San Lorenzo, California Weather Forecast Discussion

583
FXUS66 KMTR 262124
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
224 PM PDT Fri Sep 26 2025
...New SHORT TERM, LONG TERM, MARINE...
.KEY MESSAGES...
Updated at 222 PM PDT Fri Sep 26 2025
- Rain and thunderstorms return Monday
- Wet weather Monday through Wednesday
&&
.SHORT TERM...
Issued at 222 PM PDT Fri Sep 26 2025
(This evening through Saturday)
Surface analysis shows that high pressure has nosed into the Pacific
Northwest with a cutoff low in the Colorado River Valley. This has
created a northerly gradient which supports northerly (drying) winds
down the Sacramento Valley. Fortunately, onshore flow and a shallow
marine layer will return tonight,easing fire weather concerns and
providing free air conditioning. Heights begin to fall tomorrow as a
longwave upper-level trough approaches. As a result, temperatures
will return to near normal and the marine layer will deepen. This
weekend will be a great weekend to clean out your gutters and
properly store any outdoor items that are sensitive to rain and/or
wind as there is a stark pattern change in the long term forecast.
&&
.LONG TERM...
Issued at 222 PM PDT Fri Sep 26 2025
(Saturday night through next Thursday)
A negatively tilted upper-level longwave trough will approach the
West Coast Sunday. The first of two embedded upper-level shortwave
troughs and cold fronts will bring rain and the chance for
thunderstorms Monday. Taking a look at thunderstorm potential and
the three essential ingredients of lift, instability, and moisture,
they seem to all be there. The upper-level longwave trough and cold
front will act as lifting mechanisms themselves with further lift
from both the Polar jet stream and Subtropical jet stream as they
nose into the region. MUCAPE will be up to 200 J/kg with
conditionally unstable lapse rates of less than 7 degrees
Celsius/kilometer. This system is forecast to tap into subtropical
moisture with the GEFS and ECMWF Ensemble IVT Forecast ensemble
means in the 250-500 kg/ms range. The second embedded upper-level
shortwave trough and cold front will approach the West Coast Tuesday
afternoon. The moisture associated with the second system is coming
all the way from the Northwest Pacific Ocean and Tropical Storm 25W
(Neoguri). It`s moisture will overtop the subtropical ridge in the
Central Pacific Ocean and eventually get pulled into the parent gale
force low pressure system in the Gulf of Alaska. As of now, this
second system is forecast to have lower rainfall totals and a lesser
chance for thunderstorms as the Polar jet stream and subtropical jet
stream are not as strong, MUCAPE is 60 J/kg, and the moisture
content isn`t as high with the GEFS and ECMWF Ensemble IVT Forecast
ensemble means just over 250 kg/ms. As is typical with Gulf of
Alaska systems and their northwest to southeast trajectory, when all
is said and done, the North Bay will have accumulated up to 1.50"
while the Interior Central Coast may get less than 0.10". It is
important to note that there is a great deal of uncertainty when it
comes to the timing of both of these systems as the GFS is providing
an earlier arrival time as compared to the ECMWF by 12 hours. We
should have a better idea tomorrow as we come into range of high-
resolution models.
&&
.AVIATION...
(18Z TAFS)
Issued at 1044 AM PDT Fri Sep 26 2025
VFR conditions have returned to just about all of the region with
the exception of KHAF where visibilities are reducing flight
categories to MVFR. Expecting onshore winds to increase this
afternoon, yet are forecast to be weaker than yesterday afternoon.
Winds diminish after sunset and moreso into Saturday morning with
the best potential for MVFR (30%-50% probability of lowing to IFR)
ceilings to return to the Monterey Bay terminals and coastal
terminals such as KHAF early Saturday morning. West-southwest winds
increase by Saturday afternoon.
Vicinity of SFO...VFR. Onshore winds will increase early this
afternoon, yet are forecast to be weaker than yesterday. Winds ease
late tonight and into Saturday morning with moderate to high
confidence for VFR conditions to prevail through the TAF period.
West-southwest winds increase by Saturday afternoon.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR. Onshore winds will increase early this
afternoon before easing after sunset. Moderate to high confidence
for a return of MVFR ceilings around 08Z-10Z Saturday with a 30%-50%
potential of lowering to IFR around sunrise.
&&
.MARINE...
(Tonight through next Wednesday)
Issued at 222 PM PDT Fri Sep 26 2025
Gale force gusts continue across the northern outer waters with
elevated seas between 10 to 13 feet through early Saturday
morning. Fresh to strong gusts continue across the southern outer
waters and portions of the inner waters through Saturday morning.
Winds diminish and seas subside 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 3 AM PDT Saturday 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...Sarment
LONG TERM....Sarment
AVIATION...RGass
MARINE...RGass
Visit us at www.weather.gov/sanfrancisco
Follow us on Facebook, X, and YouTube at:
www.facebook.com/nwsbayarea
x.com/nwsbayarea
www.youtube.com/nwsbayarea

NWS MTR Office Area Forecast Discussion