Oyster Bay Regional Shoreline, California Weather Forecast Discussion

066
FXUS66 KMTR 181821
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1121 AM PDT Thu Sep 18 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 304 AM PDT Thu Sep 18 2025
- Showers with isolated to scattered thunderstorms today into
   Friday
- Cooler temperatures today and Friday, with a weekend warm up
- Rainfall chances increase again for the beginning of next week
&&
.UPDATE...
Issued at 953 AM PDT Thu Sep 18 2025
Interesting start to the day around the Bay Area and Central Coast
weather wise with a mix of marine stratus and thunderstorms. Odd
to say that typically, but that`s what happens with upper level
forcing and moisture. Ft Ord and Bodega Bay profilers put the
marine layer depth 1300-1600 feet. Even with that depth, coverage
is mainly over the waters and locally inland. Despite the stable
stratified lower levels we`re popping thunderstorms over the
coastal waters. It`s because we have forcing and moisture that is
well above the surface. This is what meteorologists call high-
based convection. The other interesting wrinkle this morning;
there is moisture at the surface and aloft, but what about in
between? It`s actually pretty dry. Morning soundings shows a solid
dry pocket from roughly 650mb down to just above the marine
layer. This is further reflected on observations over the higher
terrain in the Bay Area with current humidity readings in the
10-25% range.
So what`s the rest of the day look like? Stratus will linger
along the coast. The bigger impact will be the showers and
thunderstorm chances. Moisture lurking to the south (per PWATS on
satellite) will continue to advect northward. The increasing
moisture will overlap with meager upper level instability and
daytime heating. Therefore, carrying a low chance for
thunderstorms in the forecast remains valid. The question on where
they`ll form is the tricky part. Latest Hi-res and CAMS still
struggle to handle the left over circulation from Mario. Some
models bring the track farther inland, meaning eastward push of
storms and less impact to the Bay Area, while others bring the
track up the coast/farther west, meaning better chances for
thunderstorms this afternoon. Given the amount of activity
currently on satellite/radar west of Big Sur coast thinking the
westward track may be playing out.
All that being said, as stated below think there is a still a
chance this afternoon and evening for showers and thunderstorms.
No update needed at this time.
MM
&&
.SHORT TERM...
Issued at 304 AM PDT Thu Sep 18 2025
(Today and tonight)
Forecast for areas of showers and thunderstorms continuing to move
north into our area out ahead of the remnants of Mario remains on
track as we head into early Thursday morning. Mario is a loosely
defined upper level disturbance at this point, currently moving
slowly north offshore over the waters of SOCAL. The heaviest
rainfall as of 3am PDT is over the Channel Islands with the
leading edge of the precip just south of Paso Robles. Light to
occasionally moderate showers offshore of our area, associated
with an upper level disturbance out ahead of Mario will continue
to drift northwest over our open waters for the remainder of the
morning.
Isolated to scattered thunderstorms remain in the forecast,
especially for later this afternoon and evening, however the trend
for enough instability for impactful convection has been steadily
declining. The good news is that, the unusually high PWATs (near
record for this time of year) may lead to some welcomed wetting
rainfall across much of the Central Coast, and portions of the South
Bay and East Bay. Guidance has struggled with the position of Mario
the past fews days leading up to its arrival on our southern
doorstep this morning, with no dominant synoptic feature forcing its
progress. Deterministic guidance both HiRes and synoptic scale,
indicate a more eastern projection inland than this time 24hrs ago,
and that trend is indicated within the National Blend of Models
(NBM).
The best window for isolated to scattered thunderstorms is this
afternoon and evening, mainly across the Central Coast and South
Bay. Both thermal and kinematic instability has weakened with most
recent CAM updates. Ensemble mean CAMs MUCAPE (Most Unstable
Convective Available Potential Energy) values are generally less
than 100J/kg, with worst-case values struggling to eclipse
200J/kg. Meanwhile dCAPE (downdraft Convective Available Potential
Energy) values are generally 4-5x higher. That means that the
strongest updrafts aren`t likely to last long, and those that do
are more likely to produce moderate to strong wind gusts (but less
than 50mph) as they collapse. Any cloud-to-ground lightning over
a dry area continues to pose a fire weather threat, and conversely
where it does rain, brief downpours are possible. Otherwise
beneficial wetting rainfall will be the story for a majority of
our area between now and Friday morning. Portions of southern
Monterey County in the Santa Lucias may see over an inch of rain
over the next 24 hours or so, with a 0.25-0.50" farther inland into
San Benito County. Elevated areas farther inland across the South
Bay and East Bay may see up to a 0.25" with the Bay Area and North
Bay lucky to reach 0.10" over that same time frame.
