St Helena, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

767
FXUS66 KMTR 162254
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
254 PM PST Tue Dec 16 2025

...New SHORT TERM, LONG TERM, MARINE...

.KEY MESSAGES...
Updated at 251 PM PST Tue Dec 16 2025

 - Light rain will spread into the North Bay and portions of the
   rest of the Bay Area tonight into tomorrow.

 - A second system arrives late Thursday with beneficial rain
   continuing through Saturday.

 - Confidence continues to increase that a stronger, impactful
   system will arrive late weekend and continue into next week.

&&

.SHORT TERM...
Issued at 251 PM PST Tue Dec 16 2025
(This evening through Wednesday)

A weak system is currently bringing light rain to the North Bay.
Showers have been fairly isolated so only a few locations in
northern Sonoma and Napa counties reporting totals of a few
hundredths of an inch. Scattered showers will continue through this
afternoon/evening with the bulk of the precipitation to our north
over Mendocino and Humboldt counties. This precipitation will shift
southwards overnight, but, it won`t be as widespread with showers
largely diminishing/falling apart as they move into the North Bay.
Not much change to rain totals for this event, totals continue to be
highest in the North Bay with up to 0.1" in the valleys and 0.2"-
0.3" in the coastal mountain ranges. Showers will move south of the
Golden Gate Bridge by mid to late tomorrow morning with the
potential for up to 0.1" along the Bay Shoreline and San
Francisco/San Mateo peninsula. Overall, this is a light and
beneficial rain system and leaves the storm door open behind it for
additional systems to enter California.

Residents of the North and East Bays may notice it is less foggy
today than it has been for the past few days. Tule Fog has been the
talk of the last few weeks as it persisted over the Central Valley
and portions of the Bay Area. If you`re tired of the gloom of fog,
you`re in luck. This system reduces fog chances across the Bay Area
but you may not see the sun just yet as generally overcast
conditions persist while this system rolls through. Given morning
fog and overcast skies, lowered the forecast high temperatures for
the North Bay into the mid 50s from the upper 50s. This should be
the last day requiring major adjustments to the temperature given
the series of incoming systems (Long Term) and more zonal upper
level flow compressing high pressure/diminishing fog potential.

&&

.LONG TERM...
Issued at 251 PM PST Tue Dec 16 2025
(Wednesday night through next Monday)

Things remain active in the long term with a second system expected
late week and a third system expected next week. Rain from the first
system (Tuesday - Wednesday) will diminish by Wednesday afternoon
with dry weather and upper level ridging briefly rebounding. The sun
may even briefly come out Wednesday afternoon in this break between
systems. By early Thursday morning, the next system will arrive and
bring with it renewed, more widespread rain chances across the
entire Bay Area and portions of the Central Coast. This system is
associated with a surface low pressure system and slight upper level
trough moving into the Pacific Northwest. This system will help to
compress surface high pressure and replace upper level ridging with
more zonal upper level flow. PWAT values associated with this system
peak around 1.5" to 1.6" with IVT values ranging from 250 to 500
kg/ms. The PWAT plume initially stays to our north before moving
into our CWA Friday this plume then lingers over the Bay Area and
Central Coast for much of the weekend. QPF totals have gone up for
this system with between 2.0"-2.5" across the North Bay Coastal
Mountains, 1"-1.5" for interior Marin and the North Bay Valleys, 0.5-
1.0" south of the Golden Gate, and up to 0.25" across the Central
Coast. These values may be further refined as we get closer to
Friday with NBM percentile guidance suggesting the current forecast
is on the lower end. While this system is not expected to be the
strongest of the three, preparations for wet weather should be
completed by Thursday as the second and third systems do not have a
defined break between them.

The third system is expected to be the most impactful, but, there is
still some forecast uncertainty as it is a more prolonged system and
is not entirely within our 7-day forecast period. Beginning Sunday,
a stronger, deep upper level trough will approach the West Coast and
bring a renewed source of tropical moisture into the region. This
storm will bring potentially moderate to heavy rain with the highest
rain totals in the North Bay. Given that totals continue to
fluctuate, it is still too early to give exact totals for this
event. The key messages are as follows: confidence is increasing
that a potentially impactful atmospheric river will arrive Sunday
and continue for much of next week. This system will bring periods
of moderate to heavy rain and potentially impact holiday travel. If
you are traveling next week, make sure to check road conditions,
flight delays, and the most up to date weather forecast for both
here and the location that you are traveling to (especially if it is
within California). Winds look to strengthen next week with this
system but, in agreement with the previous forecaster, there is not
a clear signal that winds will meet wind advisory criteria. All in
all, this system bears keeping an eye on especially as the forecast
surrounding it becomes clearer. Make sure to stay up to date with
the forecast so you can best prepare for any upcoming holiday travel.

High temperatures rise slightly (particularly for regions that have
been foggy) with more seasonal high temperatures in the 50s to low
60s across the region. Morning low temperatures stay in the 40s to
low 50s with some patchy cooler temperatures in the interior
mountain ranges.


&&

.AVIATION...
(18Z TAFS)
Issued at 1009 AM PST Tue Dec 16 2025

Conditions vary from LIFR to VFR this morning before ceilings
lift briefly this afternoon (generally MVFR to VFR). It`s
currently raining in the far northern portions of the North Bay
this morning, with that plume of moisture spreading southward
throughout this evening and into Wednesday morning as a weak
frontal boundary approaches the Bay Area. As such, we are
expecting lowering ceilings and/or visibilities later this
afternoon across the North Bay and into early Wednesday morning
over the Bay Area as the boundary advances southward. The boundary
will begin to dissipate as it reaches the Monterey Bay terminals.

Vicinity of SFO...MVFR currently, with periods of VFR early this
afternoon. Ceilings and visibilities are forecast to gradually
lower this evening and into Wednesday morning (IFR-MVFR and
potentially LIFR) with the greatest probability for light rain to
be early to midmorning on Wednesday.

SFO Bridge Approach...Greater probability for VFR through this
afternoon across the approach. Otherwise, similar to SFO.

Monterey Bay Terminals...VFR. Ceilings and visibilities are
forecast to lower to IFR-MVFR late this evening with pockets of
LIFR early Wednesday morning.

&&

.MARINE...
(Tonight through next Sunday)
Issued at 251 PM PST Tue Dec 16 2025

Light rain is forecast over the northern waters tonight through
Wednesday afternoon as a plume of moisture arrives from the north.
Light to moderate west to northwesterly breezes will also prevail
through Wednesday afternoon. Rain chances return by Thursday night
and increase as we head into the upcoming weekend. Moderate seas
will prevail through the remainder of the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PST this evening for Pt Pinos to
     Pt Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...RGass
MARINE...RGass

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