Ben Lomond, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

350
FXUS66 KMTR 212225
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
225 PM PST Thu Nov 21 2024

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

.SYNOPSIS...
Issued at 222 PM PST Thu Nov 21 2024

Atmospheric River continues to remain stationary across the North
Bay with increasing flood risks developing. Prolonged rainfall is
additionally resulting in an increased risk of landslides, downed
trees, and downed powerlines across the North Bay. More
widespread moderate rain will spread south on Friday into the rest
of the Bay Area and Central Coast.

&&

.SHORT TERM...
(This evening through Friday)
Issued at 222 PM PST Thu Nov 21 2024

Key Messages:

-Aerial Flood Warning now in effect for central Sonoma County
-Numerous Flood Advisories cover the North Bay
-Flood Watch in effect for the North Bay through Saturday morning
-Wind Advisory in effect from 10 PM tonight through 10 PM tomorrow
-High Surf Advisory in effect through Friday morning

A stationary rain band producing moderate to at times heavy showers
continues to train over the North Bay with the highest rain totals
focused in Sonoma County. Most areas in Sonoma and Napa counties
have seen between 5" to 10" of rain in the last 48 hours. Locally
higher amounts between 10" to 15" have been observed across the
higher elevations of the coastal and interior North Bay Mountains.
Southwards in Marin County, rain totals are slightly lower with most
sites seeing between 3" to 6" over the last two days. Rain totals
drop off significantly outside of the North Bay with most sites in
the northern San Francisco Peninsula and East Bay seeing less than
an inch so far. Reports of flooding and downed trees have started to
increase across the North Bay. As such an Aerial Flood Warning is
now in effect for portions of central Sonoma County, including urban
Santa Rosa, with creeks and streams starting to rise in the vicinity
of the city. Heading into Friday, we can expect more widespread
moderate showers to spread into the rest of the Bay Area and Central
Coast late tonight through tomorrow. This will bring an additional
2" to 5" across the North Bay with locally higher totals possible in
the elevated terrain. The majority of the Bay Area and coastal
mountain ranges will see between 1.5" to 3" of rain while portions
of the South Bay and Central Coast will see up to an inch. A slight
(less than 10%) chance of thunderstorms will continue through Friday
evening. The main things to focus on heading into Friday are 1)
increased flood risk across the North Bay, 2) flood risk across the
rest of the Bay Area and Central Coast as rain spreads southward,
and 3) elevated wind threat tonight into tomorrow.

Showers have not let up over the North Bay in over a day and a half.
As such two things are happening 1) streams and creeks in Sonoma
County are starting to rise with many already reaching the action to
minor flood stages. Among the most notable, official forecast points
have the Russian River at Geyserville peaking at minor flood stage,
Russian River at Guerneville peaking at the action stage, and the
Mark West Creek near Mirabel Heights peaking at the moderate flood
stage tomorrow evening. Streams and creeks throughout the North Bay
are expected to continue rising as moderate to at times heavy rain
continues to train over the North Bay today and tomorrow. This
brings us to our second point, 2) soil saturation. Prior to this
event soils were fairly dry across the North Bay, but, after the
last two days, soils are becoming fairly saturated. As the soil
becomes more saturated, it is able to absorb less water and results
in increased surface runoff, resulting in more widespread flooding.
As such, a Flood Advisory remains in effect across the entire North
Bay through early Saturday morning and a more targeted Aerial Flood
Warning is now in effect for urban Santa Rosa. Elsewhere, flooding
potential will increase slightly across the rest of the Bay Area and
Central Coast as moderate showers spread southward Thursday night
into Friday. Soils remain dry in these regions which help to
slightly decrease flooding concern for this region. However,
nuisance flooding is still a possibility especially if moderate to
heavy showers train over a particular location for an extended
period of time.

Strong, gusty southerly winds return tonight with a Wind Advisory in
effect from 10 PM tonight to 10 PM tomorrow night along the coast
and across areas of elevated terrain. HRRR model guidance supports a
narrow frontal rainband developing overnight Thursday into Friday
and pushing southwards through the evening. Strong winds are
expected to develop ahead of and along this rain band but will
quickly ease after frontal passage occurs. Widespread wind gusts up
to 50 mph are possible but locally higher 60+ mph gusts may develop
above 2,500 feet. This will contribute to an increased risk of
downed trees and power outages, particularly across the North Bay,
as soils become saturated. Anyone who is able to should avoid being
out during the worst of the storm (early Friday morning through
Friday evening) should do so. If you have any outdoors decorations,
now is the time to secure them. If you have to be out on Friday,
make sure to leave extra time to get to your destination and avoid
driving through flood waters. Waters overtopping roadways may be
deeper and swifter than they appear and can result in a dangerous
situation developing. Flooding can be particularly hard to see in
the dark so use extra caution when traveling at night. Remember -
turn around, don`t drown.

&&

.LONG TERM...
(Friday night through next Wednesday)
Issued at 222 PM PST Thu Nov 21 2024

Friday night through Saturday, light showers will continue across
the region in the wake of cold frontal passage. Totals from these
showers will be relatively minimal (amounting to less than a few
tenths of an inch) with a 10% chance of thunderstorms persisting
through the day on Saturday. However, the storm door remains open as
we see low pressure lingering off the coast of Washington/Oregon and
increased moisture transport into Central California through
midweek. While there is still some uncertainty as to how long rainy
conditions will persist for, it is looking increasingly likely that
rain will persist through midweek. As such, an additional 1" to 2"
of rain is expected across the North Bay, Santa Cruz and San Mateo
Mountains, and Santa Lucia Mountains from Sunday to Wednesday and up
to an inch of rain elsewhere. Some potential light at the end of the
tunnel, models have trended drier beginning Wednesday afternoon into
Thursday but, given uncertainty in the forecast almost a week out,
this may change as we get closer.

&&

.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...
(Tonight through next Tuesday)
Issued at 222 PM PST Thu Nov 21 2024

Widespread gale force winds will continue the hazardous marine
conditions. Winds will range between 25 knots to 40 knots. 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.

     Wind Advisory from 10 PM this evening to 10 PM PST Friday for
     CAZ006-502>505-509-512-514-515-517-518-530.

     Flood Watch through late Friday night for CAZ502>506.

     Beach Hazards Statement through late tonight for CAZ529.

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

     Gale Warning until 9 AM PST Friday for Pt Arena to Pt Reyes 0-10
     nm-Pt Arena to Pt Reyes 10-60 NM.

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

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

     Gale Warning from 3 AM Friday to 3 AM PST Saturday for Pigeon Pt
     to Pt Pinos 0-10 nm.

     Small Craft Advisory until 9 AM PST Friday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

     Gale Warning from 9 AM to 9 PM PST Friday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...Bain
MARINE...Murdock

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