Flood Watch 2, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco CA
923 AM PST Mon Jan 29 2024

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 318 AM PST Mon Jan 29 2024

Another near record warm day today. Stage set for widespread
moderate-to-heavy rain Wednesday into Thursday, with lingering
scattered showers into the weekend. Another round of rain by the
end of the weekend.

&&

.SHORT TERM...
(Today and tonight)
Issued at 318 AM PST Mon Jan 29 2024

Once again, really not much to write home about in the short term
other than the high temps. Another gorgeous day today with
afternoon highs in the upper 60s to mid-70s across the area. Some
spots in the southern Santa Clara Valley and the Salinas Valley
may touch 80 degrees once again today, but overall most locations
should be a couple of degrees cooler than they were yesterday.

&&

.LONG TERM...
(Tuesday through Sunday)
Issued at 318 AM PST Mon Jan 29 2024

Tuesday is our transition day as cloudiness increases in response
to increasing synoptic scale lift ahead of the trough. Temps will
still be on the warmer side in the mid-60s to lower 70s. By late
Tuesday night, the surface trough associated with our storm will
be knocking on the door of the North Bay. Rain begins there
overnight and spreads southward through the day Wednesday. The
strong 150 kt Pacific jet remains the key driver in digging the
trough to the southeast. However, a jet streak of lesser magnitude
downstream of the trough axis may act to slow the progression of
the system slightly as it moves into the area initially. In
addition, strong 850 mb flow peaking at 4 to 5 standard deviations
above normal and associated moisture plume will result in robust
upslope water vapor flux along SW facing slopes. This will
ultimately result in a prolonged period of very efficient rainfall
in the coastal ranges, where locally we could see slightly above
the higher end of current predictions. The silver lining to all of
this is that the storm is still relatively progressive, as it
will be hard for anything to beat out that 150 kt jet. Once the
trough moves nearer to shore and the downstream jet weakens, the
system is progged to speed up slightly. As the system progresses,
it is also becoming dislodged from the parent deep low up near
British Columbia, ultimately resulting in weakening magnitude as
the moisture plume makes its way down the coast. WPC have retained
a slight chance (at least 15%) of exceeding flash flood guidance
for the North Bay and a marginal chance (at least 5%) elsewhere.
It is noted that an increase into a moderate risk (at least 40%)
cannot be ruled out for portions of the North Bay. We will be
assessing the extent of potential Flood Watches over the next
couple of shifts.

At this time there is very high confidence (99%) that higher
elevations of the North Bay and other coastal ranges will see at
least 3" in 24 hours, and high confidence (80-90%) that the
Sonoma Coast range, Santa Cruz Mtns, and Big Sur coast see at
least 4" in 24 hours. For lower elevations and urban areas,
generally 1-2" with medium confidence (50%) in exceeding 2" in San
Francisco. A glance at the aforementioned higher end predictions
for coastal ranges; the Sonoma Coast range, Santa Cruz Mtns, and
portions of the Big Sur coast have about a 20% chance of
exceeding 6". The City of SF has about a 15% chance of exceeding
3". Additional rainfall is expected as moisture lingers into the
weekend, but the period from Wednesday morning to Thursday morning
appears the most impactful at the moment.

In terms of impacts, soils are near saturation and well primed
from the previous storm cycle. Any heavy rainfall will have no
problem running off into creeks, streams, and roadways. There
remains about a 15-30% chance of main stem rivers reaching minor
flood stage between Wednesday and Saturday, with the higher end
of those probabilities being for the North Bay rivers. Otherwise,
the most likely impacts look to be stream and urban flooding and
numerous shallow landslides along steep slopes. Winds will also
likely be an issue with the main frontal passage Wednesday with
high confidence in gusts over 40 mph along exposed coastal
locations and ridgetops. There is a slight chance in isolated
gusts of 50-60 mph along the Big Sur coast and ridge of the Santa
Cruz Mtns. Generally 30-40 mph gusts elsewhere. It is likely that
we will be dealing with sporadic downed trees and powerlines as
well.

&&

.AVIATION...
(18Z TAFS)
Issued at 922 AM PST Mon Jan 29 2024

Currently seeing LIFR-VFR at the terminals as LIFR fog is still
lingering at STS. STS is slated to receive LIFR fog again tonight
with lower confidence on if APC will. Monterey Bay terminals will
see MVFR ceilings late tonight with moderate confidence on SNS
deteriorating to become IFR by the mid-morning hours.

Vicinity of SFO...Currently VFR and calm. VFR through the TAF
period. Winds will become onshore this afternoon before becoming
southerly in the early morning hours tomorrow.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR with breezy drainage flow at
both terminals. Winds will become onshore this afternoon. MVFR
ceilings look to arrive at the terminals after 07Z with moderate
confidence on SNS deteriorating to IFR after 11Z.

&&

.MARINE...
(Today through Saturday)
Issued at 922 AM PST Mon Jan 29 2024

Moderate to gentle winds continue today as winds build and turn
southerly into Tuesday, as a low pressure system moves over the
waters creating possible rain. Southerly winds continue to build
on  Wednesday, becoming strong with gale force gusts over much of
the  waters as rain showers begin. Moderate to heavy rain will be
widespread across the waters Wednesday and Thursday, but becomes
lighter into the weekend as chances diminish. Moderate period
westerly swell persists through the middle of the work week. A
new,  moderate period, NW swell train moves into the waters
starting late  week.

&&

.CLIMATE...
Issued at 830 PM PST Sun Jan 28 2024

Record highs at the long term climate sites for Monday, January
29:

Climate Site       Jan 29 Record

Santa Rosa           72 (1940)
San Rafael           71 (2015)
Kentfield            69 (1931)
Napa                 73 (1962)
Richmond             78 (2015)
Livermore            70 (1906)
SF Dwntn             71 (1899)
SF Airport           66 (2015, 2009, 1989)
Redwood City         72 (1931)
Half Moon Bay        71 (1984)
Oakland Dwntn        70 (2015, 2009)
San Jose             73 (1931)
Gilroy               76 (1976)
Salinas Airport      77 (1931)
King City            82 (1976)

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 AM to 3 PM PST Wednesday for SF Bay
     N of Bay Bridge.

     Small Craft Advisory from 9 AM to 3 PM PST Wednesday for Mry Bay.

     Small Craft Advisory from 3 PM Tuesday to 3 AM PST Wednesday for
     Pt Arena to Pt Reyes 0-10 nm.

     Gale Watch from late Tuesday night through Wednesday afternoon
     for Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60
     NM.

     Small Craft Advisory from 9 PM Tuesday to 9 AM PST Wednesday for
     Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60
     NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to
     Pigeon Pt 0-10 nm.

     Gale Watch from Wednesday morning through Wednesday afternoon
     for Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-
     60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to
     Pigeon Pt 0-10 nm.

     Small Craft Advisory from 9 AM Tuesday to 3 AM PST Wednesday for
     Pt Arena to Pt Reyes 10-60 NM.

&&

SHORT TERM...Behringer
LONG TERM....Behringer
AVIATION...Sarment
MARINE...Kennedy

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