San Francisco Bay Area SKYWARN®

There will be series of light rain storms starting tonight and persisting off and on through Sunday. There is also a high surf advisory in effect through Sunday afternoon.

.SYNOPSIS...
Issued at 841 AM PST Mon Dec 23 2024

While most areas will be dry today, some light showers will be
possible especially across North Bay. More widespread significant
rainfall will return to the region later tonight and into Tuesday
morning. Very hazardous beach conditions will continue through
Tuesday. While Christmas Day may be dry, unsettled and wet weather
will continue through the latter part of the week and weekend.

&&

.UPDATE...
Issued at 950 AM PST Mon Dec 23 2024

Water vapor imagery shows weak ridging over the Northern
California this morning, with the next system poised near 40 North
and 135 West. As a result, high clouds are starting to spread across
the region, ahead of the warm front. These high clouds have not
really disrupted the fog formation that we saw early this morning,
but visibilities are starting to improve at this time. Some
drizzle has also been reported this morning, with just a
hundredths of an inch reported the region. Temperatures are also
pretty mild out there this morning, with most areas seeing in the
mid 50s to lower 60s already. Given these mild conditions and the
fact the sun is peaking through the relatively thin cloud cover at
this time, expect temperatures today to warm into the 60s most
locations, with the warmest spots topping out around 70 degrees.
This is 5 to 10 degrees above normal for this time of year.

Main concern for today remains with the building surf and
potential for coastal flooding. Buoys indicate there is a lot of
water moving out there, and we are just starting to see the
impacts. If you are headed out to the coast today, stay off the
rocks, jetties, and beaches, as these large breaking waves can
sweep across the beach without warning.

Otherwise, light rain will be possible, mainly across North Bay
today, as the warm front starts to drape across the region.
However, the brunt on the rain will spread from north to south
across the region late tonight and on Tuesday, with the cold
front. There is enough instability with this system, that there is
a slight chance of thunderstorms possible as well. While this
rain will mostly be beneficial for the region, North Bay will
once again see the brunt of the rainfall. Given the wet weather
and moist antecedent conditions, believe streams and creeks will
rapidly rise once again. For now, the forecast remain below flood
stage, so we will need to watch them closely.

Other lesser concerns with this system are with the winds. Breezy
south southwesterly winds will develop late tonight and into
Tuesday morning, as 925 mb and 850 winds increase to 20 to 30 kts,
before shifting to westerly behind the front. At this time,
cannot rule out some gusts up to 40 mph, but do not see the need
for widespread wind products. That said, with the saturated soils,
even a gust to 40 mph could down a tree.

Current forecast looks on track, and no updates are planned at
this time. Palmer

&&

.SHORT TERM...
(Today and Tuesday)
Issued at 119 AM PST Mon Dec 23 2024

Key Messages:
*Train of storm systems through next week
*Most impactful storm appears to be late tonight into Tuesday
*Rainfall totals between today and Sunday: North Bay: 4-10" | Bay
 Area: 1-6" | Central Coast: 1-5" | Salinas Valley: Up to 1"
*Wind gusts up to 40 mph (isolated 45 mph in spots) on Tuesday
*Life-threatening beach and ocean conditions linger into next week

High resolution model output points to scattered coastal drizzle and
some scattered showers in the North Bay, particularly in
orographically favored areas, through the course of the day with
probabilities ticking upwards starting in the late morning and early
afternoon hours. Otherwise most of the region remains dry with high
temperatures reaching the upper 60s to near 70 in the inland valleys
of the Bay Area and Central Coast, the mid 60s near downtown San
Francisco and Oakland, the lower 60s in the North Bay valleys and
along the Pacific coast, and the middle 50s to the lower 60s in the
higher elevations. Very hazardous marine conditions continue through
the day with a Coastal Flood Warning and a High Surf Warning in
effect through noon on Tuesday (see BEACHES for more information).

Tonight into Tuesday morning, the next band of rain is expected to
be the most impactful rainband of this active weather pattern so
far. Intense rain is set to begin impacting the North Bay late
tonight with the band moving through the Bay Area and Central Coast
overnight into the morning hours of Tuesday. Rainfall totals are
expected to reach 0.5-1" across the North Bay valleys and coastal
ranges south of the Golden Gate, up to 2" in the North Bay
mountains, 0.3-0.6" in the Bay Area valleys and Monterey Bay region,
and up to 0.25" across the interior Central Coast. The Weather
Prediction Center has issued a Marginal Risk for Excessive Rainfall
(at least a 5% chance that rainfall totals exceed flash flood
guidance) for tonight through 4 AM Tuesday across the northern and
western halves of Sonoma County. The main threats will be the
possibility for rapid rises in the creeks, streams, and small rivers,
especially as the last couple of days have moistened the soils.