&&
.LONG TERM...
Issued at 304 AM PDT Thu Sep 18 2025
(Friday through Wednesday)
Once the mostly beneficial wetting rainfall is realized through
the remainder of Friday we`ll see temperatures warm up over the
weekend into Monday. While we warm up over the weekend, extended
guidance shows an upper level disturbance well offshore diving
south from the Pacific Northwest down to SOCAL, becoming almost
stationary Sunday into Monday, then taking aim once again for the
Central Coast on Tuesday with more wetting rain likely with that
system. We`re fortunate to be seeing this rainfall at this time of
the year.
&&
.AVIATION...
(18Z TAFS)
Issued at 1121 AM PDT Thu Sep 18 2025
It`s VFR inland and a mix of VFR and LIFR-IFR in lingering stratus
and fog along the coastline. Mid level clouds (tropical moisture from
the remnants of TS Mario) continue to advance northward. It remains
a very complex forecast in the near term as demonstrated in varied
model solutions, not only with respect to potential areal coverage
of wet weather, but also with respect to marine stratus and fog.
Thunderstorms are not mentioned in the current TAFs since robust
mid level instability so far appears to be over the coastal waters
west and southwest of Monterey, where 700-500 mb thermal troughing
and instability are aligned. 500 mb thermal troughing is present over
the remainder of the forecast area but not so much aligned with any
significant 700 mb troughing at this time, this is helping to keep
t`storm potential at a minimum over land so far today. 18z TAFs
mention vicinity showers, and are otherwise a mix of model forecasts,
monitoring radar, satellite and surface observations.
Vicinity of SFO...Near high confidence VFR with a few showers
possible by evening and overnight. Surface winds mainly onshore 5
to 10 knots.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Highly variable weather continues with a
mix of marine stratus, fog and mid to high level clouds this morning.
Monitoring a band of mid-high based light rain to the south and
southeast, however it remains dry in the sub-cloud layer with
virga likely at least until greater precipitable water arrives
from the south later today, tonight and Friday morning. High
resolution model forecasts show 700-500 mb cyclonic circulation
embedded within the higher precipitable water arriving tonight,
will this result in a more e.g. widespread wetting rain remains
the question. There remains some indication thermal troughing will
be present at the 700 mb to 500 mb levels tonight and Friday morning,
therefore can`t rule out showers to possibly a band of a wetting
rain (much like what the models have shown, but with varied to highly
varied solutions as to where this may/may not verify).
Otherwise it`ll remain a mix of IFR and VFR through late morning
and afternoon. While mid level moisture is present (water vapor
present slows outgoing nocturnal radiative cooling, thickening mid
level clouds otherwise slows incoming diurnal radiative heating.
There`s an ongoing high probability of further disruption to marine
stratus and fog through tonight into Friday. Surface winds will
remain mainly light and variable with onshore winds 5 to 10 knots
more likely during the afternoon and early evening.
&&
.MARINE...
(Today through Tuesday)
Issued at 1027 AM PDT Thu Sep 18 2025
Strong northerly winds, occasional gale force gusts, and very
rough seas will prevail for the northern outer waters today.
Elsewhere, a moderate northerly breeze and moderate to rough seas
will prevail. Isolated rain showers with embedded thunderstorms
remain possible today which will pose the risk of locally heavy
rainfall and erratic outflow gusts. Conditions improve tomorrow
as winds diminish, seas abate, and unsettled weather exits the
region.
&&
.FIRE WEATHER...
Issued at 813 PM PDT Wed Sep 17 2025
Threat for dry lightning is diminishing as deeper tropical
moisture moves in from the south. Thunderstorm chances are
uncertain Thursday afternoon across the Central Coast.
Thunderstorms that form over the Santa Lucia may experience dry
air entrainment due to strong offshore winds off the Gabilan
Range. Deep moisture will still be in place, so this isn`t a
typical "offshore wind event", but may be just enough to snub much
of the thunderstorm activity across the Central Coast. However,
this is all dependent on the track of the low and if it shifts
eastward. For the rest of the region, isolated thunderstorm
activity will spread northward through Thursday night into Friday
morning. Even though most thunderstorm cores will produce wetting
rain, there is always the chance of a stray strike or two outside
of the main core, in addition to gusty and erratic winds.
Behringer
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 AM PDT Friday for Pt Arena to Pt
     Reyes 10-60 NM.
&&
$$
SHORT TERM...JM
LONG TERM....JM
AVIATION...Canepa
MARINE...Canepa
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NWS MTR Office Area Forecast Discussion