Tornado and Flooding in California, December 14 to 16, 2024

Area Forecast Discussion
National Weather Service San Francisco CA
943 PM PST Fri Dec 13 2024

...New AVIATION, MARINE, BEACHES...

.SYNOPSIS...
Issued at 126 PM PST Fri Dec 13 2024

Strong winds and periods of moderate to heavy rainfall will occur
later tonight across the North Bay and Bay Area. The strongest
winds and heavy rain are expected to occur overnight in the pre-
dawn hours through late tomorrow morning. Secure or bring inside
any holiday decorations or other objects that could be lifted and
blown away.

&&

.UPDATE...
Issued at 909 PM PST Fri Dec 13 2024

The much talked about storm is beginning to make its presences
known across the Bay Area and Central Coast. The afternoon was the
proverbial calm before the storm featuring steady rain showers
feeding into the North Bay with breezy/gusty southerly flow. Fast
forward a few hours and conditions are beginning to change. The
offshore vortlobe at 500mb embedded in the longwave trough is
definitely energizing the atmosphere. Satellite imagery reveals
this feature showing up as a small compact comma cloud.
Additionally, NLDN and GLM picking up a decent amount lightning
activity near this comma cloud. Ahead of this feature, and closer
to the North Bay coast, are enhanced radar returns. Latest KMUX
scans are showing the earmarks of a NCFR off the North Bay coast.
At this time, no lightning yet with that feature, but heavy radar
returns and decent winds on velocity. Speaking of winds, southerly
flow has really ramped up across the region with widespread gusts
of 30-50 mph being reported with 77 mph at Cobb Ridge in N Sonoma
being the top so far. Simply put, the Wind Advisories and High
Wind Warnings are verifying. While not as strong as the "sting
jet" scenario from Feb '24 tonight will still be windy with
potential for downed powerlines and trees.

On the hydrology front two changes from the afternoon. First:
Flood Watch issued for Carmel River Lagoon in Monterey county.
Given the combination of abnormally high tides, building surf,
potential runoff, and expected river flow flooding is possible
Saturday into Sunday morning. Second: A special river statement
was issued for Napa River near St Helena as max height of 16.6 ft
exceeds the Action stage of 16 ft. In other words, will watch
closely, but not expected to reach flood stage.

More details on the next 12 hours below. The biggest challenge or
area of concern will be the potential for an NCFR and its impacts.
Some hi-res model stretch out the front/NCFR and slow it down
slightly near the Golden Gate/SF Downtown overnight. If this
happens flooding in urban areas will increase rapidly.

MM

&&

.SHORT TERM...
(This evening through Saturday)
Issued at 249 PM PST Fri Dec 13 2024

Key Messages:

- Conditions will begin to deteriorate rapidly later this evening
with strong winds and periods of heavy rainfall across the North Bay
and Bay Area, and will continue through the overnight, then begin to
gradually ease by late Saturday morning.
- Interior locations across the East Bay and the remainder of the
Central Coast will also see periods of strong winds and moderate
rainfall, but not as bad as our coastal locations.
- Timing for the worst conditions will be from approx 2AM-8AM PST
for the North Bay and Bay Area.
- As the cold front shifts south and east through the early morning
hours, the strong winds will continue to be a hazard from the Santa
Cruz coastal range south to the Santa Lucias through late Saturday
morning.

A strengthening midlatitude trough continues to track toward our
region this afternoon. The North Bay and the Bay Area have seen a
steady fetch of scattered light rain out ahead of the system all
morning and early afternoon. Rain totals have been light thus far,
but the North Bay will begin to see rivers and streams swell
overnight as moderate to heavy rainfall spread across that area
overnight through late tomorrow morning. Flooding concerns in low
lying areas and known trouble spots across Sonoma, Napa and Marin
counties won't end with the rainfall, as the response will lag as
water moves downstream.

The timing for the strongest winds and heaviest rainfall for the
North Bay and Bay Area will be from approximately 1AM-9AM PST.
Southerly wind gusts out ahead of the cold front are expected to
reach 60 MPH at times. However, there is also a slight chance of
embedded thunderstorms from around 3AM-7AM which could vertically
mix even stronger winds down to the surface during that time,
possibly in excess of 70 MPH. Elevated locations away from the
coast, mainly above 1000ft, may see wind gusts in excess of 70 MPH
from this evening through late tomorrow morning.

After sunrise, the fetch of strong winds with periods of moderate
rain will continue to track south from Santa Cruz coastal range
through Monterey Bay and eventually the Santa Lucia's by mid
morning. Rainfall totals across inland portions of the East Bay and
remainder of the Central Coast won't be as impressive as farther
north, with a range of 0.5" to 1.0" inland, and 2-3" for the
elevated coastal ranges. Light rain and moderate winds will persist
through the remainder of Saturday afternoon into the evening as the
trough axis pivots inland and the cold fronts moves east out of our
area.

