003
FXUS66 KMTR 250313
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
813 PM PDT Sun Aug 24 2025

...New UPDATE, FIRE WEATHER...

.KEY MESSAGES...
Updated at 1224 PM PDT Sun Aug 24 2025

 - Seasonal temperatures persist through the forecast period.

 - Small chance for elevated convection Monday - Tuesday.

 - Drier air aloft after midweek, but still looking at a chance
   for deeper Pacific moisture by the weekend.

&&

.UPDATE...
Issued at 717 PM PDT Sun Aug 24 2025

While the surface temperatures continue to cool and the typical marine
layer clouds filter in, there doesn`t seem to be anything out of
the ordinary to mention. However, in the mid to upper troposphere,
well above the highest coastal mountains, there is an interesting
development worth exploring that has a chance to generate thunderstorms
over the next couple days. In a word: instability. Some of this
can get complicated, so I will explain this situation in two
ways; first to a fellow meteorologist, and then to my young
nephew.

Good evening Dr. Storm, the visible loop shows all organized
convection is confined to the highest terrain across the state,
with a significant down trend in activity over the last 24 hours.
Despite this, the potential for elevated thunderstorms remains
over the next 48 hours with lingering monsoon moisture and
persistent instability. NAM point soundings in particular show
some very interesting statistics. Analyzing the 00Z run over OAK
shows MUCAPE topping out at over 1,300 J/kg at 26/09Z. That would
put it in the top 5 values in the climatological record (over
57,000 observed soundings going back to 1948). It`s all elevated
with most unstable parcels originating from around 775 mb. Diving
deeper shows the potential energy is supported by 850 temps around
23.8C, which is above the top 10th percentile for this time of
year. In contrast, 500 temps are simulated to be -11.6C, which is
actually in the bottom 10th percentile. These conditionally
unstable lapse rates in excess of -8 C/km are widely supported
across various model guidance. The big question is moisture,
specifically the vertical distribution. NAM has PW at 1.2", well
above the 90th percentile (1.04"), and critically there is a nose
of moist air as low as 775 mb. This is why the MUCAPE starts at
775 mb. While this all sounds promising, you know as well as I do
that NAM is notorious for having a high moisture bias. However,
even if I modify the model sounding profile in AWIPS to be more
in-line with the actual 00Z upper air observation (roughly 40-45%
RH at 775 mb), it still produces 852 J/kg of MUCAPE. Importantly,
however this also increases the amount of dry adiabatic cooling
needed before the parcel can follow a moist adiabat, thus
bringing CIN up from -43 to -121. With no strong forcing to
overcome this CIN, it may be the limiting factor preventing the
potential energy from being realized. That said the pattern is
gradually shifting to a weak trough at 500mb, which could produce
a weak boundary. PoT thunder is around 5-10% for inland portions
of the Bay Area and Central Coast from Monday- Tuesday, which
seems reasonable for now, but don`t be surprised if that increases
as more high resolution guidance resolves the instability. By
Wednesday the moisture should start to clear out as the lapse rate
shallows.

Hey Bear, what`s up dude? Tell your dad he still owes me five
bucks. Anyway, guess what? There`s a chance for thunder and
lightning over the next couple days! Let me explain why. The air
above Mt. Diablo is super warm compared to the air way up where
airplanes fly. Just like a hot air balloon, this warm air wants to
rise. As it rises, it cools and forms a cloud. If it rises fast
and high enough, this cloud will grow really tall, and it will
start to make rain drops and hail stones. When these hailstones
bump into each other, the friction creates static electricity,
just like when you rub your socks on the carpet before petting the
dog. That little shock you give Simba is exactly what the cloud
is doing to the earth with lightning, its just a much more
powerful shock! We`re not sure it`s going to happen, but there is
a chance, so keep your eyes and ears open and let me know if you
see any lightning or hear any thunder!

