006
FXUS66 KMTR 201112
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
412 AM PDT Wed Aug 20 2025

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 245 AM PDT Wed Aug 20 2025

 - Heat Advisory Thursday - Friday across much of the interior

 - Moderate HeatRisk extends into Saturday

 - Elevated Fire Weather conditions across interior Thursday into
   the weekend

&&

.SHORT TERM...
Issued at 213 AM PDT Wed Aug 20 2025
(Today and tonight)

Overnight satellite fog product indicates pretty quiet weather
across the forecast region. Only some patchy stratus over the
coastal waters and along portions of the coast. There will be
some additional filling in of the stratus through early morning,
but not expecting drizzle/fog. Definitely a change from last
week.

A notable and impactful pattern change will kick in today. It`s
been teased a few times since late July; a large 4-corners ridge
of high pressure nosing into CA and impacting the Bay Area.
Previously, the ridge would build west and then get buffeted or
even replaced by an upstream upper level trough of low pressure.
That`s why we`ve only had "tastes" of summer heat and it`s been
mainly confined to interior locations. The pattern developing
today and lasting into the weekend will bring more widespread heat
with a 4-corners ridge of high pressure becoming the main dominate
weather feature. As 850mb temps climb (showing airmass potential)
and 500mb heights rising max temps today will warm 5 to 10
degrees over Tuesday. That means highs in the 60s to near 80
coast/bays and 80s to near 100 degrees interior. This will
initially nudge our HeatRisk up across the interior. For tonight,
solid thermal belts in place given the warming 850mb temps (low
20s deg) leading to temps in the upper 60s to lower 70s in the
hills.

&&

.LONG TERM...
Issued at 331 AM PDT Wed Aug 20 2025
(Thursday through Tuesday)

No big changes from previous forecast for Thursday and Friday.
Did nudge temperatures upward on Thursday across the interior
based on ensemble spread and where the deterministic NBM fell,
which is on the lower end toward the 25th percentile.
Additionally, if the EC is right with 850mb temps around 25C that
would yield fully maxed temps to the surface of low 100s. The
immediate coast/bays are still spared from the bigger heat given
lingering onshore flow/sea breeze. There will be a few warm
pockets at the coast, but with lack of offshore flow it`s hard to
beat the natural AC. As the airmass continues to warm thermal
belts will remain locked in with the hills staying mild each
night. Given the time of year, multi day heat both day and night,
HeatRisk values remain elevated away from the coast with Moderate
to Major (interior Monterey has a few pockets of Extreme).
Therefore, Heat Advisory remains in effect for interior locations
for Thursday and Friday.

Friday into Saturday becomes a little more interesting as some
monsoonal moisture begins to ride the western periphery of the
ridge. Latest guidance continues to paint a wetter picture
moisture wise with PWATs over an inch or 150-200% percent of
normal. While an influx of monsoonal moisture after a round of
heat is usually a watch out situation we`re not there just yet.
700-500mb RH is decent with values increasing Friday and then
lasting into Saturday. Upper level lapse rates are good for ascent
with values GT 7 C/km. Modified Total-totals values start out slow
on Friday, but then increase to critical values by Saturday. We do
have some MUCAPE during this timeframe too, but it`s accompanied
by weak CINH. Not a slam dunk, but moisture and instability are
there. Would like to see more of a defined trigger. 1.5 PV does
shows a subtle ripple on the dynamic tropopause, but would like
to see something more pronounce. Therefore, will keep forecast dry
for now. Non-zero chance for high based convection Friday and
Saturday with greatest chances over the Central Coast.

Given the influx of moisture on Saturday it will feel a tad muggy
around here with some afternoon dewpoints reaching the mid 60s.
It will be similar to last weekend. May need to extend the Heat
Advisory across interior Monterey/San Benito on Saturday given
expected temperatures. However, with uncertainty regarding
potential clouds impacting temps will hold off for now.

Finally, relief begins on Sunday with the ridge sliding farther
east and more clouds. FWIW, one lone model shows rain across the
interior Central Coast form the monsoon. Not biting on it yet,
but we`ll be watching it.

More widespread cooling expected on Monday and Tuesday.

&&

.AVIATION...
(12Z TAFS)
Issued at 412 AM PDT Wed Aug 20 2025

Patchy coastal stratus and fog /IFR-LIFR/ this morning and again
tonight and Thursday morning otherwise VFR. Mainly onshore winds
at the terminals during the period.

Vicinity of SFO...VFR. West wind near 10 knots except 15 to 22
knots in the afternoon and early evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Coastal stratus and fog /IFR-LIFR/ this
morning then VFR during the afternoon and early evening. Coastal
stratus and fog /IFR-LIFR/ redevelops tonight and Thursday morning.
Mainly onshore winds 5 to 15 knots.

&&

.MARINE...
(Today through Monday)
Issued at 407 AM PDT Wed Aug 20 2025

Breezy to gusty northwest winds especially across the outer
waters and coastal jet regions near Point Sur and Point Reyes
will prevail through late week. Winds and seas will ease over the
weekend.

&&

.FIRE WEATHER...
Issued at 338 AM PDT Wed Aug 20 2025

Will keep fire weather headline going. As noted below, heightened
fire weather concerns will develop on Thursday and last into the
weekend. The warm and dry conditions will persist both day and
night, especially interior locations and higher terrain. Do want
to mention a non-zero chance for dry lightning Friday and Saturday
across the Central Coast.

The warming and drying trend expected through the later part of
the week is causing elevated fire weather threats across the
region. In particular, the inland regions will see poor to very
poor daytime humidity retention, compounded with poor overnight
humidity recoveries in the higher elevations, leading to
widespread curing of fuels. The latest ERC and Burning Index
forecasts all show drying fuels across the region and the
possibility of large fires to take hold and spread rapidly.
Although a gradual cooling trend will begin over the weekend, the
fire weather conditions will be slow to recover, and elevated fire
weather threat continues through the early part of next week.

DialH/MM

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory from 10 AM Thursday to 10 PM PDT Friday for CAZ504-
     510-512>518.

PZ...Small Craft Advisory until 9 PM PDT Thursday for Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt
     Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PDT
     Thursday for Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory from 3 PM to 9 PM PDT Thursday for Pigeon
     Pt to Pt Pinos 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
     evening for Pigeon Pt to Pt Pinos 0-10 nm.

&&

$$

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

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