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Area Forecast Discussion

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126
FXUS66 KMTR 211800
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1000 AM PST Sat Dec 21 2024

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 250 AM PST Sat Dec 21 2024

A parade of systems will bring compounding impacts to the Bay Area
and Central Coast. Life-threatening bay, ocean, and beach
conditions linger through next week.

&&

.UPDATE...
Issued at 946 AM PST Sat Dec 21 2024

Cloudy skies continue to overspread the Bay Area and Central Coast
this morning ahead of our next upper storm system forecast to
impact the area. The primary upper trough axis continues to lift
northward, displacing a bulk of the large scale ascent to northern
California and adjacent portions of Oregon. At the surface, the
attendant frontal boundary will stall and likely set up across the
Bay Area. What this means for us is a gradual downturn in
precipitation coverage and wind intensity this afternoon and into
the early evening. PoPs will be greatest across the North Bay and
portions of the SF Peninsula at around 80% through the early afternoon,
before falling down to around 30-50% later this afternoon and into
the early this evening. This current band of light to moderate
rain across the North Bay has the potential to result in rises on
creeks and streams. In addition, brief bursts of 25-35 mph winds
(largely across the higher terrain) should be anticipated with
heavier precipitation as downdrafts assist with momentum transfer
down to the surface. Instability doesn`t look overly favorable for
thunderstorm activity, at least through the afternoon. In fact,
probability of CAPE above 50 J/kg is around 40-50% across extreme
northern Sonoma Co.

For tonight, largely zonal upper level flow is expected. This
will induce gradual ascent atop the diffuse surface frontal
boundary and should be the focus (outside of terrain) for an
additional round of overnight largely light rain showers. Forecast
mid-level RH of over 70% will largely be confined to areas near
and north of Monterey Bay and 20-40% PoPs are advertised tonight
into the pre-dawn hours on Sunday. Outside of these regions, it`s
likely that shallower moisture will be more conducive to
drizzle/fog. Some of the drizzle/fog may reduce visibilities down
to below 1-2 miles early Sunday morning, so those across the
region will want to be cognizant of rapid changes in visibility.

The rest of the forecast remains unchanged and updated products
have already been shipped.

&&

.SHORT TERM...
(Today and Sunday)
Issued at 250 AM PST Sat Dec 21 2024

The first shortwave and its associated surface cold front will move
through the region this morning. Rainfall totals will be as high as
1.25" in the higher terrain of the North Bay with amounts tapering
off significantly from north to south. Southerly winds will increase
through the morning as the cold front encroaches the area with
occasional gusts up to 40 mph likely in the higher terrain and
along the coast, particularly in the North Bay. While this first
system is largely forecast to be unimpactful in terms of rain and
wind, it will have an impact on what transpires with the more
significant systems next week by providing the antecedent
conditions. The region is also coming off the heels of an impactful
storm last weekend, so while winds are not expected to be anywhere
near what was experienced last weekend, soils approaching saturation
with winds from an abnormal direction may allow for trees that were
sufficiently damaged last weekend to lose damaged branches or come
down altogether.

SPC has highlighted the North Bay for general or non-severe
thunderstorms through 4 AM Sunday. Let`s take a deeper dive into the
ECMWF point sounding forecast for STS (Charles M. Schulz County
Airport) at 15Z (7 AM) this morning. The three essential ingredients
for convection are lift, instability, and moisture. The primary
lifting mechanism will be the surface cold front with the secondary
lifting mechanism being orographic lift. Surface based CAPE
(Convective Available Potential Energy) hardly rises above 0 J/kg.
When surface based CAPE does not show instability we look at most
unstable CAPE which in this case shows 40 J/kg. Moisture will be
provided from preceding rainfall. All that being said, the
environment will be that of low CAPE and high shear which is typical
for the area.

A High Surf Advisory will go into effect at 6 AM Saturday and run
through 4 PM Sunday with life-threatening bay, ocean, and beach
conditions lingering into next week.

Sunday`s rainfall totals will largely be the product of warm frontal
processes. QPF (Quantitative Precipitation Forecast) has trended
down with this second shot of rainfall with totals up to 0.75" in
the higher terrain of the North Bay with no wetting rains (0.10"
or greater) expected south of the Golden Gate Bridge.

