Livermore, California Weather Forecast Discussion

604
FXUS66 KMTR 182152
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
252 PM PDT Thu Sep 18 2025
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 304 AM PDT Thu Sep 18 2025
- Showers with isolated thunderstorms today into Friday
- Cooler temperatures today and Friday, with a weekend warm up
- Rainfall chances increase again for the beginning of next week
&&
.SHORT TERM...
Issued at 209 PM PDT Thu Sep 18 2025
(This evening through Friday)
The richer and juicier tropical air has finally arrived into the
Central Coast and continues to push northward this afternoon.
ACARs soundings out of SJC show the influx of upper level
moisture. However, its impact isn`t panning out as advertised on
some of the guidance the last few days. We`ve mentioned how low
confidence and how nuanced the forecast would be and that is still
the case even as events unfold. Showers and thunderstorms have
been most prolific for our area over the ocean and that was well
advertised. Where the forecast is suffering is over land. Rainfall
amounts over the land have been slashed from previous forecast as
the focus for most precip this afternoon through Friday is either
over the ocean or east of the forecast area. Rainfall amounts are
still forecast to be highest over the Central Coast, but for the
Bay Area not expecting much with only a few hundredths to a tenth.
So what`s happening? We have a few things at play. For one, the
moisture continues to surge north, but ACARS still indicate a
rather robust dry layer still hanging around. While the radar
shows echoes over land they`re evaporating before reaching the
ground. This will ultimately change later this afternoon and this
evening as the column becomes more saturated. Starting to see this
trend with a few gages finally showing a few tips in
Monterey/Carmel and East Bay Hills. Second, a weak surface low has
remained west of Monterey Bay and continues to drift northward.
This explains the better convection over the ocean. Third, the
Mario left over circulation is trending eastward focusing the
forcing to the Central Valley. Fourth, the northward movement of
the initial moisture was slightly delayed. As such, it`s now
battling the approaching upper level trough seen on water vapor
to the northwest. The upper trough may also be helping to push the
Mario circulation eastward as well.
All that being, still expecting some showers and isolated
thunderstorms across portions of the Central Coast and Bay Area
through Friday. The convective threat has decreased, but cannot
rule out an isolated rumble or two. Will still feel muggy given
the higher dew points and tropical nature. Would not be surprise
to near record moisture on the Oakland sounding this evening.
&&
.LONG TERM...
Issued at 239 PM PDT Thu Sep 18 2025
(Friday night through next Wednesday)
Friday night into Saturday is looking much drier than previous
forecast. There may be one or two lingering showers early Friday
due to wrap around moisture with the departing circulation.
Over the weekend the approaching upper trough deepens enough at
500 to eventually become cut-off. The upper low meanders off the
coast before moving to SoCal early next week. As the trough/and
low move through there is a sliver of upper level moisture/lift
Saturday into Sunday. Maybe some cu building on the hills.
Forecast is dry for now as precip seems more likely over the
Sierra and mt ranges north of here.
Early next week gets interesting once again as the upper low that
meandered to SoCal pulls in some higher PWAT air and moves
northward to the forecast area. This feature will bring rain
chances, possibly thunderstorms, back to the forecast area
Tuesday/Wednesday.
&&
.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.
&&
.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...MM
LONG TERM....MM
AVIATION...Canepa
MARINE...Canepa
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NWS MTR Office Area Forecast Discussion