 11 am as the marine layer clears back to the 
coast.

2)Expanded today's heat advisory to now include the Santa Clara
Valley and Santa Clara Eastern Hills. Upon further analysis and
fine tuning of temperatures HeatRisk coverage of Moderate to near
Major for Heat Advisory expansion. Keep in mind heat impacts can
be cumulative and today some of these locations have been over 90
degrees since Saturday.

MM


&&

.SHORT TERM...
Issued at 148 AM PDT Tue Aug 4 2026
(Today and tonight)

The southerly surge of stratus that arrived earlier on Monday, has 
allowed for stratus to become more widespread this morning. Nearly 
all of our coastline has low clouds along it, with the stratus 
penetrating inland. The Monterey Bay along with the majority of the 
Salinas Valley has filled in, is slowly creeping into Bay Area. 
Expect the marine layer to about 800-1000ft, which should keep high 
temperatures for coastal communities and areas in or under the 
marine layer reasonable. Clouds will begin retreating to the coast 
by mid to late morning, leaving mainly clear skies across the 
region. In terms of the upper level pattern, high pressure will 
slowly take hold of the forecast. 850mb temperatures today, look to 
to be between 24-29C, warmest across the interior. Expect 
temperatures to a bit cooler than yesterday for areas influenced by 
the marine layer, while other locations remain hot. Patchy Moderate 
HeatRisk exists interior North Bay and interior Monterey, with areas 
to widespread Moderate HeatRisk for interior East and South Bays. 
Opted to hold off an issuing a Heat Advisory for these locations 
tonight until we get a better visual on the marine layer expansion 
since that could put interior temperatures at risk.

&&

.LONG TERM...
Issued at 148 AM PDT Tue Aug 4 2026
(Wednesday through Monday)

No major changes to the forecast as high pressure will continue to 
build into the region and then hold through at least Saturday. 
Temperatures will may decrease a more degrees on Wednesday, but it 
will be dependent on the marine layer. Interior locations will still 
be warm, but likely have temperatures in the 90s, perhaps cracking 
100 in a few spots. Warmer and dryer weather starts to return on 
Thursday as the ridge strengthens. Looking at models, 850mb temps 
don't really deviate too much. We're still looking at that airmass 
being warm, around 27-30C, depending on the model. Overnight lows 
would be mild, especially in the thermal belts/mountains. And day 
time highs would climb back into the 90s and low 100s. The HeatRisk 
projection looks to have widespread Moderate HeatRisk for interior 
slots, with pockets of Major HeatRisk. Heat Advisories may be 
needed. 

&&

.AVIATION...
(12Z TAFS)
Issued at 427 AM PDT Tue Aug 4 2026

Stratus is expanding inland along through the interior valleys and
is expected to reach SFO and OAK shortly. Confidence is lower that
stratus will reach APC and SJC but the potential for brief
overcast conditions cannot be fully ruled out at either airport
this morning. IFR to LIFR conditions persist along the coast
through late this morning, briefly clear during the afternoon, and
returns during the late afternoon/early evening. Stratus looks to
be widespread again tonight and will reach most interior sites
(excluding SJC and LVK). Diurnally breezy onshore winds continue
with light, offshore at times, winds expected overnight.

Vicinity of SFO...Stratus is approaching SFO via the SFO Bridge
Approach (southern SF Bay) and is expected to reach SFO between
12-13Z. This is most likely to bring MVFR to IFR conditions with
the marine layer between a depth of 500-1000 feet. Any cloud cover
that does develop should clear by 17-18Z. Moderate confidence that
stratus will return late tonight and persist through the remainder
of the TAF period.

SFO Bridge Approach...Stratus is filling in faster over the south
SF Bay and gradually building northwards towards SFO. High
resolution guidance suggests clearing times for the approach will
be similar to SFO but patchy clouds may linger along the approach
path for longer than over the terminals. Otherwise, similar to 
SFO.

Monterey Bay Terminals...IFR-LIFR conditions continue through 
late this morning. Moderate confidence that a brief period of VFR 
will return during the afternoon/evening before stratus returns 
(high confidence) late afternoon/early evening. Confidence is 
slightly lower as to the exact CIG height but for now kept the 
stratus arriving as IFR and lowering to LIFR overnight. Not seeing
any major impacts to visibility forecasted for tonight.

&&

.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 452 AM PDT Tue Aug 4 2026

Elevated fire weather conditions continue through the remainder of
the week due to dry daytime conditions across the interior with
near critical conditions returning late week into the weekend.
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 (20-30%) 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. 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. 

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory until 10 PM PDT this evening for CAZ510-513>515.

PZ...None.
&&

$$

SHORT TERM...KR
LONG TERM....KR
AVIATION...Kennedy
MARINE...DialH

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Prepared by Boulder-Creek.com Weather at: Tue Aug 4 10:30:02 PDT 2026