Annapolis, California Weather Forecast Discussion

903
FXUS66 KMTR 081159
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
459 AM PDT Mon Sep 8 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1222 AM PDT Mon Sep 8 2025
- Chance for light rain Monday through Wednesday.
- Below normal temperatures through the week.
&&
.SHORT TERM...
Issued at 1222 AM PDT Mon Sep 8 2025
(Today and tonight)
The marine layer will expand through the day as low pressure
approaches the coast. The Ft. Ord profiler shows the depth currently
around 1,500 feet, and the PGE WRF suggests it will expand to
around 3,000 feet tonight before mixing out as colder air aloft
moves in Tuesday morning. The typical marine layer stratus is
expected to blanket much of the Bay Area and Central Coast this
morning before mixing out around mid-day. Temperatures will be a
few degrees cooler than yesterday, and well below normal for this
(typically warm) time of year.
Most of our attention is on the cold front slowly moving in from
the NE Pacific. The surface front should arrive along the coast
early Tuesday morning, though the latest trend has been delaying
its arrival. Ahead of the front, a moist air mass will be lifted
by the cooler denser air, creating a good chance for prefrontal
stratiform rain. While the precipitable water should be around
1.0" in the preceeding air mass, we are only expecting around
1/10" of rain in the warm sector due to lack-luster lift from a
front that`s more cool than cold, and may not actually make
landfall intact. This prefrontal rain will begin to fall in the
North Bay around mid-day, slowly spreading south through Tuesday
morning. No significant impacts are expected, but still note-
worthy that this is our first rain producing cold front of the
season.
&&
.LONG TERM...
Issued at 1222 AM PDT Mon Sep 8 2025
(Tuesday through Sunday)
After the front moves through, the combination of colder air
aloft, high moisture, moderate deep layer shear, and daytime
surface heating will keep rain chances going in the form of
convective showers through Thursday afternoon. A deeper dive shows
that these ingredients are not all lined up perfectly in space
and time. The coldest air is moving in over the Bay Area Tue-Wed,
while the jet stream moves through the Central Coast Tuesday and
then pushes into southern California Wednesday. The instability
really doesn`t develop until Wednesday with a stubborn elevated
inversion between 850-700 mb lingering on Tuesday. By the time
the colder air eliminates the inversion and starts bringing more
instability to the column, the jet stream will have pushed far to
the south, cutting off the shear and upper level divergence. The
low-level moisture has some staying power, however. That won`t be
an issue. With long, skinny CAPE, the instability is highly
dependent on surface temperature. For example, the 00Z NAMBufr
model sounding for OAK has the CAPE peaking at a paltry 97 J/kg at
10/14Z (7 AM Wed). The surface temperature at that time is
simulated to be 16.25C with a dew point of 15.58C. Increasing both
the T and Td by just 1C increases the surface based CAPE from 97
to 600 J/kg. But again, this is still fighting the fact that the
shear is basically gone by Wednesday. 0-6 km shear is a whopping
12 kts at this time. While there`s a chance that some of these
ingredients could align in a more favorable way for a brief period
(best chance Tuesday afternoon), the thunderstorm chances in the
post frontal environment are slight. Most likely we will get some
shallow cumulus that exist entirely below the freezing level.
Perhaps a few showers as well, but I don`t have much confidence in
a big thunderstorm event this time.
&&
.AVIATION...
(12Z TAFS)
Issued at 459 AM PDT Mon Sep 8 2025
Conditions are highly variable from inland to the coast, varying
from VFR to LIFR. There`s plenty of water vapor in the air resulting
in low clouds, fog and coastal drizzle. A few spots have already
lightly measured 0.01" possibly more at the moment. KHAF has been
1/4 mile or less in drizzle and fog. General duration of cloud cover,
fog and drizzle is a low confidence forecast in the 12z TAFs. Onshore
winds continue during the 12z TAFs.
Vicinity of SFO...MVFR-IFR for the morning, improving chance of
VFR in the afternoon. Onshore wind near 5 knots, except 10 to 20
knots in the afternoon and evening. Low confidence if KHAF drizzle
will reach KSFO this morning, otherwise light showers tonight and
Tuesday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...IFR-LIFR in low clouds, fog and possibly
drizzle. Low confidence MVFR-VFR in the afternoon. IFR in low
clouds, fog and drizzle tonight and Tuesday morning. Onshore winds
5 to 10 knots.
&&
.MARINE...
(Today through Saturday)
Issued at 444 AM PDT Mon Sep 8 2025
Winds increase to become fresh with breezy conditions Tuesday
through Thursday. Light rain is expected tonight into Tuesday
morning with convection possible during the afternoons of Tuesday
through Thursday, chances increase with latitude.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Canepa
MARINE...Canepa
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NWS MTR Office Area Forecast Discussion