&&

.LONG TERM...
(Monday through Friday)
Issued at 250 AM PST Sat Dec 21 2024

The first widespread impactful system of the forecast period will
take place late-morning Monday through Tuesday afternoon. Rainfall
totals up to 2.50" can be expected in the higher terrain of the
North Bay with a gradual taper from north to south. Southerly winds
will increase with the passage of the cold front with the
probability of at least briefly reaching Wind Advisory criteria
(wind gusts 45+ mph) increasing to 40%. There is also up to a 20%
chance for thunderstorms with this system. WPC has highlighted Napa
and Sonoma Counties for a marginal or at least 5% risk of rainfall
exceeding flash flood guidance between 4 AM Monday and 4 AM Tuesday.
This means isolated flash floods are possible and that they will be
localized and primarily affecting places that can experience rapid
runoff with heavy rainfall. As the rivers rise so will flooding
concerns with forecasts showing a rapid rise beginning Monday night.
Life-threatening bay, ocean, and beach conditions will linger
through Tuesday with a High Surf Warning likely issued on the heels
of the 6 AM Saturday through 4 PM Sunday High Surf Advisory.

Wednesday will see a break in the rain, giving rivers and their
creeks and streams about 24 hours or so to at least somewhat recede.
Thursday and Friday will bring the second widespread impactful
system with rainfall totals up to 3.50" in the higher terrain of the
North Bay with a more abrupt taper from north to south with WPC
highlighting the North Bay for heavy precipitation Thursday and
Friday. Impacts from this system will be heavily dependent on what
happens preceding it and the antecedent conditions. The compounding
effects of soils approaching saturation will make it more likely
that trees will fall, mud and rock will slide, and creeks and
streams will rise.

Below are mainstem forecast points and the ensemble forecast of
exceeding (action/monitor stage), (minor flood stage) between Monday
and next Sunday.

Sonoma County:
Russian River - Guerneville (GUEC1): 39%, 25%
Russian River - Healdsburg (HEAC1): 9%, <2%

Napa County:
Napa River - St. Helena (SHEC1): 25%, 16%
Napa River - Napa (APCC1): 16%, 9%

&&

.AVIATION...
(18Z TAFS)
Issued at 959 AM PST Sat Dec 21 2024

The rain continues to move through the terminals creating some
reduced visibility and low ceilings. Therefore, a mix of
VFR/MVFR/IFR/LIFR conditions across the area. Expect MVFR/IFR
conditions to continue through the TAF period, with the possibility
of LIFR conditions overnight. Mostly light southerly winds through
the TAF period, with moderate speeds during the afternoon hours.
Showers should taper off after the afternoon for most of the
terminals, with chances of drizzle overnight.

Vicinity of SFO...MVFR as the rain continues over the terminal.
Expect conditions to remain MVFR/IFR through the TAF Period.
Southerly winds will remain light to moderate through the TAF
period. Most of the heavier rain has diminish, but there is still
a chance for light VCSH into the afternoon hours.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR but conditions become MVFR as the rain
moves closer to the terminal. Expect MVFR to linger through the TAF
period. Southerly winds will remain light through the TAF period.

&&

.MARINE...
(Today through Thursday)
Issued at 959 AM PST Sat Dec 21 2024

Southerly winds continues over the coastal waters and will briefly
become calmer today before becoming gusty Sunday afternoon,
especially in the northern waters. Chances of rain, isolated
thunderstorms, gusty winds, and building swell are expected
through the remainder of the weekend. Wave heights will remain
elevated through late next week with 15 to 20 foot waves Monday
through Wednesday. Expect dangerous conditions for small crafts
through the weekend into the beginning of next week. The elevated
seas will continue to produce enhanced shoaling at harbor
entrances and bars.

&&

.BEACHES...
Issued at 357 AM PST Sat Dec 21 2024

Elevated surf of 20 to 26 feet, with occasional breakers up to 30
feet at favored spots such as Mavericks, will occur this morning to
Sunday afternoon as another system approaches the West Coast, with a
High Surf Advisory issued for the Pacific coast from 6 AM Saturday
through 4 PM Sunday.

Then, a very large and powerful, long period west swell train will
likely arrive along our entire coastline Monday into Tuesday next
week. Current global wave models are forecasting swell heights to
20+ feet with very energetic swell periods of 20+ seconds, this will
lead to much higher breakers including localized coastal flooding
impacts. As mentioned above, this will result in life-threatening
bay, ocean, and beach conditions lingering into next week. Stay
tuned for further updates.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory until 4 PM PST Sunday for CAZ006-505-509-529-
530.

PZ...Small Craft Advisory until 3 PM PST this afternoon for SF Bay N
of Bay Bridge.

Small Craft Advisory from 3 PM this afternoon to 3 AM PST Monday
for Mry Bay.

Small Craft Advisory until 3 AM PST Monday for Pigeon Pt to Pt
Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt
Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt
Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

&&

$$

SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...SO
MARINE...SO

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