Asti, California Weather Forecast Discussion

145
FXUS66 KMTR 261203
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
503 AM PDT Fri Sep 26 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 300 AM PDT Fri Sep 26 2025
- Warmer and drier heading into the weekend
- Rain and thunderstorms return Monday
- Wet weather Monday through Wednesday
&&
.SHORT TERM...
Issued at 249 AM PDT Fri Sep 26 2025
(Today and tonight)
What a difference 24 hours makes. Last night at this time we were
dealing with very active weather with showers and thunderstorms.
And tonight, radar is quiet with stratus impacting portions of the
region. The upper low that was responsible for the active weather
is finally moving southeast into SoCal/AZ. Behind the upper low
more stable/drier air is moving in with higher pressure. At the
surface, high pressure is building across OR/NorCal while low
pressure remains over SoCal. As such, the N-S gradient from ACV-
SFO has increased to -5mb. The stronger N-S gradient plus drier
air is eating away the stratus deck/marine layer that was
reforming. This is seen on the overnight satellite fog product.
That being said, the clearing from N to S will be most evident
along the coast and locally inland. Clouds farther inland will
linger into the morning hours and be less impacted by the N-S
gradient. Regardless, do expect clouds to clear all areas by this
afternoon. Ample sunshine this afternoon with building high
pressure will lead to a noticeable warm up. 24 hour trends will
be 5 to 15 degrees with interior locations back into the 90s.
This will be about 5 to 15 degrees above normal for this time of
year. Not a bad way to finish the work week.
For tonight, marine layer will return, but be compressed with low
clouds along the coast and locally inland.
&&
.LONG TERM...
Issued at 349 AM PDT Fri Sep 26 2025
(Saturday through Thursday)
The warm up will quickly end for the weekend as high
pressure/warmer air is replaced by broad upper level
trough/cooler. At this point the weekend looks dry, but
temperatures dropping 5-10 degrees each day with highs 5 to 15
degrees below normal by Sunday. Current forecast for Sunday will
have highs in the 60s and 70s most areas.
As the weekend ends and the work week begins eyes turn west to an
active Pacific. A robust and anomalously deep upper low begins to
move through the Gulf of AK. The associated upper trough to the
south of the low inches closer to CA Sunday night. Model guidance
is in pretty good consensus with this longwave pattern. An upper
level jet streak on the back side of the trough will help to
"push" the trough eastward by Monday. At the surface, an
associated cold front will approach late Sunday night and early
Monday before ultimately passing through the region by early
Tuesday. Model guidance has been pretty consistent in advertising
some decent moisture associated with this fropa. Global models
show a solid moisture plume extending southward to HI. Definitely
has some "Pineapple Express" vibes going on with PWATs 1-1.5". In
addition to the moisture the fropa will have some instability
associated with it leading to thunderstorm chances Monday
afternoon/evening. In classic early season fropa characteristics,
the front will have the greatest impact over the North Bay and
slowly fade in moisture/intensity by the time it reaches interior
Central Coast.
As the first cold front exits SE Monday into Tuesday a second cold
front will quickly follow. The upper low over the Gulf of AK will
begin to drift S on Tuesday as a result of the aforementioned jet
streak dropping southward. While there will be a brief break (sct
showers) in precip will pick back up by Tuesday afternoon and
linger into Wednesday. In similar fashion this front will lose
steam by the time it reaches the Central Coast. What makes this
fropa a little more interesting will be some of its moisture
source. Global models shows some tropical moisture from TS Neoguri
currently in the WPac being entrained.
One other potential moisture source in the mix will be for both
Monday and Tuesday will be the leftovers from Hurricane Narda. As
of right now, this moisture looks to stay south of the region, but
we`ll be watching it closely.
All in all decent rainfall is expected across the North Bay for
both system from Sunday night through Wednesday - late Sept/Oct
standards. WPC does have this region in a Excessive Rainfall
Outlook. North Bay rainfall amounts: 1-1.5 inches, Hollister to SF
0.25-1.0", Monterey Bay southward less than 0.25"
&&
.AVIATION...
(12Z TAFS)
Issued at 503 AM PDT Fri Sep 26 2025
Stratus continues across the interior with patchier stratus coverage
along the coastline. As of 11Z the SFO-ACV gradient clocked in at
-5.4 hPa. This correlates to a moderate northerly gradient bringing
drier air southward which helped to break up stratus coverage along
the coastline. Moderate confidence that HAF and MRY will stay clear
through the remainder of the morning as incoming drier air eroded
stratus coverage. Model guidance is more pessimistic and shows
stratus persisting through late morning. Models do not appear to
picking up well on the drier air mass, decreasing confidence in
them, and instead have a more widespread stratus bank offshore. For
more interior airports, stratus will last through 16-18Z before
clearing after that. Tonight - a brief period of shortwave ridging
will cause the marine layer to compress. This will keep any stratus
that is able to develop along the coastline with low confidence in
stratus spreading to the interior. Fog may become more likely for
HAF, MRY, and SNS as the marine layer compresses.
Vicinity of SFO...VFR with temporary MVFR CIGs possible through late
morning. Satellite shows stratus filling in across the SF Bay which
increases confidence that SFO will at least briefly see stratus at
the airport and along the bridge approach this morning. Any stratus
that does develop should dissipate by late morning with VFR through
the remainder of the TAF period. Low confidence that stratus will
return again tonight. Winds strengthen during the afternoon/evening
with gusts to around 23 knots expected this afternoon and evening.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Stratus has cleared out of MRY with low
confidence that it will redevelop this morning. Satellite shows
clearing starting in vicinity of SNS so may need to adjust SNS to
allow for an earlier clearing time similar to MRY. Low to moderate
confidence in timing of stratus return tonight with probabilistic
guidance indicating a higher likelihood of stratus developing after
08Z. IFR CIGs are likely tonight given that the marine layer is
expected to compress and keep moisture confined to the coastline.
Winds pick up slightly during the afternoon and evening before
decreasing, becoming locally variable at times, overnight.&&
.MARINE...
(Today through Wednesday)
Issued at 503 AM PDT Fri Sep 26 2025
Gale force to near gale force gusts continue across the northern
outer waters with elevated seas between 10 to 13 feet. Fresh to
strong gusts continue across the southern outer waters and
portions  of the inner waters. Winds decrease and seas abate late
Saturday  before unsettled weather returns early next week. A
strengthening  low pressure system and surface cold front will
bring showers, a  chance for embedded thunderstorms, and building
seas to our coastal  waters and bays early next week.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Pt Arena to
     Pt Reyes 0-10 nm.
Gale Warning until 9 PM PDT this evening for Pt Arena to Pt
     Reyes 10-60 NM.
Small Craft Advisory until 3 AM PDT Saturday for Pigeon Pt to Pt
     Pinos 10-60 NM.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...Kennedy
MARINE...Kennedy
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NWS MTR Office Area Forecast Discussion