Day Valley, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

190
FXUS66 KMTR 111808
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1008 AM PST Wed Dec 11 2024

...New AVIATION...

.SYNOPSIS...
Issued at 859 AM PST Wed Dec 11 2024

The first of several rounds of rainfall over the next week will
begin this evening across the North Bay, and gradually increase in
coverage to the south and east across the remainder of our area
during the overnight. The next wave of impactful rainfall is
expected Friday night through Saturday. Along with the rainfall
this weekend, expect coastal flooding associated with combined
king tides and high surf leading to hazardous beach conditions.

&&

.UPDATE...
Issued at 859 AM PST Wed Dec 11 2024

No changes to the current forecast this morning. The frost
advisory for much of the interior has expired as we await the
first of several rounds of rainfall set to arrive across the North
Bay later tonight. There is still some uncertainty in
deterministic guidance wrt to storm evolution for the Friday-
Saturday time frame that will impact expected rainfall totals.
Although too far out at this time frame, also narrowing in on
whether and where a wind advisory may be necessary Friday evening
through at least Saturday morning.

&&

.SHORT TERM...
(Today and tonight)
Issued at 134 AM PST Wed Dec 11 2024

Key Messages:
- Cold temperatures expected through the morning.
- Frost Advisory for the inland Bay Area and Central Coast, in
effect through 9 AM.
- Light rain expected later today and Thursday, mainly in the North
Bay.
- More significant rainfall expected Friday and Saturday with rain
chances continuing through early next week.

High clouds blanket the region tonight, with light offshore flow and
chilly temperatures expected, especially in the interior valleys
with lows in the low to mid 30s. A Frost Advisory is in effect until
9 AM across the North Bay interior valleys, the interior East Bay,
the eastern Santa Clara hills, the southern Salinas Valley, and the
interior mountains of Monterey and San Benito Counties.

Remember the Four P`s of Winter Weather:

People: Dress in layers and wear a hat that covers the ears. Pay
special attention to children and the elderly, who are more
vulnerable to the cold.
Plants: Bring potted plants indoors and keep outdoor plants and
trees protected.
Pets: Bring all pets indoors. Pets and other animals that have to be
kept outdoors should have an enclosed shelter.
Pipes: Make sure all external water pipes are insulated. Keep an
interior faucet running at a drip to prevent ice buildup.

In addition to the above, fog and low stratus over the Central
Valley is entering into the eastern reaches of Contra Costa and
Alameda counties and the Carquinez Strait, and a potential for fog
to develop in the North Bay valleys through the morning. Stations in
the interior East bay are already reporting visibilities as low as 2
1/2 to 4 miles, and visibilities as low as 1/4 of a mile have been
reported in the Sacramento metro area. Probabilistic NBM guidance
suggests a 40-50% probability of visibilities below 1 mile across
the far eastern reaches of Contra Costa County (east of Antioch and
Brentwood). If your morning commute does take you through a foggy
area, remember to allow extra time, drive slow, and use low beam
headlights.

A pattern change will take place today, as ridging over the western
coast migrates towards the northwest and dissipates, allowing a
series of low pressure systems to come into the western United
States. The first of these systems arrive later this evening, with a
cold front passing through the state. The most intense rainfall is
expected during the overnight hours into Thursday morning, with a
few lingering showers through the rest of Thursday. The North Bay
will see 0.5-1" of rain over the two days, with localized totals up
to 1.5" in the higher elevations, with the Bay Area, the Monterey-
Salinas region, and the Santa Lucias seeing 0.25-0.75" of rain, and
the interior regions of southern Monterey and San Benito counties
seeing around 0.1-0.25". The highest peaks of the Santa Lucias and
the eastern Santa Clara hills might even see some snowflakes
Thursday morning, as the post-frontal air gets cold enough that the
wet bulb temperatures supports snow. The NBM is not showing
accumulating snow in these regions. Winds will also turn to the
northwest, with gusts up to 20-30 mph along the coast, the higher
elevations, and NW-SE aligned valleys.

&&

.LONG TERM...
(Thursday through Tuesday)
Issued at 134 AM PST Wed Dec 11 2024

A stronger low pressure system is expected to come through late on
Friday into Saturday, with more significant rainfall expected across
the region. Some uncertainty remains on how exactly the system will
evolve in the deterministic models, with the ECMWF showing a cutoff
surface low developing in the eastern Pacific, moving easterly to
come into the North Coast, while the GFS does not develop that low
until it is just about to enter the Bay Area. As for the parent low
off the coast of the Pacific Northwest and British Columbia, the GFS
is showing more deepening in the low than what the ECMWF shows.

