913
FXUS66 KMTR 221937
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1137 AM PST Fri Nov 22 2024

...New UPDATE...

.SYNOPSIS...
Issued at 453 AM PST Fri Nov 22 2024

An atmospheric river continues to inundate northern California this
morning, with the North Bay region currently seeing the bulk of the
rain. Rain chances will increase throughout the day for the service
area as a cold front works its way across the Bay Area and north
Central Coast. Prolonged rainfall will result in an increased risk
of flooding, an increased risk of landslides, and downed trees and
powerlines across the North Bay.

&&

.UPDATE...
Issued at 1112 AM PST Fri Nov 22 2024

Short term, high resolution models are beginning to show the
system slow to a craw this afternoon, potentially increasing the
rain time for potions of the Bay Area and Santa Cruz Mtns, and
also increasing the rainfall amounts for those areas. The
forecasting team is currently reviewing these numbers.

&&

.SHORT TERM...
(Today and tonight)
Issued at 453 AM PST Fri Nov 22 2024

Key Messages:

-Widespread moderate to heavy rain will spread southeast across
 the rest of the Bay Area and north Central Coast today.
-Large waves and strong winds will lead to dangerous marine and
 beach conditions today.
-Heavy rain may lead to the flooding of low lying areas and
 roadways, which may cause them to not be passable.
-Gusty winds, especially where soils are saturated, may lead to
 downed trees and power lines.
-Saturated grounds may lead to an increase risk of land slides.

An atmospheric river will continue to impact northern and central
California today. As of this morning, the majority of the rain has
been up in northern California reaching as far south as the North
Bay. The area of low associated with this AR spins west of the PacNW
via satellite. The Ocean Prediction Center placed the cold front
around 846UTC between 130W and 120W. This cold front is expected to
gradually work its way through the region, bringing widespread to
moderate heavy rain for the Bay Area and the north Central Coast.
Areas south of there, chances for rain decrease. Given the prolonged
rain for our northern most areas, the Weather Prediction Center has
placed the North Bay in a Moderate Risk (level 3 of 4) for
Excessive Rainfall and the rest of the Bay Area and the north
Central Coast in a Slight Risk (level 2 of 4) which matches well
with the listed flooding hazards listed above in the key points.
Elsewhere in the Central Coast, particularly for the rest of Santa
Clara, northwest San Benito, and areas along and west of the
Salinas Mountains in Monterey County, a Marginal Risk (level 1 of
4) of Excessive Rain is forecast. In addition to the rain threat,
southerly winds will increase today ahead of the cold front. The
Wind Advisory covers much of the region, where sustained wind
speeds will range from 15 to 30 mph. Gusts up to 40 to 50 mph are
expected for coastal communities and for elevations up to 2500
feet. There is a decent shot at some of the ridges above 2500 feet
seeing winds exceed 50 mph, perhaps getting close to 65 mph.
Areas that are excluded from the winds are the valley zones where
they should be sheltered from the stronger gusts. Given the
saturated grounds and increasing winds in the North Bay, there
will be an increased risk of landslides, flood roads, especially
in low lying and poor drainage areas, downed trees, and downed
powerlines. There may be a couple of rumbles of thunder,
particularly in the North Bay; however, confidence is fairly low.

&&

.LONG TERM...
(Saturday through Thursday)
Issued at 453 AM PST Fri Nov 22 2024

Wet weather continues through the weekend and into early next week.
Showers will linger across the region in wake of the cold front on
Saturday. Several additional rounds of rain will then work their way
through the region beginning on Sunday and continuing through
Tuesday as low pressure sits off the coast. The upper level trough
will slowly work through the PacNW and then enter intermountain west
by midweek. Conditions begin to dry out by then and may hold for a
day or two. There`s some split guidance in the cluster analysis as
to where ridging sets up and if an upper low forms west-southwest to
southwest of southern CA.

&&

.AVIATION...
(18Z TAFS)
Issued at 1030 AM PST Fri Nov 22 2024

Flight categories and higher end wind gusts will be tied to the
occurrence of rain and showers. The timing of precipitation
remains tricky and overall the latest TAF iterations shows a
slower arrival and end time to precipitation. Overall, there is
low to medium confidence in the forecast due to timing uncertainty.
In general rain will continue to overspread from west to east
resulting in largely IFR visibility. Ceilings may improve briefly
with the passage of individual downpours, though gusty and
erratic winds of over 25 knots may transpire. Some uncertainty
does exist regarding the cessation of precip, but current
indications are that most of the activity will exit Bay Area and
KHAF TAFs between 08-10 UTC, but linger through Saturday morning
at Monterey Bay sites. A brief wind shift to the north, associated
with a weakening front, will transpire during the pre-dawn hours
on Saturday. This may support a brief window of VFR. Upper level
troughing to the west will encourage winds attaining more of a
southerly component and potentially additional -RA or DZ. -RA/DZ
will result in reduced ceilings/visibility.

Vicinity of SFO...MVFR to at times IFR with periods of RA are
forecast this afternoon. Wind gusts near or just under 30 knots
will accompany RA. A return to a west plan airport config seems
most probable around 02Z when winds become more westerly and
weaken with the approach of the surface trough/front. Brief VFR
after 07Z is expected to yield to MVFR around 13Z as gradual
ascent atop the cooler surface airmass results in stratus.
Confidence is low to medium on the details and forecasts will be
updated as additional information arrives.

SFO Bridge Approach...Similar to SFO, though higher winds may
take place for aircraft on approach.

Monterey Bay Terminals...VFR currently with largely mid/upper
level clouds cluttering the Central Coast airspace. The onset of
RA has been pushed back a few hours based on trends and the latest
model guidance. At current time, RA is anticipated to arrive after
06 UTC and bring MVFR visibility and ceilings. RA is anticipated
to continue, albeit on an intermittent basis, through the end of
the valid TAF period.

&&

.MARINE...
Issued at 1030 AM PST Fri Nov 22 2024

Hazardous marine conditions will persist through the rest of
today and into Saturday with Gale force wind gusts continuing for
Pacific waters south of the Farallons. Seas of 7 to 12 feet are
anticipated this afternoon before easing down to 5 to 8 feet on
Saturday. Seas will rebuild briefly late Sunday and into early
Monday. Boating conditions will improve this weekend, but still
remain quite hazardous, particularly to smaller vessels and
crafts.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 10 PM PST this evening for CAZ006-502>505-
     509-512-514-515-517-518-530.

     Flood Watch through Saturday morning for CAZ502>506.

PZ...Small Craft Advisory until 3 AM PST Saturday for Mry Bay-SF Bay
     N of Bay Bridge.

     Small Craft Advisory until 3 PM PST this afternoon for Pt Arena
     to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.

     Gale Warning until 9 PM PST this evening for Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Reyes to Pigeon Pt 0-10 nm.

     Gale Warning until 3 AM PST Saturday for Pigeon Pt to Pt Pinos 0-
     10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...KR
LONG TERM....KR
AVIATION...Bain
MARINE...Bain

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