578
FXUS66 KMTR 172059
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
159 PM PDT Sun Aug 17 2025

...New SHORT TERM, LONG TERM...

.KEY MESSAGES...
Updated at 159 PM PDT Sun Aug 17 2025

 - Patchy fog and/or drizzle along the coast Sunday morning

 - Warming trend this week

&&

.SHORT TERM...
Issued at 159 PM PDT Sun Aug 17 2025 (This evening through
Monday)

Afternoon satellite shows some remnants of a weak surface cold
front draped across the the coastal reaches of the Bay Area. This
has kept clouds along coastal areas of North Bay down to Half Moon
Bay. By all accounts, this front has stalled. This stalled
boundary will provide a focus for some drizzle to potentially
develop. Confidence was not high enough to paint it into the
forecast, so left it with patchy fog for now. This may need to be
updated if conditions change, namely the frontal boundary tries to
hang on longer than expected. The higher probability is that the
boundary fades into obscurity, allowing for upper level ridging to
start to flex again. Overall, tonight through Monday will see
relatively non-impactful weather.

&&

.LONG TERM...
Issued at 159 PM PDT Sun Aug 17 2025 (Monday night through next
Saturday)

Now to the impactful portions of the forecast. Heat. High pressure
over the 4 corners region of the desert southwest will start to
nose into our service area on Monday, gradually strengthening
through the week. The challenge with the forecast is what the area
of low pressure over the Gulf of Alaska will do. So far this
summer it has been sending troughs across our area, keeping us on
the moderate side with onshore winds and ample moisture. This
trend looks to continue for the start of the week. However,
ensemble models are indicating a strengthening of the 4 corners
high to the point that there is increasing probability the high
pressure forces the low pressure out to sea. The way to watch this
over the week will be in the temperatures over the southern half
of the state and even souther NV. As those heat up, we`ll be
seeing the ridge build towards us. It`ll first consume interior
Monterey County and San Benito County, then creep into interior
East Bay and march into interior North Bay. We should see the apex
of the heat Thursday and Friday. The two challenges to the
forecast will magnitude of the heat and westward extent of the
heat. Right now the NBM has less than a 3 degree spread in
probabilities over the most impacted areas. That said, the entire
spread could be on the low side, meaning the NBM is underdoing the
magnitude. At this point I would give the forecast a 70% chance of
being underdone. As we get more data and see how this is evolving,
expect the high temperature forecast for Thurs and Fri to increase
by a few degrees. Taking more areas into the upper 90s and low
100s. This plays directly into the second challenge, the westward
reach of the ridge. Models are not in good agreement with how far
west the ridge can nose and ensemble means are simply washing it
out. If the patterns of this summer hold true, the coast will
reamin the sanctuary from the interior heat. However, we are reach
the climatological time of the year we start to see heat reach the
coast. While it is not a true nature vs nurture argument,
climate, or "nature" in this analogy, does play a role and may end
up being the arbiter of coastal heat. The Ocean Prediction
Center`s 96 hour forecast chart shows a thermal (or inverted)
trough Thursday morning just off the coast of the Bay Area. This
screams heat moving closer to the coast. All that said, be ready
for heat wherever you are located. From Santa Cruz and San
Francisco to Brentwood and Pinnacles, it will get hot. HeatRisk
at this time is mostly moderate for Thurs and Fri, but don`t be
surprised to see it go major with areas of extreme. We will be
watching this closely for any issuance of heat products. Either
way it will be a jarring shift from being below normal for months
to being 5 to 15 degrees above normal. Prepare now for a sharp
warm up.


&&

.AVIATION...
(18Z TAFS)
Issued at 1100 AM PDT Sun Aug 17 2025

The main themes for this TAF package will be a gradual
improvement of flight categories through the afternoon with
MVFR/IFR ceilings returning tonight into Monday morning.
Thereafter, VFR seems most probable for most sites in the 17-18Z
time frame on Monday. While stratus isn`t quite as expansive as
yesterday, a stream of moisture through the Golden Gate Gap will
continue to support intermittent MVFR ceilings at KOAK and KHAF
through 21-22Z. Winds through the delta and the various gaps will
promote W`ly wind gusts to around 20-22 knots (KAPC, KOAK, and
KLVK), though a gust or two near 30 knots cannot be discounted.
Moisture will gradually return after sunset and as a result,
there`s low confidence in the specific onset time of MVFR/IFR at
the terminals. Confidence isn`t quite high enough to include a
ceiling at KLVK (opted for a SCT group here) or KSJC, so VFR is
advertised at these sites. If moisture return is more aggressive,
then inclusion of MVFR cigs will be warranted at these sites.
Confidence is moderate to high that VFR will return by 16-17Z on
Monday.

Vicinity of SFO...The clearing forecast remains a challenge at
KSFO with satellite trends suggesting an eventual return to VFR.
At present time, intermittent stratus may linger as late as 21Z
over the terminal with gusty winds being funneled through the San
Bruno Gap. Peak gusts are forecast near 26 knots, with a less than
20% chance of gusts exceeding 30 knots. VFR is expected for the
evening push with MVFR cigs returning near/just after midnight
Monday. Confidence in the timing of MVFR cigs is low to medium.
VFR is anticipated to return around 17Z Monday, though the timing
may need to be fine tuned in future TAFs.

SFO Bridge Approach...Current web camera and satellite imagery
suggest non-VAPS and intermittent cigs are anticipated through at
least 21Z today. Cigs return around 07Z with a possible return to
VAPS by 17Z Monday.

Monterey Bay Terminals...Stratus dissipated quickly thanks to
shallow moisture and a diffuse eddy across the bay. As a result,
the return of MVFR stratus may be delayed, with some guidance
suggesting VFR through the entire TAF period at KMRY and KSNS.
The overall consensus, however, is that cigs around FL015 will
initially arrive at KMRY during the pre-dawn hours on Monday, with
intermittent MVFR cigs at KSNS around daybreak. The threat for
cigs diminished rapidly after 15-16Z on Monday.

&&

.MARINE...
(Today through Friday)
Issued at 934 AM PDT Sun Aug 17 2025

Fresh to strong breezes are anticipated, largely for waters
within 10 nautical miles of the shoreline including San Francisco
and Monterey Bay regions today and tonight. This will equate to
hazardous boating conditions, particularly for smaller vessels.
Moderate northwesterly seas are forecast through late Monday with
wave heights subsiding briefly on Tuesday. Moderate to rough seas
are advertised mid-week and onward as breezes increase. Southerly
secondary swell with periods of 12 to 16 seconds are forecast.
Heights are forecast to remain at or below 2 feet.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Mry Bay-
     Pigeon Pt to Pt Pinos 0-10 nm-SF Bay N of Bay Bridge.

     Small Craft Advisory until 3 AM PDT Monday for Pt Arena to Pt
     Reyes 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 9 AM PDT Monday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...BFG
LONG TERM....BFG
AVIATION...Bain
MARINE...Bain

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