&&

.LONG TERM...
(Saturday night through next Thursday)
Issued at 249 PM PST Fri Dec 13 2024

Shortwave ridging moves across our area in the wake of the exiting
storm system on Sunday with dry conditions and easing surface
pressure gradient resulting in light winds, and weak offshore flow
near the coast. Another weaker storm system will push toward NorCal
Sunday night into early Monday. Gusty winds with light to moderate
rainfall across the North Bay early Monday morning will spread
southeast across the Bay Area and East Bay Monday afternoon, with
another 0.25"-0.50". Interior locations and southern portions of the
Central Coast may see a trace to 0.25" of rain by Monday evening.
Longwave ridging will quiet things down for the remainder of the
work week beginning Tuesday. However, there are signs of another
potentially unsettled pattern taking shape by next weekend.

&&

.AVIATION...
(06Z TAFS)
Issued at 943 PM PST Fri Dec 13 2024

Rain, heavy at times, mainly in the North Bay, however rain and
winds will continue to increase and spread southeastward tonight
and Saturday morning. Currently VFR-MVFR however expect increasing
areas of IFR developing in the heaviest rain tonight through
Saturday morning. There's a slight chance of thunderstorms in the
vicinity of an eastward moving energetic 500 mb trough moving over
the Bay Area Saturday morning. Low level wind shear (LLWS)
continuing tonight to Saturday morning except where gusty winds
mix down to the surface. Gusty southwesterly to westerly post
frontal winds and scattered showers late Saturday morning and
afternoon. Drier and VFR conditions Saturday evening.

Vicinity of SFO...Rain has reached the terminal, expect
conditions to lower to IFR in the heaviest rain overnight. Low
level wind shear continuing to 07z Saturday when the lower level
wind has an increasing probability of mixing vertically to the
surface. South to southeast wind gusting to 35 knots 07z-13z
Saturday, cold frontal passage 13z Saturday with heavy rain /IFR/
continuing during the morning. Lingering showers from late
Saturday morning through the afternoon with conditions improving
to MVFR-VFR. West wind diminishing by early Saturday evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Near high confidence VFR continues
through late evening with south to southeasterly (downsloping)
winds. However, as the cold front and increasing lower level
convergence of winds approaches from the west later tonight expect
areas of lowering ceilings to MVFR with rain developing. Low
level wind shear developing at KMRY by 09z Saturday, otherwise
increasingly strong and gusty south to southeast winds developing
tonight and Saturday morning, especially focusing around cold
frontal passage Saturday morning. Showers, heavy at times Saturday
morning. Conditions improving to VFR by Saturday afternoon and
evening.

&&

.MARINE...
(Tonight through next Thursday)
Issued at 928 PM PST Fri Dec 13 2024

A cold front will move eastward across the coastal waters and bays
late tonight and Saturday morning with southerly winds out ahead
of the boundary and then northwesterly winds in its wake. Storm
force gusts up to 50 kt, perhaps 55 kt, will be possible tonight
and continuing into late Saturday morning. Winds ease considerably
for Saturday night, yet seas will remain hazardous to small craft
through early Sunday. Dry weather returns for Sunday then another
low brings rain to the coastal waters and bays Monday.

&&

.BEACHES...
Issued at 941 PM PST Fri Dec 13 2024

Forecast is generally on track. Still expecting a larger swell
well offshore currently to enter the northern waters through this
evening. People should use caution if they venture to the
beaches/coastline/bay shoreline through the upcoming weekend.

Previous details: There will be multiple hazards across the Bays,
Pacific Ocean, and their associated beaches as three storm
systems take aim at the West Coast and King Tides return. A
Coastal Flood Advisory has been issued through 1 PM Monday along
the bayshore of the San Francisco Bay and San Pablo Bay with the
highest tides: 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. These very high tides in tandem with long period
northwesterly swell will allow for large sets of waves to run a
lot farther up the beaches. A High Surf Advisory has been issued
through 4 AM Friday for all west and northwest facing beaches for
breaking waves of 18-22 feet. Other impacts will be rip current
risk, longshore current risk, gale-force gusts, slight
thunderstorm chances, and widespread rainfall. There are going to
be poor conditions everywhere, especially on the water or at the
beach.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 1 PM PST Monday for CAZ006-506-508.

     Wind Advisory until 10 PM PST this evening for CAZ006-502-503-
     505-509-512-515-529.

     High Wind Warning until 7 AM PST Saturday for CAZ006-502-503-505-
     509-512-515-529.

     High Surf Advisory until 7 PM PST Saturday for CAZ006-505-509-
     530.

     Flood Watch through Sunday afternoon for CAZ502>506.

     Wind Advisory until 7 AM PST Saturday for CAZ504-506-508-510.

     Wind Advisory until 10 AM PST Saturday for CAZ513-514.

     Wind Advisory from 1 AM to 10 AM PST Saturday for CAZ516>518-528.

     Wind Advisory until 4 AM PST Saturday for CAZ530.

     High Wind Warning from 4 AM to 10 AM PST Saturday for CAZ530.

PZ...Gale Warning until 9 AM PST Saturday for Mry Bay-Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt
     Piedras Blancas 0-10 nm-SF Bay N of Bay Bridge.

     Gale Warning until 3 AM PST Saturday for Pigeon Pt to Pt Pinos 0-
     10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Reyes to Pigeon Pt 0-
     10 nm.

     Storm Warning from 3 AM to 9 AM PST Saturday for Pigeon Pt to Pt
     Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Reyes to
     Pigeon Pt 0-10 nm.

&&

SHORT TERM...JM
LONG TERM....JM
AVIATION...Canepa
MARINE...Canepa

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