National Weather Service United States Department of Commerce

Heavy Rain and Severe Storms in the Southern U.S.; Atmospheric River to Impact California

Heavy rain, flash flooding and severe thunderstorms are expected tonight into early Thursday from southern Louisiana into western Georgia. An Enhanced Risk (level 3 of 5) of severe thunderstorms is in effect. Across the West, an atmospheric river will bring a heavy rainfall and flash flooding threat to central and southern California through Thursday. Heavy snow is forecast in the Sierra Nevada. Read More >

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878
FXUS66 KMTR 130005
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
405 PM PST Wed Feb 12 2025

...New AVIATION...

.SYNOPSIS...
Issued at 330 PM PST Wed Feb 12 2025

Potentially major impacts from a strong storm system incoming
tonight and lasting into Thursday. Local flooding and widespread
damaging winds are the main threats through Thursday morning.
Slight chance for thunderstorms Thursday afternoon. Drier
conditions on Friday into the weekend. Potential showers for the
North Bay on Sunday.

&&

.SHORT TERM...
(This evening through Thursday)
Issued at 330 PM PST Wed Feb 12 2025

KEY POINTS
* Flood Watch in effect 10 PM Wed to 10 PM Sat (area-wide)
* High Wind Warning (coastal) and Wind Advisory (inland) in
effect from 10 PM Wed to 10 AM Fri (coastal)
* Widespread shallow landslides possible Thursday morning
* Downed trees and power outages likely
* Highest impact areas include Santa Cruz Mtns and Big Sur Coast
* High Surf Warning in effect 3 PM Thu to 3 AM Sat

METEOROLOGY:

Major winter storm still on track to impact the region beginning
tonight. Beautiful visible satellite representation of the storm
currently centered about 1,000 miles to our northwest. One can
clearly make out features such as the nearly stationary warm front
draped along the CA coast, as well as the intense surface cold
front wrapping into the storm from the south and west. The storm
will continue translating southeastward guided by an intense 170
kt jet streak aloft. Latest advanced scatterometer pass over the
storm from about 1 PM today shows estimated sustained surface
winds of 40-45 kts, which is in line with latest forecast wind
intensities. Current surface pressure has been analyzed at 986 mb
as of 1 PM this afternoon. Expecting these winds to hold or
slightly increase their intensity as the storm nears the CA coast
and deepens to around 981 mb by late tonight. As the storm nears
the coast tonight, rainfall will likely start out as a gradual
warm frontal overrunning in which rain will be widespread, but
only light to moderate in intensity, lacking the robust cold
frontal processes and enhanced upslope moisture transport.
Progressing further after midnight tonight, the warm front moves
ashore and the cold front enters the North Bay, moving quickly
down the coast through the early morning hours. This will be the
period in which the heaviest rainfall rates will occur...on the
order of 1.0 to 1.5 in/hr, especially in the Santa Cruz Mtns and
Big Sur coast. In addition to the intense rainfall rates, damaging
winds will be a factor through the morning hours as well. Once
the mid-level trough axis comes ashore around midnight, the strong
mid-to-upper level flow begins to impinge on the CA coast. 850 mb
flow out of the southwest at 50 to 60 kts spreads across the
region from north to south from about midnight in the North Bay to
11 AM along the Big Sur coast. This will likely result in surface
wind gusts of 40 to 50 mph across the area, with coastal areas
and higher elevations seeing gusts in the 60 to 70 mph range.
Isolated areas along the coast and coastal mountain ridgetops may
end up seeing gusts closer to 80 mph as the more intense winds mix
down with the passage of the cold front. Behind the cold front,
the environment remains supportive for thunderstorms well into the
afternoon. Immediately after the cold frontal passage, any
thunderstorms that initiate will likely be over powered by robust
speed shear and limited instability. There will also be a bit of
dry air mixing into the storm top level per RAP forecast soundings
along the coast, so there is some limited downburst potential
with a few storms that are able to stay intact along the coast.
Beyond the morning hours, instability increases a bit as the
surface warms slightly, and speed shear decreases a bit as well
as the upper level jet start to move out of the region. This will
likely provide a window of time where storm ingredients are better
suited to lead to more prolonged updrafts and better potential
for rotation should any storm deviate to the right of the mean
storm motion. For more details on the severe threat, see
spc.noaa.gov, which places us in a general risk for thunderstorms
for Thursday.

IMPACTS:

