FXUS66 KMTR 220527
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
927 PM PST Thu Nov 21 2024
...New AVIATION...
.SYNOPSIS...
Issued at 828 PM PST Thu Nov 21 2024
Update: Flood Warning is in effect from Friday afternoon until
further notice for the Russian River at Johnsons Beach at Guernville.
At 8 pm this evening the stage was 21.9 feet and it is forecast
to continue rising to a crest of 33.7 feet early Saturday morning.
Flood stage is 32.0 feet.
We continue to closely monitor a strong atmospheric river affecting
mainly 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. Widespread
moderate to heavy rain will spread southeast on Friday across the rest
of the Bay Area and north Central Coast.
&&
.UPDATE...
Issued at 921 PM PST Thu Nov 21 2024
As mentioned in the synopsis: Flood Warning is in effect from
Friday afternoon until further notice for the Russian River at
Johnsons Beach at Guernville. At 8 pm this evening the stage was
21.9 feet and it is forecast to continue rising to a crest of 33.7
feet early Saturday morning. Flood stage is 32.0 feet.
The North Bay continues to receive the heaviest rainfall, expect
additional moderate to heavy rain to continue overnight and Friday
with the very moist atmospheric river aligned with the warm front
and subsequent cold front passage Friday. The cold front will
begin moving forward Friday producing moderate to heavy rain
across the rest of the forecast area Friday. In addition to
ongoing and developing areas of flooding and road closures in the
North Bay, anticipate area-wide disruptions during Friday's
morning and afternoon commutes. Rain rates have also been
increasing and shifting southward along the San Francisco
Peninsula to the Santa Cruz Mountains with rain gauges reporting
a few tenths to 1-2 inches so far. We're getting closer to when
the entire pattern moves eastward pushing the frontal rain forward
due to the arrival of a strong west to east flowing polar jet
stream setting up over the offshore and coastal waters Friday,
pushing forward across northern California Friday night and
Saturday.
A brief check on instability forecasts for Friday and Friday evening
does show at least weak mid level convective potential; grid forecasts
currently carry a slight chance of thunderstorms North Bay for Friday
morning. The mid shift forecasters as time allows may be able to look
in to this further, adding updates as needed.
A big travel week is ahead with Thanksgiving soon. Unsettled wet weather
continues over the weekend and early next week then there's potential
amplification in the upstream pattern next week, however with limited
north-south splitting in the jet stream winds, will have to monitor
to see if the jet stream is able to undercut the forecast Pacific 500
mb ridge and reach the West 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...
(06Z TAFS)
Issued at 926 PM PST Thu Nov 21 2024
Radar shows some of rain are becoming lighter, which can give an
opportunity for conditions to slightly improve for the North Bay,
but rain is expected to increase again overnight keeping
conditions in the MVFR/IFR range. Otherwise, a mixed bag of VFR
over KMRY, KSNS, KLVK, and KSJC with the remaining terminals
seeing MVFR/IFR conditions. As the heavier rain bands move over
some of the terminals in SF Bay, East Bay, and South Bay ceilings
will lower to MVFR with a chance of IFR conditions starting early
Friday morning going into the afternoon. Models show agreement
that KHAF will see mostly IFR conditions through the TAF period.
The remaining aviation concern will be the southerly winds. Winds
will continue to fluctuate between breezy and gusty to moderate
but the overall winds will remain pretty gusty and strong through
most of the TAF period, until Friday night where gusty winds seem
to diminish for SF Bay and North Bay.
Vicinity of SFO...MVFR through most of the TAF period with
occasional rain starting near 12-13Z. There is moderate
confidence that VFR will return towards the end of the TAF period
(late Friday night), as some models show MVFR/IFR lingering
through the TAF period. Southerly winds have calmed to relatively
breezy but will rebuild to gusty as the rain bands start to move
over the terminal early Friday morning. Gusty winds diminish
towards Friday night but sustained winds will remain breezy and
will slightly turn SW.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR but as the showers start to move closer
to the terminals, expect MVFR conditions to build. The chances for
heavier rain increases after 00Z. Southerly winds will build to
relatively breezy, but moderate confidence on gusty winds building
towards Friday afternoon.
&&
.MARINE...
(Tonight through next Wednesday)
Issued at 848 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 through
Friday. 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. Slight chance for thunderstorms tonight into early Friday
morning for the outer northern 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 until 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 until 6 AM PST Friday 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...SO
MARINE...SO
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Prepared by Boulder-Creek.com Weather at: Fri Nov 22 02:30:02 PST 2024
From the National Weather Service
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