Gusty winds will also spread with the rainband with gusts around 20-
30 mph in the valleys, reaching up to 35-45 mph in the ridgelines
and through the gaps and passes. As for convective probabilities,
model output is showing relatively robust CAPE values of around 200-
400 J/kg on Tuesday, but the 0-6km shear remains low (around 10-20
knots) during the passage of the highest CAPE values, thus
minimizing one of the three key ingredients (instability, shear, and
lift) for convection. The robust instability (for this part of the
world) may still foster some robust updrafts and thus a gusty wind
(up to 45-50 mph) and lapse rates on the order of 7-7.5 C/km per
forecast sounding would support coin-sized hail (pennies or
smaller).

Temperatures are expected to drop on Tuesday with the highs in the
middle 50s to near 60 in the lower elevations and the lower to
middle 50s in the higher elevations.

&&

.LONG TERM...
(Wednesday through Sunday)
Issued at 119 AM PST Mon Dec 23 2024

The Christmas Day forecast remains dry and partially sunny, although
hazardous marine and beach conditions continue to persist. Rain
returns to the forecast on Thursday with some uncertainty in the
exact totals. Analysis of the ensemble model clusters continues to
suggest that groupings that mainly consist of runs from the Canadian
and GFS ensemble models show shortwave ridging and "drier" scenarios
while those drawing from the European EPS model offer "wetter"
solutions.

Regardless of the actual rainfall totals, impacts from these
forthcoming systems could be notable. The Weather Prediction Center
has issued Marginal Risks for Excessive Rainfall across Sonoma
County for Thursday and Friday. In addition, with saturating soils
across the Bay Area, the risk for shallow landslides is increasing,
especially in the North Bay. We`re continuing to use the
deterministic NBM in our forecast, although additional refinements
may be necessary through the next couple of days.

&&

Key Messages:
*Parade of storm systems through next week
*Most impactful storm appears to be Monday into Tuesday
*Rainfall totals between today and Saturday: North Bay: 2"-6",
 locally up to 9", rest of the Bay Area: 1-4", Central Coast:
 0.25"-1.50", locally up to 5" in the Santa Cruz Mountains and
 locally up to 3" along the Big Sur Coast
*Wind gusts up to 45 mph on Tuesday
*Life-threatening beach and ocean conditions linger into next week.

A widespread impactful system will move through the region Monday
afternoon through Tuesday afternoon. Rainfall totals up to 2.50" are
forecast for the higher terrain of the North Bay with a gradual
taper in rainfall amounts from north to south with higher rainfall
totals expected in higher terrain. The greatest impacts are expected
in the North Bay as portions of Marin, Napa, and Sonoma Counties
have been highlighted by the Weather Prediction Center (WPC) for a
marginal/at least 5% risk of rainfall exceeding flash flood guidance
between 4 AM Monday and 4 AM Tuesday. This means that isolated flash
floods will be possible and that their nature will be localized and
primarily affecting places that can experience rapid runoff with
heavy rainfall. Fortunately, no mainstem river forecast points are
forecast to reach minor flood stage according to deterministic
forecasts; however, rivers and their creeks and streams will have no
problem responding quickly to rainfall. While it is too early to
issue any wind headlines, there is increasing probability of wind
gusts reaching or exceeding 45 mph during this system. Whether
southerly winds meet Wind Advisory criteria or not, soils
approaching saturation in addition to tree damage from last
weekend`s storms will allow for plentiful tree debris. Isolated
thunderstorms will also be possible with this system with the Storm
Prediction Center (SPC) highlighting all counties with the exception
of Monterey and San Benito for general or non-severe thunderstorms
between 4 AM Tuesday and 4 AM Wednesday. Remember
that lightning and flooding threats exist with all thunderstorms. An
ECMWF point sounding forecast for STS at 10 AM Tuesday shows that
the environment will be that of low CAPE and high shear, but that
the parcel will have to overcome a stable lapse rate in the
850-700 mb layer.

