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303
FXUS66 KMTR 210837
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1237 AM PST Thu Nov 21 2024

...New SYNOPSIS, SHORT TERM, LONG TERM...

.SYNOPSIS...
Issued at 1232 AM PST Thu Nov 21 2024

Day 2 of the atmospheric river. The heaviest rain will reamin
focused in the North Bay before spreading south on Friday.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1232 AM PST Thu Nov 21 2024

Key Messages:

-Numerous Flood Advisories cover the North Bay
-Flood Watch in effect for the North Bay through Saturday morning
-Wind Advisory in effect through early Thursday
-High Surf Advisory in effect through Friday morning

A classic atmospheric river is in full display as a narrow band of
water vapor, clouds, and rain stream across northern California.
This feature is more or less unmoving, keeping the major impacts
focused north of the Golden Gate. Once it started, the rain really
never backed off in the North Bay. 24 hour rainfall totals are
quite impressive, with 7.07" at the Santa Rosa airport and a few
reports over 10" in the coastal mountains of Sonoma County. As
expected, the rain totals drop off quickly south of the Golden
Gate, and Downtown San Francisco has only recorded 0.39" so far.

This trend will continue through Thursday. There may be a slight
decrease in the amount of North Bay rain today, but it`s still
looking like another washout by any measure, with 1-3" in the
valleys and up to 6" in the mountains over the next 24 hours. This
prolonged rain will continue to fill creeks and streams and flooding
concerns persist in the North Bay.

Strong southerly winds will also continue, and the combination with
heavy rain will lead to downed trees and potential power outages.
The winds are expected to decrease a bit today as the gradient
relaxes, but they won`t improve substantially until Friday evening.

For the rest of the Bay Area, the southerly flow is causing warm air
advection, and tomorrow will be surprisingly warm and humid under
overcast skies. Some rain is expected for areas north of San Jose,
but will be much lighter and more spotty. The Central Coast will
stay dry, with a few sun breaks. Your time will come on Friday as
the cold front finally pushes through and brings more widespread
impacts.

&&

.LONG TERM...
(Friday through Wednesday)
Issued at 1232 AM PST Thu Nov 21 2024

If we zoom out on the current satellite loop, a new surface low is
being born in the cold air mass out in the Eastern Pacific. This
system is under a ~130 kt jet stream, which is causing plenty of
divergence aloft and thus lowering the surface pressure. This system
will undergo explosive cyclogenesis as it moves towards the
Pacific Northwest and occludes by Friday. The associated cold
front will come ashore on late Friday or early Saturday, bringing
another bout of heavy rain and strong winds in the prefrontal
environment through the day Friday. After the front passes, the
atmospheric river will end, but the rain won`t stop. Convective
showers are expected Saturday, though the threat of thunderstorms
has decreased to around 10%. Moderate rain is expected Sunday and
Monday as well, but the uncertainty is much higher. The active
weather pattern looks like it will continue into next week with a
series of weaker low pressure systems and fronts brining periods
of rain through the week. We don`t have a dry day in the full 7
day forecast.

&&

.AVIATION...
(06Z TAFS)
Issued at 951 PM PST Wed Nov 20 2024

A nearly stalled frontal boundary associated with a strong atmospheric
river continues to bring moderate to heavy rain to the North Bay.
North Bay conditions include strong lower level southerly to
southwesterly winds and lower level wind shear, KSTS and KAPC
weather is persistence for the 06z TAFs. Surface to lower level
southerly winds are upsloping winds for the North Bay, enhancing
rain rates not only through upslope cooling, air parcel volume
expansion and lower pressure, but enhancing frontal development
(frontogenesis) via ingesting initially drier more distant surface
air, air parcels partially cooled via evaporation in contrast to
the incoming lower level warm air advection with the nearly stalled
atmospheric river. South to southeast surface winds across much
of the rest of the forecast area are resulting in downsloping
(drying) winds limiting rainfall away from the North Bay causing a
sharp gradient to the current copious rainfall/no rain line, though
this line is shifting slowly southward a bit with surface winds
easing tonight (increasing nocturnal stability). It`s a high
confidence IFR forecast for the North Bay, to the south a moderate
to high confidence MVFR-VFR forecast.

Vicinity of SFO...MVFR, periodic light rain and gusty southeast wind
through tonight and Thursday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR, a passing shower/sprinkle or two possible
otherwise periodic gusty southeast winds, especially in the Salinas
Valley.

&&

.MARINE...
(Tonight through next Tuesday)
Issued at 842 PM PST Wed Nov 20 2024

Heavy rain and strong winds continue primarily across the northern
waters tonight into early Friday morning before pushing
southeastward Friday. Very rough seas up to 20 feet in the
northern zones are propagating south through the coastal waters.
Gusty gale-force winds to near storm force winds are expected
across the waters tonight and linger into late week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory until 6 AM PST Friday for CAZ006-505-509-530.

Wind Advisory until 6 AM PST early this morning for CAZ006-
502>506-508-509.

Flood Watch through late Friday night for CAZ502>506.

Beach Hazards Statement through late tonight for CAZ529.

PZ...Gale Warning until 3 AM PST Friday for Pigeon Pt to Pt Pinos 0-
10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-
10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to
Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge.

Small Craft Advisory until 3 AM PST Friday for Mry Bay.

Gale Warning until 3 AM PST early this morning for Pt Arena to
Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Canepa
MARINE...Canepa

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