Technical NOAA Weather Discussion

FXUS66 KMTR 241827
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1127 AM PDT Mon Aug 24 2026

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 1200 AM PDT Mon Aug 24 2026

 - Temperatures near to slightly below seasonal averages persist
   within the marine layer through the rest of the week

 - Warm and dry conditions continue above 1000 feet

 - Pattern change Friday into the weekend brings cooler
   temperatures and possible tropical moisture

&&

.SHORT TERM...
Issued at 1200 AM PDT Mon Aug 24 2026
(Today and tonight)

For the last couple of days we've written the tale of a battle 
between the eastern Pacific trough and the Desert Southwest ridge 
that has dominated our weather pattern. Today it's the troughing 
that will take the upper hand as a cutoff low inches towards the 
Pacific Northwest, leading to slightly cooler temperatures and 
cloudier mornings across the region. Another interesting factor 
today is a weak shortwave trough coming into the region through the 
morning. The two main global weather models, the European ECMWF and 
the American GFS, are showing that there could be enough 
instability, moisture, and lift to maybe, just maybe, support a few 
high-based thunderstorms developing across the North Bay, 
particularly over Napa County. This is a very low probability 
forecast (around 5% chance through around 11 AM), along with a low 
confidence for storm development. The higher resolution NAM, HRRR, 
and RRFS models do not show any signs of developing storms, lapse 
rates aloft are marginal at best, and K-Index values below 20 
degrees Celsius suggest no chance for developing thunderstorms. Any 
precipitation that does develop is also likely to end up as virga, 
evaporating within the vast dry low layer where relative humidity 
values as of the 0Z (5 PM PDT) OAK sounding fell into the single 
digits.

Today's highs range from the 80s into the lower 90s for the inland 
valleys, the middle 90s to the triple digits across the warmest 
interior spots, the upper 60s to the upper 70s near the Bays, and 
the upper 50s to the middle 60s along the Pacific coast. The higher 
elevations above the marine layer influence will remain rather day, 
especially during the afternoon and evening hours. Breezy onshore 
pattern winds will develop this afternoon and evening with wind 
gusts reaching 20 to 30 mph along coastal jet regions, through 
favored gaps and passes, and within the northern Salinas Valley. 
Smoke from the Timber Fire will continue impacting the immediate 
area around the actively burning fire through the day, with winds 
aloft lofting the smoke plume over Salinas and Hollister. For air 
quality information, refer to products issued by the Monterey Bay 
Air Resources District.

&&

.LONG TERM...
Issued at 1200 AM PDT Mon Aug 24 2026
(Tuesday through Sunday)

Temperatures rebound Tuesday and Wednesday as the ridge reasserts 
itself, with the inland valleys reaching highs in the middle 80s to 
the middle 90s on Wednesday and Thursday while the Bays reach the 
lower 70s to the lower 80s. A pattern change will begin later on 
Thursday as a deep trough builds into the West Coast while the ridge 
begins to inch its way eastwards towards the Southern Plains. It 
will be interesting to see what the trough will do as the upcoming 
weekend comes closer and we are better able to refine the forecast. 
Also worth a mention is the possibility of tropical moisture coming 
into the region, with our colleagues at the National Hurricane 
Center tracking Tropical Storm Iselle and three other areas of 
interest off the coast of Mexico. While the circulations remain far 
to the south of our waters, the resultant moisture will be captured 
by the developing trough with a wide variation in just how far north 
the moisture goes. By way of example, the deterministic European 
ECMWF model brings a swath of tropical moisture into the San 
Francisco Bay Area by Friday morning, whereas the deterministic GFS 
keeps the moisture plume to the south of Monterey County. In 
addition, wave models are also showing an increase in southerly 
swells as the tropical eastern Pacific remains active into the early 
part of next week.

&&

.AVIATION...
(18Z TAFS)
Issued at 1126 AM PDT Mon Aug 24 2026

A blanket of stratus has retreated to the coastline resulting in VFR 
conditions for most terminals, except for HAF. HAF may briefly 
scatter out this afternoon, but will mainly remain IFR through the 
TAF period. Moderate onshore flow persists during the day which will 
allow for a more prominent inland push of stratus tonight. The 
marine layer is expected to be around 1000 to 1200 feet overnight 
which will be sufficient enough to reach most terminals. High 
confidence for coastal sites (HAF, MRY, and SNS) to develop IFR to 
LIFR ceilings as early as sunset. Medium confidence on the exact 
arrival of IFR to MVFR ceilings elsewhere, though they may arrive as 
early as 04-06Z Tuesday.

Vicinity of SFO...VFR through the afternoon as westerly winds 
increase to around 15 kt. Borderline MVFR-IFR ceilings are expected 
tonight with the shallow marine layer and persistent onshore push, 
it just becomes a matter of when. High confidence OAK will develop 
ceilings before SFO, though medium confidence on the arrival as they 
may roll in a few hours later than specified.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...IFR to LIFR ceilings will continue to lift 
this morning with VFR conditions to prevail this afternoon. An early 
return of IFR conditions is likely with ceilings gradually lowering 
to LIFR by late this evening/overnight. Some potential for 
reductions in visibility early tomorrow morning but that will depend 
how much the marine layer compresses and if fog is able to develop. 
Breezy onshore winds continue this afternoon/evening before winds 
ease overnight. 

&&

.MARINE...
(Today through Saturday)
Issued at 1126 AM PDT Mon Aug 24 2026

Northwest light to moderate breezes continue into Tuesday and
gradually increase to fresh by the middle of the week. Wind driven
seas remain slight to moderate, though a long period swell is
expected to approach our coastal waters by the end of the week.
Smoke from the Timber Fire continues to impact visibility to as
low  as 2 miles along the Big Sur coast.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Navarrete
MARINE...Navarrete

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Prepared by Boulder-Creek.com Weather at: Mon Aug 24 12:30:03 PDT 2026
From the National Weather Service