805 
FXUS66 KMTR 091908
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1208 PM PDT Tue Sep 9 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 220 AM PDT Tue Sep 9 2025

 - Slight (15%) chance for thunderstorms during the afternoons
   through Thursday

 - Below normal temperatures and rain shower chances will continue
   through Thursday

&&

.UPDATE...
Issued at 915 AM PDT Tue Sep 9 2025

Bands of showers, some relatively intense, are making their way
through the Bay Area and Central Coast. There may be a little
pause later this morning and into the early afternoon before 
showers return to the region late in the afternoon and through the
evening and overnight hours, with a slight chance for
thunderstorms in the northern parts of Sonoma and Napa counties.

DialH

&&

.SHORT TERM...
Issued at 220 AM PDT Tue Sep 9 2025
(Today through Thursday)

While it's not technically Water Year 2026 yet (October 1st, 2025), 
the "first rain of the season" is underway via a cold front. The 
parent feature is a cold and moist upper-level low as noted by the 
September 9th 00Z balloon. It recorded a 1.24 inch precipitable 
water value which is the second highest for this date and time (1.37 
inches in 1998) and an 850 millibar temperature of 11.95 degrees 
Celsius which is near the 10th percentile (11.8 degrees Celsius). 
While the cold air aloft is notable for this time of year, well 
below normal temperatures at the surface will likely do little to 
capitalize on the situation i.e. the atmosphere will be less 
inclined to "flip" cool air over cold air than hot air over cold 
air. Still, there's a slight (15%) chance for thunderstorms during 
the afternoons of today, tomorrow, and Thursday as the essential 
ingredients of lift, instability, and moisture will be there. The 
best chance for timing will be Wednesday afternoon as the core of 
the upper-level low will be over our area. The best chance for 
location will be the North Bay (namely northern Napa and Sonoma 
Counties) where most unstable convective available potential energy 
(MUCAPE) (in this case it will be surface based) will reach 
approximately 500 Joules/kilogram. With the exception of orographic 
lift induced thunderstorms, maximum temperatures will be the 
limiting factor as model point soundings illustrate a long, skinny 
CAPE profile. Lightning/flooding threats exist with all 
thunderstorms: when thunder roars, go indoors! and see a flash, dash 
inside! Outside of thunderstorms, rain showers will be beneficial.

&&

.LONG TERM...
Issued at 220 AM PDT Tue Sep 9 2025
(Friday through Monday)

Global ensemble clusters suggest it is likely that the upper-level 
low will exit to the northeast Friday, raising heights and 
temperatures at least briefly. Troughing returns to the West Coast 
Sunday with another cold front and light rainfall chances.

&&

.AVIATION...
(18Z TAFS)
Issued at 1208 PM PDT Tue Sep 9 2025

The front has passed, leaving us with mainly VFR to MVFR conditions 
and lingering showers. These conditions will likely continue over 
the TAF cycle, with perhaps drier conditions for the afternoon and 
evening hours. The latest model guidance shows chances for showers 
increasing tomorrow morning, near or just after sunrise. Adjusted 
the forecast towards that outlook, carrying either VCSH or -SHRA. 
Thunder chances remain negligible  for the TAF sites and confidence was 
too low to add VCTS. 


Vicinity of SFO...Mainly VFR conditions should prevail until later 
this evening, when low clouds make a return. Showers may skirt by 
the airspace over the next couple of hours, with dry conditions 
expected through the overnight hours. Given the timing change for 
rain, opted to keep the mention of -SHRA towards mid to late 
Wednesday morning. Confidence is medium to low on precipitation 
timing, and medium on CIGs tonight. 

SFO Bridge Approach...Similar to SFO. 

Monterey Bay Terminals...Mainly VFR conditions should prevail this 
afternoon unless some rogue showers sneak by, which may bring brief 
periods of MVFR conditions. Come the evening, we should see a return 
of MVFR to IFR CIGs with showers potentially arriving around or 
after sunrise. Medium confidence in the CIG forecast, low to medium 
confidence in the precip timing. 

&&

.MARINE...
(Today through Sunday)
Issued at 905 AM PDT Tue Sep 9 2025

Scattered light rain showers are expected across the coastal
waters  through Thursday as an upper level low moves into the West
Coast. Isolated thunderstorms are possible during the afternoons
Tuesday through Thursday. Thunderstorm chances peak on Tuesday
across the northern coastal waters. A gentle to moderate breeze
will  continue Tuesday with a moderate to fresh breeze developing
over  the outer waters Wednesday into the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...KR
MARINE...KR

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