FXUS66 KMTR 051211
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
509 AM PDT Wed Aug 5 2026

...New AVIATION, MARINE...

.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.

&&

.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 425 AM PDT Wed Aug 5 2026

Stationary and weak surface low pressure will remain over the
coastal waters today through Thursday morning before dissipating.
This low will combine with generally onshore breezes including
more locally gusty winds across the San Francisco Bay and the
Delta. After the low dissipates Thursday, onshore breezes will
continue  late in the week and over the weekend. Northwest winds
will begin to increase out over the northern coastal waters during
the weekend resulting in building seas and 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...None.
PZ...None.
&&

$$

SHORT TERM...KR
LONG TERM....KR
AVIATION...Canepa
MARINE...Canepa

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Prepared by Boulder-Creek.com Weather at: Wed Aug 5 08:30:02 PDT 2026