In any case, the North Bay sees higher rainfall totals, leading our
colleagues at the Weather Prediction Center to issue a marginal risk
of Excessive Rainfall (at least a 5% probability of flash flooding)
for the North Bay. Rainfall totals lie around 1-3" across the
valleys, with minor flooding possible along the low lying trouble
spots. Across the rest of the forecast region, the current forecast
calls for around 0.5-1.5" of rain in the Bay Area valleys, around 1-
2" in the Santa Cruz and Santa Lucia ranges, and 0.25-0.75" in the
Central Coast, with the higher totals expected closer to the coast
and the higher elevations.

Wind gusts will also ramp up with the Friday-Saturday system. The
ECMWF`s Ensemble Forecast Index is showing EFI values of 0.7-0.8
across the coastal ranges of the North Bay and northern San Mateo
Peninsula, with a shift of tails value above. In layman`s terms:
ensemble model runs are showing good agreement on wind gusts above
the model climate (previous model forecasts from this time of year),
and there are some model runs that go beyond the most extreme
members of the model climate. If all that doesn`t mean anything to
you, the current forecast is showing wind gusts up to 40-50 mph
generally along the coast, at the higher elevations, and through the
gaps and passes. Wind Advisories and Gale Watches may need to be
issued later today or tomorrow. There`s even the possibility of
convection along the coastal ranges, with the ECMWF showing a band
of MUCAPE up to 200-300 J/kg through Saturday. It`s a very low
chance, but it`s nonzero.

The current forecast shows a lull in the rain on Sunday before
another round arrives Monday, but the strength of this incoming
system is uncertain enough that there`s not really any value in
trying to note any specific rain totals. CPC products continue to
suggest that for the seven days before Christmas, temperatures above
seasonal averages are likely (50-70% probability) and that there`s
still an ever-so-slight lean towards precipitation totals above
seasonal averages (33-40% probability).

&&

.AVIATION...
(18Z TAFS)
Issued at 1007 AM PST Wed Dec 11 2024

A cold front will bring strong winds and widespread rain to the
region. Winds will initially be out of the south, strengthening as
the cold front approaches, transitioning to become northwesterly in
its wake. Ceilings will lower into the evening with rain set to
begin late tonight. Steady rain overnight will be followed by
post-frontal rain showers tomorrow morning.

Vicinity of SFO...Currently VFR with easterly flow. Winds will
slowly start to back, strengthening and becoming southerly ahead of
the cold front. Steady rain is expected 07Z-13Z with post-frontal
rain showers expected through the afternoon.

SFO Bridge Approach...Slant range visibilities will likely be
reduced due to haze.

Monterey Bay Terminals...Currently VFR and calm at MRY and VFR with
drainage flow at SNS. Steady rain is expected 10Z-16Z with post-
frontal rain showers continuing through the morning.

&&

.MARINE...
(Today through Monday)
Issued at 326 AM PST Wed Dec 11 2024

Southerly flow will prevail over the coastal waters today as a
storm system approaches from the west. Unsettled weather returns
tonight and Thursday as a storm system sweeps through with rain
and gusty winds. Winds shift to northwesterly early Thursday with
a cold frontal passage. The gusty winds will result in hazardous
conditions with steep wind waves and fresh swell. Even stronger
winds are still expected over the weekend as another storm moves
through across the coastal waters. Wave heights will remain
moderate today, but build Thursday and again over the weekend due
to east Pacific storms.

&&

.BEACHES...
Issued at 134 AM PST Wed Dec 11 2024

Global wave models continue to show a pulse of high surf coming into
the West Coast sometime around the upcoming weekend. These models
show that northwest swell could rise to 12-18 feet or more across
the outer waters, enhancing the risk for hazardous surf conditions,
especially along northwest facing beaches. Adding to the dangers,
this period also coincides with the peak of the December "King
Tides", the highest astronomical tides of the season, and the
existing tidal flood impacts to the low lying coastal regions could
be enhanced by runup from the intense surf. Historically, king tide
flooding peaks within a couple of hours of the highest high tide of
the day. The first coastal flood products for the king tide flooding
will likely be issued later today.

High tide heights and times at San Francisco during the King Tides:
Thursday, December 12: 6.78 ft at 812 AM
Friday, December 13: 6.98 ft at 854 AM
Saturday, December 14: 7.07 ft at 937 AM
Sunday, December 15: 7.02 ft at 1022 AM
Monday, December 16: 6.82 ft at 1108 AM

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory from 6 AM Thursday to 1 PM PST Monday for
     CAZ006-506-508.

PZ...Small Craft Advisory from 9 PM this evening to 3 PM PST Thursday
     for SF Bay N of Bay Bridge.

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

     Small Craft Advisory from 9 PM this evening to 9 PM PST Thursday
     for Pt Arena to Pt Reyes 0-10 nm-Pt Reyes to Pigeon Pt 0-10
     nm.

     Small Craft Advisory until 9 PM PST Thursday for Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Sarment
MARINE...MM

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