402
FXUS66 KMTR 121056
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
356 AM PDT Wed Mar 12 2025

...New AVIATION...

.SYNOPSIS...
Issued at 1212 AM PDT Wed Mar 12 2025

An active weather pattern arrives today with periods of rain,
strong wind, thunderstorm chances, and some snow to the higher
regional mountains. While the bulk of the impacts arrive today
through Thursday, disturbed weather will continue into early next
week.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1212 AM PDT Wed Mar 12 2025

Prefrontal rain over the coastal waters is just now getting into
radar range. This first round will arrive in the North Bay bay
around mid to late morning and continue sliding south through the
afternoon. Strong southerly winds in the warm sector will
coincide with the main rain band today, making for nasty
conditions outside. The wind has trended stronger on the latest
high resolution guidance. The official forecast leans heavily on
the 12/00Z local WRF, and brings a several hours of 30-55 mph
gusts to the entire Bay Area and Central Coast. As such, we`ve
issued a wind advisory from 8 AM through 8 PM today. The rain
forecast has changed slightly with a later arrival, but overall
similar accumulation as the previous few forecasts. Coastal areas
will see 1-1.5", with 2-4" in the coastal mountains. Inland
valleys will see anywhere from 0.5-1", with 1-2" in the inland
mountains. After the cold front pushes through this
afternoon/evening, cold air aloft will settle in, creating an
unstable atmosphere. This will change the precipitation mode from
steady stratiform rain to convective showers. Most of the
precipitation will fall as rain today, with higher elevations
switching to snow overnight after the cold air starts to settle in
behind the front. There is good agreement that the 500 mb height
will drop to around 536-538 dm by Thursday morning. While that`s
enough to bring some high elevation snow, it`s not quite getting
cold enough for anything extreme. For reference, the last time
Highway 17 was shut down due to snow (2/24/23), the 500 mb height
was 533 dm. The last time it snowed in San Francisco (2/5/76), the
500 mb height was 528 dm. The biggest concern for impacts today
is the combination of wind and rain, which could lead to some
downed trees and related impacts to roadways and power grids. We
are not expecting the large rivers to flood, but urban and small
stream flooding, as well as shallow landslides, are possible as
the narrow band of high intensity rain pushes through today.

&&

.LONG TERM...
(Thursday through Tuesday)
Issued at 1212 AM PDT Wed Mar 12 2025

Thursday looks like the best day for thunderstorm chances, but
it`s not a slam dunk. As the cold air continues to rush in
through the morning, sun breaks will allow the surface to warm
moderately. This combination of cold over warm is unstable. The
warm air near the surface will rise, expand and cool to the dew
point, and condense out cloud droplets. With relatively low
humidity at the surface Thursday, the cloud bases will be higher
than normal, and this cloud formation process will happen mostly
above the freezing level. That`s important because it means the
clouds will contain a significant amount of ice. Ice obviously can
mean hail, but it`s also a necessary ingredient for thunderstorm
development. It`s the static buildup between colliding ice
particles that builds the electrical charge necessary for
lightning strikes. The cold air aloft and steep lapse rates (as
high as 9 C/km in the afternoon) are favorable, but the moisture
content is relatively low and decreasing in the post-frontal
environment (PW around 0.35"). This could be the limiting factor
for thunderstorm development and may skew any activity earlier in
the day when the humidity is still elevated from Wednesday`s rain.

There looks to be a brief lull in showers as the dry air filters
in late Thursday. It won`t last long, however, as the next system
comes rolling in from the Eastern Pacific Friday morning. This
will bring another round of stratiform rain, but the impacts look
less than the first system. The third, and weakest, system will
bring another round of rain Monday. The rest of the week is more
complicated, with a lot of uncertainty beyond next mid week but
the pattern will likely remain active.

&&

.AVIATION...
(12Z TAFS)
Issued at 355 AM PDT Wed Mar 12 2025

Currently a mixed bag across the board with MVFR ceilings and pre-
frontal rain showers already being observed at some terminals.
Conditions will deteriorate through the morning as the cold front
approaches the region as strong and gusty southerly winds develop,
ceilings lower to MVFR, and rainfall increases in coverage and
intensity with steady rainfall expected at each terminal for several
hours. Behind the cold front, while remaining strong and gusty,
winds shift to a southwesterly regime. Scattered rain showers will
succeed the cold front with even a slight chance for thunderstorms
this evening through the end of the TAF period which would pose the
risk of locally heavy rainfall and gusty winds, lightning, and pea
sized hail.

Vicinity of SFO...Currently VFR with southerly flow. Several wind
shifts are anticipated through the TAF period overall following a
veering pattern. Southerly winds will increase through the morning
with gusts up to 40 knots possible from mid-morning through the
afternoon. As the cold front passes, winds will veer to become
southwesterly and eventually westerly. Steady rainfall can be
expected by the afternoon with a transition to rain showers tonight
which will present a slight chance for thunderstorms.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR with drainage flow at both
terminals. Winds will veer to become southerly, strong, and gusty
through the morning. Steady rainfall can be expected by the
afternoon with a transition to rain showers tonight which will
present a slight chance for thunderstorms.

&&

.MARINE...
(Today through Monday)
Issued at 301 AM PDT Wed Mar 12 2025

Winds are increasing across the coastal waters as a cold front
approaches. Gale warnings are in effect through the day. Rough
seas will build through the day. Overnight, there is a 30 percent
chance for thunderstorms and associated strong gusts. Seas will
remain rough into next week as a series of storms moves through.

&&

.BEACHES...
Issued at 420 AM PDT Tue Mar 11 2025

Hazardous beach conditions are expected Wednesday and Thursday
as moderate period westerly swell builds to support breaking
waves up to 15 feet. As always, please remember to stay off of
rocks and jetties, keep pets on a leash, and never turn your back
on the ocean!

Sarment

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 8 AM this morning to 8 PM PDT this evening
     for CAZ006-502>506-508>510-512>518-528>530.

PZ...Small Craft Advisory until 9 AM PDT this morning for Mry Bay-
     Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60
     NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to
     Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge.

     Gale Warning from 9 AM this morning to 9 PM PDT this evening for
     Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos
     10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to
     Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge.

     Gale Warning until 3 PM PDT this afternoon for Pt Arena to Pt
     Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Sarment
MARINE...Flynn

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