Lytton, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

642
FXUS66 KMTR 200537
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
937 PM PST Fri Dec 19 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 315 AM PST Fri Dec 19 2025

 - Light to moderate rain tonight and Saturday for the North Bay,
   Bay Area and East Bay

 - Focus of moderate rainfall shifts south into Bay Area to Santa
   Cruz Mountains on Sunday with a slight chance of thunderstorms

 - Rain chances persist on Monday, but Monday appears to be the
   least impactful day in the extended forecast

 - Tuesday through Friday will bring strong winds and periods of
   heavy rainfall for much of our area, expect extensive travel
   delays

 - By the middle of the week into next weekend, our coastal
   beaches and coastal waters will be exceptionally dangerous
   whether you`re from here or visiting friends and family, stay
   away

&&

.UPDATE...
Issued at 652 PM PST Fri Dec 19 2025

The first push of rain has moved into the central Bay Area, and is
now tapering off. While this was never going to be the main
event, this first round over performed in the North Bay. Santa
Rosa recorded 0.47" while a CALFIRE site in the coastal mountains
near Sea Ranch received an impressive 2.79". As a result, the
chance for river flooding has increased, particularly the small
streams and creeks. Santa Rosa Creek surprised us by reaching
action stage tonight. The larger rivers are still expected to
remain in their banks, but the probability of flooding has
increased here as well. The upper 10% peak exceedance probability
would now bring moderate flooding to both the Russian River and
Napa River. The 25% exceedance probability would bring minor
flooding to the Russian, and put the Napa in action stage. Again
the official deterministic forecast isn`t explicitly calling for
flooding of these large rivers yet, but the probabilities are
creeping up. Rain intensity will ramp up Saturday afternoon and
continue through the day Sunday. Again this next push will be
focused on the North Bay, where a Flood Watch is in effect. The
other update is to the wind forecast Tuesday-Wednesday. As the
strong low pressure system develops and approaches the coast, a
very strong low-level jet is likely to develop. This is a result
of the strong pressure gradient force enhanced by the terrain
effects of the coast-parallel SSE wind direction. Non-covective
wind gusts along the coast are likely to reach 50 mph or more
Tuesday night due to this effect.

&&

.SHORT TERM...
Issued at 406 PM PST Fri Dec 19 2025
(This evening through Sunday)

Light rain has been ongoing across the North Bay since earlier this
morning with some of the higher elevation areas already seeing an
inch of rain, and lower lying areas seeing mostly between 0.25"-
0.50" as of late afternoon today, which is a little ahead of current
guidance. HiRes guidance indicates the moisture plume will gradually
shift south toward the Bay Area tonight with mostly light scattered
rain, also extending into the East Bay. By day break Saturday, light
scattered rain, off-and-on in nature will persist through the
morning hours over the same area. Much of the Central Coast will
remain dry Saturday with the exception of the coastal ranges. The
next wave of rainfall from the atmospheric river arrives once
again along the coast of the North Bay and Bay Area late Saturday
afternoon under embedded mid-level shortwaves and surface
convergence from a quasi-stationary front draped across the North
Bay. While that wave of precip moves over the northern half of our
forecast area Saturday night into early Sunday, a developing but
progressive trough begins to deepen to our north, pushing onshore
into NorCal and Oregon by late Sunday afternoon.

The lift associated with the progressive trough axis pushing onshore
during the day Sunday into early Monday morning will result in more
widespread rainfall across our entire area, with the higher amounts
still focused on the coastal ranges from Sonoma south to the Santa
Cruz mountains looking at 2-3" over that same time period. Inland
portions of the North Bay and Bay Area will see approximately 1-2"
with locally higher amounts for higher elevations inland by early
Monday morning. Farther south for the South Bay and remainder of the
Central Coast are looking at between 0.5"-1.5" of rain with limited
impacts for those areas. There is a slight chance of embedded sub-
severe thunderstorms Sunday that may produce brief downpours and
gusty winds up to 40mph.