.LONG TERM...
(Wednesday through Monday)
Issued at 407 AM PST Tue Dec 17 2024

The offshore boundary level flow will persist through Thursday
evening with high temperatures in the 60s to mid-70s across interior
valleys. Thursday into Friday a shift in the pattern will result in
increased southerly flow up to 30kts at H85 Friday morning into the
afternoon, as H50 heights begin to drop out ahead of a broad
deepening trough to our west. The first of two waves of
precipitation will arrive late Friday night, brushing the the North
Bay with light rain, expected to be under a tenth of an inch. The
second wave associated with the trough axis moving onshore across
NorCal and southern Oregon will bring more impactful rainfall, with
the North Bay now highlighted by the WPC in the Day 5 Excessive Rain
Outlook for moderate to heavy rain possible Saturday afternoon
through late Saturday night. The active longwave synoptic pattern
looks to continue through and beyond the extended forecast. Get out
enjoy the pleasant weather the next few days.

BULLETIN - EAS ACTIVATION REQUESTED
Tornado Warning
National Weather Service San Francisco CA
551 AM PST Sat Dec 14 2024

The National Weather Service in The San Francisco Bay Area has issued
a

* Tornado Warning for...
  Northwestern San Mateo County in northern California...
  San Francisco County in northern California...

* Until 615 AM PST.

* At 551 AM PST, a severe thunderstorm capable of producing a tornado
  was located near Daly City, moving northeast at 45 mph.

  HAZARD...Tornado.

  SOURCE...Radar indicated rotation.

  IMPACT...Flying debris will be dangerous to those caught without 
           shelter. Mobile homes will be damaged or destroyed. 
           Damage to roofs, windows, and vehicles will occur.  Tree 
           damage is likely.

* This dangerous storm will be near...
  San Francisco, Daly City, South San Francisco, and Pacifica around 
  555 AM PST.

Other locations impacted by this tornadic thunderstorm include Colma,
Broadmoor, and Brisbane.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

TAKE COVER NOW! Move to an interior room on the lowest floor of a
sturdy building. Avoid windows and protect yourself from flying
debris. If you are outdoors or in a vehicle, move to the closest
substantial shelter and protect yourself from flying debris.

Torrential rainfall is occurring with this storm, and may lead to
flash flooding. Do not drive your vehicle through flooded roadways.

&&

LAT...LON 3779 12250 3779 12249 3781 12248 3781 12242
      3783 12242 3779 12239 3775 12239 3773 12236
      3772 12236 3772 12238 3771 12238 3771 12239
      3769 12239 3762 12250 3768 12249 3778 12252
TIME...MOT...LOC 1351Z 224DEG 39KT 3768 12258 

TORNADO...RADAR INDICATED
MAX HAIL SIZE...<.75 IN

$$

Coastal Hazard Message
National Weather Service San Francisco CA
154 PM PST Wed Dec 11 2024

CAZ506-508-121400-
/O.CON.KMTR.CF.Y.0013.241212T1400Z-241216T2100Z/
North Bay Interior Valleys-San Francisco Bay Shoreline-
154 PM PST Wed Dec 11 2024

...COASTAL FLOOD ADVISORY REMAINS IN EFFECT FROM 6 AM THURSDAY TO
1 PM PST MONDAY...

* WHAT...Minor coastal flooding expected.

* WHERE...Bayshore along the San Francisco Bay and San Pablo Bay.

* WHEN...From 6 AM Thursday to 1 PM PST Monday.

* IMPACTS...Flooding of lots, parks, and roads with only
  isolated road closures expected.

* ADDITIONAL DETAILS...Tidal levels are increasing as a result
  of higher astronomical tides. Low lying areas within the San
  Francisco Bay Area may see minor flooding as a result during
  high tide. San Francisco highest tides are 6.74 ft at 8:12 am
  Thursday, 6.98 ft at 8:54 am Friday, 7.07 ft at 9:37 am
  Saturday, 7.02 ft at 10:22 am Sunday, and 6.82 ft at 11:08 am
  Monday.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If travel is required, allow extra time as some roads may be
closed. Do not drive around barricades or through water of
unknown depth. Take the necessary actions to protect flood-prone
property.

&&

The Tsunami Warning for the SF Bay Area has been called off.