068
FXUS66 KMTR 211836 AAA
AFDMTR

Area Forecast Discussion...UPDATED
National Weather Service San Francisco CA
1036 AM PST Thu Nov 21 2024

...New AVIATION, MARINE...

.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.

&&

.UPDATE...
Issued at 915 AM PST Thu Nov 21 2024

Over the last twelve hours cities across northern Sonoma and Napa
counties have seen an additional 2-4" inches of rain. This brings
our thirty-six hour rainfall totals to between 5-8" across most of
the North Bay with locally higher totals up to 13" across the
coastal North Bay Mountains. Notably, Venado (VENC1 at an elevation
of 1266 feet)is reporting 13.83" in the last 36 hours and Austin
Creek - Russian River (XACC1 at an elevation of 1230 feet) is
reporting 12.95" in the last 36 hours. Minor flooding has been
reported across portions of the North Bay with several smaller
creeks/rivers showing local rises. Remember to avoid flooded
roadways and to slow down/take extra time while out traveling today
so that you arrive at your destination safely. The main band of rain
over the North Bay is expected to shift northward (primarily
impacting far northern Sonoma and Napa counties). Scattered light to
moderate showers will then continue throughout the rest of the North
Bay and portions of the SF Peninsula/East Bay today.

&&

.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...
(18Z TAFS)
Issued at 1034 AM PST Thu Nov 21 2024

Quite the spread with regard to flying conditions across the
region. North Bay and SF Bay terminals will experience poor flying
conditions as widespread cloud cover and rain continues to stream
in from the SW. IFR ceilings and visibility can be anticipated
with variable conditions in/around precipitation. The first round
of more RA/SHRA is anticipated to continue through at least 0300
UTC when activity tapers off for a handful of hours during the
pre-dawn hours on Friday. Through the afternoon, precipitation
coverage is anticipated to diminish such that VCSH seems to be
appropriate. KLVK and KSJC appear to be too far south to warrant
VCSH this afternoon. By 0300 UTC, we`ll likely see a bit of a
break in precip, before additional moisture streams in and eases
southward through the Bay Area. The other sensible aviation hazard
will be S`ly winds. Sustained speeds between 12 and 15 knots are
anticipated at most terminals with gusts in excess of 25 knots
forecast. Surface flow should remain elevated to preclude LLWS in
the TAFs, but if flow does diminish rapidly at the surface,
LLWS inclusion may be needed in future TAF iterations.

The second round of SHRA is anticipated to get underway prior to
sunrise Friday when a front is projected to slice southward
through the entire area. Ahead of this feature, cloud bases are
forecast to range between IFR and MVFR at all terminals except
KLVK and KSJC. Intermittent MVFR and IFR visibility can be
anticipated in bouts of +RA, though confidence in the timing of
these features is uncertain. Winds will begin to turn more SW`ly,
but remain elevated above 10 knots at all terminals.

Vicinity of SFO...MVFR ceilings with occasional -SHRA are forecast
this afternoon. Unfortunately southerly winds are probable to
persist with gusts near and above 20 knots forecast in the
afternoon through the early evening hours. There does appear to be
a period where southerly winds subside to below 10 knots to allow
for some flexibility in takeoff/landing configs, but confidence
is low to medium in this transpiring. S`ly wind gusts to near 25
knots are anticipated Friday morning, with SHRA likely resulting
in a messy and impactful morning push as a return to the south
plan becomes probable (high confidence in this). Late Friday
morning will see widespread RA with MVFR visibility (and a low
chance for IFR visibility). Winds shift, but not until 00 UTC
Saturday, likely forcing the south plan through the first part of
the Friday afternoon/evening push.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR currently, though a medium chance for
MVFR stratus late this afternoon/evening. Elevated S`ly winds
continue at SNS as flow accelerates through the Salinas Valley
with gusts just under 30 knots. As flow just above the surface
increases and becomes more onshore, MVFR ceilings will make an
attempt to infiltrate both terminals, but confidence is low to
medium. Most guidance continues to support MVFR in the extended
portion of the TAF and I`ll advertise cigs around FL025. I`ve also
introduced VCSH at KMRY early on Friday, though there`s the
potential that activity is delayed and activity doesn`t arrive
until closer to 18 UTC.

&&

.MARINE...
(Today through Tuesday)
Issued at 956 AM PST Thu Nov 21 2024

Hazardous marine conditions are forecast to continue over the next
24 hours. Winds will range between 25 knots to 40 knots,
particularly over northern coastal waters. Seas up to and just
above 15 feet are anticipated with long period swell continuing to
roll in. Widespread rain showers will result in gusty and erratic
outflows and subsequent rough waters. Boating conditions will
improve this weekend, but still remain quite hazardous,
particularly to smaller vessels and crafts.

&&

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

     Flood Watch through late Friday night for CAZ502>506.

     Beach Hazards Statement through late tonight for CAZ529.

PZ...Small Craft Advisory until 11 AM PST Friday for SF Bay N of Bay
     Bridge.

     Small Craft Advisory until 9 PM PST Friday for Mry Bay-Pigeon Pt
     to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

     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 Reyes to Pigeon Pt 0-10
     nm.

     Small Craft Advisory until 7 AM PST Friday for Pigeon Pt to Pt
     Pinos 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Bain
MARINE...Bain

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