General flooding impacts will begin in the North Bay as early as
tonight and spread across the rest of the region shortly after
midnight. It may appear as a slow start for those areas that start
to see rainfall first, as it will mostly be associated with a
warm/stationary front, thus not terribly heavy. However, after
midnight, rainfall rates are expected to increase substantially
with the passage of the main cold front. This will exacerbate
flooding concerns, especially in the Santa Cruz Mtns and Big Sur
coast. By the time the cold front moves through, soils will be
nearly saturated in the Santa Cruz Mtns, so we do expect to see
issues with quick runoff and shallow landsliding. Not only will
this alone be an issue, but winds are expected to gust up to 60
mph (as high as 80 mph along the ridge of the SC Mtns). Combining
the high winds with saturated soils is a recipe for downed trees
and power lines, leading to potential structure damage and power
outages. The Flood Watch has been expanded to cover the entire
region, and the High Wind Watch has been upgraded to a High Wind
Warning for coastal areas, and a Wind Advisory for inland areas.
We will likely see notable rises on many major rivers, but the
Russian River at Guerneville and the San Lorenzo River have the
highest chances of going into flood stage. The San Lorenzo perhaps
being one of the more worrisome ones given how flashy the terrain
is known to be in that region. Other coastal areas outside of the
Santa Cruz Mtns may not see the same degree of shallow landsliding,
but nonetheless will still have impacts due to downed trees and
powerlines, especially areas near the Marin headlands, Big
Sur/Carmel coast, and Monterey Peninsula. The commute Thursday
morning will undoubtedly be nasty and downright dangerous in some
cases (thinking Hwy 17 in particular). When all is said and done,
the North Bay will likely end up with 2 to 4 inches, and the Bay
Area will see a range of 1.5 to 3 inches depending on if you`re in
the South Bay rain shadow or not. The Santa Cruz Mtns and Big Sur
coast will be the winners with 4 to 7 inches, locally up to 8-10.
Despite some locations seeing some of the lower end of rainfall
amounts, wind is still expected to pose a threat to life and
property through Thursday.

&&

.LONG TERM...
(Friday through next Tuesday)
Issued at 330 PM PST Wed Feb 12 2025

Shower activity will be coming to an end Friday morning with dry
conditions into Saturday. Medium range ensemble guidance is showing
the potential for another much weaker storm system on Sunday that
would brush the North Bay, but the upcoming storm system this
Thursday morning is the only majorly impactful period that we see in
the 7 day forecast period. There will likely be a lull in storm
activity along the West Coast through next week.

&&

.AVIATION...
(00Z TAFS)
Issued at 404 PM PST Wed Feb 12 2025

A mixed bag of MVFR and VFR conditions as the storm system continues
to move into our area. Some vicinity showers will occur until
tonight where the heavier bands of rain will move over the terminals
creating reduced visibility and low ceilings. CIGs and VIS may drop
to IFR if rain intensifies overnight. There is a chance for low
level wind shear/upper air turbulence across most terminals
overnight into early Thursday morning. Rain will start to diminish
towards Thursday morning but expect MVFR to prevail through the TAF
period for all terminals. Winds will start to build into strong
gusty southerly winds overnight and prevail through the rest of the
TAF period.

Vicinity of SFO...MVFR with light westerly winds. These winds will
turn to southerly direction shortly after 00Z and will build to
strong and gusty winds with a chance for AWW speeds near 10-11Z.
LLWS/turbulence is possible as well with speeds up to 50-55 knots
at 2000 feet near 10-11Z. High confidence that winds will overall
remain strong and gusty through the remainder of the TAF period.
As the rain becomes heavier, CIGs/VIS may drop to IFR conditions
overnight.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR/MVFR conditions. Higher confidence that
MVFR conditions will prevail after 06Z through the remainder of the
TAF period. Southerly winds will build to strong and gusty with a
chance of LLWS between 12-16Z.

&&

.MARINE...
(Tonight through next Monday)
Issued at 330 PM PST Wed Feb 12 2025

Winds are increasing widespread gale conditions with isolated
storm force gusts possible. Moderate period westerly swell will
build through Friday morning with very rough to high seas
expected. Light rain showers will continue through the day ahead
of moderate to heavy rainfall that is expected Wednesday night
through Friday morning with a slight chance for thunderstorms
Thursday.

&&

.BEACHES...
Issued at 1101 PM PST Tue Feb 11 2025

A High Surf Warning is in effect from 3 PM Thursday through 3 AM
Saturday for all Pacific Coast beaches. Westerly swell with a period
up to 15 seconds will build through Friday. As a result, west-facing
beaches will see dangerously large breaking waves of 20 to 30 feet
and up to 45 feet peak waves, non-west facing beaches will see
dangerously large breaking waves of 15 to 20 feet and up to 30 feet
peak waves. Everyone should remain out of the water due to life-
threatening surf conditions. Stay off of jetties, piers, and other
waterside infrastructure, keep pets on a leash and away from the
water, and never turn your back on the ocean!

Sarment

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from 10 PM PST this evening through Saturday evening
for CAZ006-502>506-508>510-512>518-528>530.

High Wind Warning from 3 AM Thursday to 10 AM PST Friday for
CAZ006-502-503-505-509-512-529-530.

High Surf Warning from 3 PM Thursday to 3 AM PST Saturday for
CAZ006-505-509-529-530.

Wind Advisory from 3 AM Thursday to 10 AM PST Friday for CAZ504-
506-508-510-513>518-528.

PZ...Gale Warning from 9 PM this evening to 3 AM PST Friday for Mry
Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-
60 NM-Pt Reyes to Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge.

Gale Warning until 3 AM PST Friday for Pt Arena to Pt Reyes 0-10
nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt Piedras
Blancas 0-10 nm.

Small Craft Advisory until 9 PM PST this evening for Pigeon Pt
to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Reyes
to Pigeon Pt 0-10 nm.

&&

$$

SHORT TERM...Behringer
LONG TERM....Behringer
AVIATION...SO
MARINE...Murdock

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