Lakeshore Plaza Station San Francisco Post Office, California Weather Forecast Discussion

656
FXUS66 KMTR 011113
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
413 AM PDT Wed Oct 1 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1213 AM PDT Wed Oct 1 2025
- Beneficial rain today
- Warming and drying trend starts Friday
&&
.SHORT TERM...
Issued at 1213 AM PDT Wed Oct 1 2025
(Today and tonight)
Light rain showers are ongoing across the Bay Area ahead of a cold
front located about 200 miles west of San Francisco. This front is
connected to a mature, cut off low pressure system way up in the
southern Gulf of Alaska. This separation from the parent low is
causing a very slow forward progression of the cold front. A
ribbon of moisture on the latest water vapor imagery is already
showing signs of stalling. As a result, the prefrontal rain will
continue through the day. Fortunately the rain intensity looks to
remain light outside of a mid day shower or two in the North Bay.
The West-WRF ensemble shows, within 1 standard deviation, the IVT
will remain between 250-500 kg of water vapor per meter per
second today. Thunderstorm chances are less than 10% today. The
lapse rates are pretty weak (around 5-6 C/km). With the upper
level low so far removed from the surface front, there is a
notable lack of cold air aloft needed to create the necessary
buoyancy. Based on the 00Z sounding, the 850-500 mb lapse rate is
5.0 C/km, which is actually in the bottom 10th percentile for
this time of year. The 850 temp isn`t the problem (9.35 C, close
the bottom 10th percentile), but the 500 mb temp (-11.75C) is
close to the median for this time of year. Furthermore, the slow
motion of the cold front doesn`t support robust updrafts. Finally,
most of the current showers are below the 12,500 foot freezing
level, limiting the amount of ice particles necessary for the
charge buildup in thunderstorms. When all is said and done we`re
expecting up to 1" in the coastal mountains of the North Bay with
up to 1/4" across the Bay Area and 1/10" across the Central Coast.
With the rain spread out over the day, this is mostly a
beneficial event and will help put a damper on fire season.
&&
.LONG TERM...
Issued at 1213 AM PDT Wed Oct 1 2025
(Thursday through Tuesday)
The prefrontal rain will finally taper off by Thursday morning as
the moisture plume slides south. The associated 500 mb trough
will swing through Thursday-Friday. This will cause winds to shift
from SW to NW, and may support some convection each afternoon,
though the lack of moisture will be a major limiting factor for
thunderstorm development. This trough will bring lower surface
pressure to southern California. Meanwhile higher surface
pressure will build over the northern Rockies through the weekend.
The gradient across the pressure field will support some
northerly, offshore wind from Saturday - Monday. Typically these
winds pose a fire weather concern. Fortunately the antecedent
conditions and wind strength are mitigating factors. The PGE-WRF
ensemble shows the SFO-WMC gradient bottoming out around -2 to -6
mb. Enough for offshore winds, but not a particularly strong
event.
&&
.AVIATION...
(12Z TAFS)
Issued at 413 AM PDT Wed Oct 1 2025
Generally mid-level ceilings prevailing as pre-frontal and frontal
showers continue throughout most of the TAF period, with some IFR
stratus in the Monterey Bay region and the Salinas Valley.
Conditions remain generally VFR with mid-level ceilings through the
day with some patches of MVFR associated with the strongest showers,
with the rain chances tapering off through Thursday morning. Winds
will increase this morning from a southerly direction, shifting to
the west and northwest as the cold front passes through the region.
Vicinity of SFO... VFR expected through the TAF period with possible
periods of MVFR ceilings during intense showers over the terminal.
Winds from the south will turn to the west as the front passes
through with gusts up to 25 knots in the afternoon and evening
hours. Chances for showers taper off Thursday morning.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... MVFR-IFR stratus should persist through
sunrise, clearing to VFR through the morning. Chances for showers
begin this afternoon and evening with low to moderate probabilities
(20-30%) that showers will directly impact MRY or SNS. Winds begin
to increase from a southerly direction before shifting to the west
and northwest by the afternoon.
&&
.MARINE...
(Today through Monday)
Issued at 413 AM PDT Wed Oct 1 2025
An upper level trough and associated surface cold front results
in scattered showers across the coastal waters through late
tonight. A moderate to rough northwest swell associated with this
system arrives tonight and will bring waves between 10 to 11 feet
across the outer waters. Wave heights will diminish below 10 feet
Thursday and Friday before building again late weekend into next
week. Winds are expected to strengthen to fresh to strong on
Friday resulting in a continuation of rough seas and hazardous
conditions for small crafts into the weekend.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 PM Thursday to 9 AM PDT Friday for
     Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.
Small Craft Advisory from 3 PM Thursday to 9 AM PDT Friday for
     Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory from 9 AM this morning to 3 PM PDT Thursday
     for Pt Arena to Pt Reyes 10-60 NM.
Small Craft Advisory from 3 PM this afternoon to 9 AM PDT
     Thursday for Pigeon Pt to Pt Pinos 10-60 NM.
Small Craft Advisory from 3 AM to 9 AM PDT Friday for Pigeon Pt
     to Pt Pinos 10-60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...DialH
MARINE...DialH
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NWS MTR Office Area Forecast Discussion