518
FXUS66 KMTR 201143
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
443 AM PDT Sun Jul 20 2025

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 133 PM PDT Sat Jul 19 2025

 - Cooling trend begins Monday with a widespread return to below
   normal temperatures.

 - Localized elevated fire weather threat for the East Bay Hills
   Sunday and Monday due to dry conditions and gusty winds.

 - Gusty winds return late Sunday and continue through Monday
   along the coast, the higher terrain, and mountain gaps/passes.

&&

.SHORT TERM...
Issued at 217 AM PDT Sun Jul 20 2025
(Today and tonight)

The forecast for the most part is starting to sound like a broken
record: morning stratus layer clearing to the coast by the
afternoon with seasonably cool temperatures. While the overall
weather picture is the same there are some details that differ:
high temps, marine layer depth, and winds. The upper level ridge
that brought some inland warmth Saturday has weakened and is being
replaced by an upper level trough/lower 500mb heights. The
overnight satellite fog product shows the marine layer has better
coverage than 24 hours due to the building upper level trough.
Have added some coastal drizzle/patchy fog to the forecast given
the current marine layer set up. The upper level trough will also
help to knock inland temperatures down a few degrees. The biggest
weather change for today will be the winds. For several days now
we`ve been mentioning increasing winds late Sunday into Monday.
Latest guidance continues to advertise a solid push of winds. In
fact, onshore gradients (SFO-SAC) are projected to be 4 to 5 mb
later this afternoon and evening yielding some robust winds for
the East Bay gaps/passes and West Delta region. Borderline for a
wind advisory, but gusts in the 30-40 mph with a few isolated up
to 50 mph will be possible. It will be windy along the coast,
higher terrain, and favored valleys, but not as strong as the East
Bay.

It goes with out saying, but the uptick in winds will create
increased fire weather concerns for the East Bay. While winds
increase humidity values are lowering to the 25-40% range. While
not critical RH the wind will override it. ERC chart values take
a dip today the more wind based index of the Burn Index reaches in
the 90th percentile. So that means, flashier fuels and more
flammable live fuels could carry fire. Speaking of fire weather
concerns a classic nocturnal drying event is taking place over
the Santa Lucia range. As stratus rolled through the Salinas
Valley rapid drying occurred above it with a sensor on Chews
Ridge dipping into the single digit RH.

Sunday night into Monday: Solid onshore flow remains with breezy
conditions lingering in the East Bay. The marine layer will
deepen with more coastal drizzle.

&&

.LONG TERM...
Issued at 254 AM PDT Sun Jul 20 2025
(Monday through Saturday)

To kick off another work week more of the same - night/morning
clouds with afternoon inland sunshine. Onshore winds ease
slightly early Monday before ramping back up Monday
afternoon/evening favoring the inland gaps/passes.

Thereafter pretty benign weather continues into next weekend.
Marine layer is locked in with cooler the seasonal temperatures
thanks to an anomalously low upper level trough overhead. Some
longer range guidance even deepens the trough and has an a cut-
off low form over the region by Thursday. Simply put, if you`re
enjoying the cooler weather with some afternoon sunshine you`ll
enjoy the long term forecast.

&&

.AVIATION...
(12Z TAFS)
Issued at 443 AM PDT Sun Jul 20 2025

Onshore winds will continue to usher stratus inland this morning.
Patchy drizzle and fog are also in the mix. Conditions are mainly
IFR-LIFR with exceptions of briefly MVFR-VFR e.g. SFO Airport in
an otherwise increasing stratus cloud coverage this morning across
the much of the forecast area. HREF shows stratus slow to mix if
at all mixing out along the immediate coastline today, the best
chance of VFR will be far inland during maximum diurnal surface
heating and vertical mixing. Coastal stratus moves inland tonight
and Monday morning with increasingly gusty onshore winds and lower
level cool air advection resulting in a deepening marine layer.

Vicinity of SFO...A nearly stationary clear patch resides over
the terminal. Tempo sct-bkn stratus ceiling forecast 13z-17z this
morning, otherwise VFR from late morning through afternoon and
evening. Gusty west-northwest wind to 25 knots in the afternoon
and early evening.

SFO Bridge Approach...Stratus /MVFR-IFR/ fills the approach this
morning, clouds mixing out by 17z-18z this morning. Stratus returns
late tonight and Monday morning.

Monterey Bay Terminals...Stratus, patchy fog and drizzle /IFR-LIFR/
gradually improving to MVFR-VFR by late morning and afternoon, though
ceiling improvement may be slow. Stratus and patchy drizzle move back
inland tonight and Monday morning. The marine layer will deepen due
to the arrival of lower level cool air advection.

&&

.MARINE...
(Today through Friday)
Issued at 425 AM PDT Sun Jul 20 2025

Light to moderate northwest breezes will increase to moderate to
fresh breezes through today, before winds diminish before mid
week. Gusty winds, potentially hazardous to small craft, will
develop strongest each afternoon and evening through the Golden
Gate, into the Delta and along the favored coastal jets off Pigeon
Point and the Big Sur coastlines. Slight to moderate seas persist
through the remainder of the forecast period.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 AM this morning to 9 PM PDT Monday
     for SF Bay N of Bay Bridge.

     Small Craft Advisory from 3 PM to 9 PM PDT Monday for Mry Bay-
     Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras
     Blancas 0-10 nm.

     Small Craft Advisory from 9 AM this morning to 9 AM PDT Monday
     for Pt Arena to Pt Reyes 0-10 nm.

     Small Craft Advisory until 9 AM PDT Monday for Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...MM
LONG TERM....MM
AVIATION...Canepa
MARINE...Canepa

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