A Flood Watch has been issued For the North Bay beginning Saturday
afternoon at 4pm through Monday afternoon at 4pm. Flood concerns are
relatively minor, centered around rivers, creeks and streams, that
may rise out of their banks. Low-lying areas and flood prone areas
may be impacted as well. Remember, turn around don`t drown and never
drive through standing or moving water.

&&

.LONG TERM...
Issued at 406 PM PST Fri Dec 19 2025
(Sunday night through next Friday)

By day break Monday, a deeper and broader midlevel trough forming to
the west over open water will provide a partial reprieve from
widespread rainfall as mid-level heights rise directly over our
forecast area. The subtle rise in mid-level heights locally will
interrupt some of the heavier rainfall rates and shift the
atmospheric river from the Santa Cruz Mountains back to the north
toward the Bay Area and North Bay. The buckling of the midlevel
height fields to the west will represent the beginning of the most
impactful portion of the forecast as strong surface cyclogenesis
initiates offshore overnight Monday into Tuesday. By Tuesday
afternoon a wide swath of strong winds will spread across our area
(especially over our coastal waters from San Francisco south
through Big Sur). Inland, winds will be gusty as well, up to 40mph
at times for low lying areas and up to 60mph (or higher) across
the highest inland peaks through early Wednesday morning. The
strengthening low offshore will shift the fetch of the atmospheric
river a little farther north with the North Bay once again
looking to see the most rain in that time period, along with the
Bay Area, with lesser amounts to the south over the Central Coast.

Unfortunately, a second rapidly deepening upper level low and its
associated trough directly on the heels of the aforementioned
strong system moving onshore Tuesday will absorb whats left of the
atmospheric river and slam several strong trough axis` in rapid
succession from the latter part of Wednesday through early Friday
across most of the California coast line. The exact timing of the
strongest winds and heaviest rain are too far out to get too
specific at this point. However, plan on extensive travel delays
Wednesday through Friday and exceptionally dangerous conditions
along the coast line and in the near shore waters. Family and
friends visiting the area will always be curious to be near the
ocean while visiting our area. This will be no time to challenge
the power of the atmosphere and the ocean. We`ve had numerous
deaths recently up and down the Pacific Coast in our waters. No
picture or memory is worth your life, just stay away for a
majority of the extended forecast.

&&

.AVIATION...
(06Z TAFS)
Issued at 926 PM PST Fri Dec 19 2025

Light rain band moving over the Bay Area will gradually weaken
through tonight, providing a bit of a break in the rainfall before
stronger, more widespread precip arrives Saturday afternoon. VCSH
added for several sites ahead of the main rain band through parts
of Saturday morning and midday. Best time of arrival for heavier,
more persistent rainfall as it stands is just after the 00Z hour
Saturday aft for North Bay terminals, spreading southward into the
SF Bay Area by midday Sunday.

Vicinity of SFO...Pause in the rain overnight, but MVFR/IFR cigs
persist as the atmospheric river moisture tap continues to advect
through the region. As of this issuance, it appears the heaviest
and most persistent rainfall will likely hold off until Sunday as
much of the stronger upper level supports stays in the North Bay
tonight and Saturday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Mostly dry conditions further south
through this weekend as the main moisture plume remains to the
north. A couple of periods of VCSH both early Saturday morning and
late evening. Heaviest rainfall will hold off until late Sunday.

&&

.MARINE...
(Tonight through next Thursday)
Issued at 652 PM PST Fri Dec 19 2025

A weak boundary is moving through the region with mainly light
southerly winds over the waters. Generally light wind and seas are
forecast through Saturday. By Sunday southerly winds may increase
slightly ahead of another warm frontal boundary but wind and seas
remain light overall despite periods of rain. A strong and more
organized cold frontal passage will occur later Tuesday and the
second half of next week with much stronger winds and building
seas that will become very hazardous to mariners.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from Saturday afternoon through Monday afternoon for
     CAZ502>506.

PZ...None.
&&

$$

SHORT TERM...JM
LONG TERM....JM
AVIATION...Behringer
MARINE...RW

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