&&

.SHORT TERM...
Issued at 1224 PM PDT Sun Aug 24 2025
(This evening through Monday)

Mostly clear skies over the region as monsoonal moisture aloft
filters out. Pretty typical Bay Area/Central Coast day with marine
stratus lingering around the peninsulas (near Pt. Reyes, SF/HMB,
and Monterey). Slightly more stable airmass and less upper level
support pretty much suppressing any threat for elevated
convection today. As we start the work week there is really not
much notable weather to mention outside of the smoke in areas of
the North Bay due to the Pickett Fire. The majority of this smoke
will continue to be blown to the NE into portions of Lake county
and the Central Valley.

&&

.LONG TERM...
Issued at 1224 PM PDT Sun Aug 24 2025
(Monday night through next Saturday)

Pretty quiet conditions anticipated through the rest of the week
as a subtle upper trough pattern takes shape off the West Coast.
Guidance has been depicting some dry air aloft wrapped up in this
setup making an advance at the Central Coast during the latter
half of the week. This mostly wouldn`t cause any issue outside of
some lower than normal RH at the highest peaks along the Central
Coast up through the Diablo Range. The good news here is that it`s
still a weakly forced pattern and onshore flow persists, so at
least we retain good marine layer moisture and shouldn`t be
dealing with any strong wind situations. Going into the weekend,
ensemble guidance is starting to trend towards a deep cutoff low
migrating from the Gulf of Alaska into the PNW region. Ensemble
cluster analysis is advertising the evolution well for being this
far out, but uncertainty remains in how far south the low will
actually make it. At the very least, it looks like at least some
limited potential for a pattern that we typically see much later
in the calendar year. Not promising rain at all (way too early for
that kind of commitment), but the first week of September
certainly has our attention.

&&

.AVIATION...
(00Z TAFS)
Issued at 419 PM PDT Sun Aug 24 2025

Stratus has retreated to the immediate coast north of the Pigeon
Point area and pulled back offshore across the Central Coast. Breezy
onshore winds will continue through the evening hours, and stratus
should return to the region overnight. With a shortwave trough
approaching the region through the TAF period, have generally kept
the previous forecaster`s optimistic persistence forecasts through
the TAF period.

Vicinity of SFO... West winds prevail through the TAF period, with
breezy and gusty winds in the afternoon and evening. IFR stratus
pushes over the terminal in the evening, persisting overnight
through the following morning.

SFO Bridge Approach... Lower confidence in stratus coverage with
some high resolution models keeping the stratus just to the
terminal area and its north. Otherwise similar to SFO.

Monterey Bay Terminals... Stratus has cleared out of both terminals
and retreated quite a ways offshore, lowering confidence in the
short-term forecast as high-resolution models do not have a good
grip on the situation. Have pushed back the development of LIFR
stratus at the terminals to around 3Z, but confidence in timing is
limited and still possible that stratus could return much earlier.
LIFR stratus persists through Monday morning. Breezy onshore winds
persist through the evening hours, turning light overnight before
resuming Monday afternoon.

&&

.MARINE...
(Tonight through next Friday)
Issued at 419 PM PDT Sun Aug 24 2025

Moderate to fresh northwesterly breezes will continue today with
strong gusts through the San Francisco Bay and across the coastal
jet regions of Point Reyes and Point Sur. Winds diminish tomorrow
with a gentle breeze prevailing through Thursday. Moderate seas
will prevail through Thursday with the exception of the northern
outer waters where seas will be rough.

&&

.FIRE WEATHER...
Issued at 717 PM PDT Sun Aug 24 2025

Fire weather conditions are steadily improving with cooling
temperatures, increasing humidity and calmer winds. RH recoveries
will be excellent under persistent onshore flow and a robust
marine layer. The exception is in the thermal belt around 1,500 -
2,500 feet elevation, where RH will stay well below 50% through
the night.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for SF Bay N of
     Bay Bridge.

     Small Craft Advisory until 3 AM PDT Monday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...Behringer
LONG TERM....Behringer
AVIATION...DialH
MARINE...Sarment

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