872
FXUS66 KMTR 310521
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
921 PM PST Thu Jan 30 2025

...New UPDATE, AVIATION...

.SYNOPSIS...
Issued at 248 PM PST Thu Jan 30 2025

The first of numerous rounds of rainfall will begin late tonight
across the North Bay and then gradually spreads south and inland
across the rest of our area tomorrow and Saturday. Scattered light
rain lingers on Sunday out ahead of a stronger, more impactful
system for the beginning of next week and beyond.

&&

.UPDATE...
Issued at 921 PM PST Thu Jan 30 2025

Stratus is developing along the coastal regions and is expected to
increase in coverage through the night. The forecast remains on
track with the main forecast issue continuing to be the pattern
change towards a rainier week ahead. With increasing confidence
in urban and small stream flooding across the Bay Area and Santa
Cruz County, the operations team has issued a Flood Watch from
Friday afternoon through Sunday evening. People living in flood
prone areas should be prepared to take action should flooding
develop over the next few days.

DialH

&&

.SHORT TERM...
(This evening through Friday)
Issued at 248 PM PST Thu Jan 30 2025

Cloud cover has increased today along the coast, with mostly sunny
skies inland. Light rain will begin to move onshore late tonight
across the North Bay and gradually spread south and farther inland
during the day tomorrow. As the increased moisture builds out ahead
of the rainfall, expect patchy fog and drizzle to increase in
coverage, especially along the coast from the North Bay south to the
Santa Cruz coastal range. Overnight lows into early tomorrow morning
will still be on the cold side across southern Monterey County and
San Benito County, while cloud cover and increased moisture will
lead to more mild, seasonal low temperatures for the remainder of
our forecast area.

&&

.LONG TERM...
(Friday night through next Wednesday)
Issued at 248 PM PST Thu Jan 30 2025

Confidence continues to increase for widespread impactful rainfall to
persist through the remainder of the extended forecast. The first
system arrives on our doorstep late tonight. Much of the North Bay
will have already seen between 0.50-1.00" of rain by late tomorrow
night and early Saturday. The focus of higher rainfall amounts will
then shift south into the Bay Area, East Bay and Santa Cruz coastal
range throughout the day Saturday. A relatively strong upper level
trough axis will shift onshore late Friday into early Saturday when
increased rainfall rates can be expected from the North Bay into the
Bay area. While the parent low reconsolidates well to our north,
zonal flow with a series of embedded shortwaves will continue to
drive widespread light rain across most of our area, with enhanced
upsloping producing periods of moderate to heavy rain along our
coastal ranges through the weekend.

The main upper low then dives south late Sunday night into Monday.
Deterministic guidance is in decent agreement for the evolution of
the primary low`s shift south, but this part of the forecast will
continue to be watched closely. Any change or retreat north for the
beginning of next week, will result in big changes for rain totals
outside of the North Bay.

The rainfall totals for tonight through early Monday morning should
only lead to minor impacts with respect to flooding across the
North Bay. However, by the time the second round arrives for
Tuesday and Wednesday, we`ll see rivers and streams begin to
respond quicker to the steady fetch of rainfall expected. Even
with the higher end totals anticipated, there is roughly a 10%
chance of flooding for normal trouble spots along the Russian Rive
in Sonoma County and other locations into Napa County and Marin
County. As we monitor the rainfall this weekend, and the evolution
of the larger scale pattern for the beginning of next week, keep
an eye on subsequent forecast updates as these river forecasts
could change considerably. Along with the potential for periods of
moderate to heavy rain leading to flooding, there will also be
moderate to strong gusty winds. The strongest winds will be across
higher elevations, beginning Saturday morning with the passage of
the aforementioned trough axis moving onshore. As some might
expect, the strongest gusts should be expected over higher
elevations like Mt Helena, up to 60 MPH after sunrise Saturday
through much of the day. We`re anticipating the need for a Wind
Advisory beginning at some point early Saturday, with elevated
wind speeds continuing through the middle of next week. While
strong gusty winds at higher elevations may approach 60 MPH, lower
lying areas along the coast and inland valleys can expect max
wind gusts between 35-45 MPH at times.

&&

.AVIATION...
(06Z TAFS)
Issued at 921 PM PST Thu Jan 30 2025

Patches of MVFR-IFR stratus along the coast, with mid-to high level
clouds streaming into the region in advance of a storm system
expected to arrive through the day Friday. Light winds through the
night, with southerly flow expected to strengthen in advance of the
approaching storm. MVFR ceilings will fill in through the night and
remain present across the region through the rest of the TAF period.
Showers and stratiform rain will arrive in the North Bay early
Friday morning, building southward through the day. Rain and mist
will lower visibilities through the rest of the TAF period.

Vicinity of SFO... MVFR stratus will build through the evening and
cover the terminal by Friday morning, when rain showers will
develop. More consistent rain arrives late afternoon and continues
into the overnight hours into Saturday morning. Light winds continue
through the night with gusty southeast winds developing through
Friday.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals... Generally MVFR stratus through the TAF
period with light winds through the night. With the terminals on the
fringes of the approaching storm, impacts from rain arrive later and
are somewhat more uncertain than in areas further north. Rain
showers develop late Friday afternoon and will intensify later in
the evening. Moderate confidence in generally light winds at the
terminals, but there is greater confidence in more intense southerly
winds west of Monterey Peninsula.

&&

.MARINE...
(Tonight through next Tuesday)
Issued at 248 PM PST Thu Jan 30 2025

Strong southerly winds with gale force gusts begin Friday and
persist through Tuesday. Significant wave heights will remain
moderate to rough. Widespread rainfall is expected Friday through
Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from Friday afternoon through Sunday evening for
     CAZ006-502>506-508>510-512>515-529.

PZ...Small Craft Advisory from 9 AM Friday to 3 AM PST Saturday for
     Pigeon Pt to Pt Pinos 10-60 NM-SF Bay N of Bay Bridge.

     Small Craft Advisory from 3 AM Friday to 3 AM PST Saturday for
     Pt Arena to Pt Reyes 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory from 3 PM Friday to 3 AM PST Saturday for
     Pigeon Pt to Pt Pinos 0-10 nm.

     Small Craft Advisory until 3 AM PST Saturday for Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...JM
LONG TERM....JM
AVIATION...DialH
MARINE...Sarment

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea