509
FXUS66 KMTR 090515
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
915 PM PST Sun Dec 8 2024

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 158 PM PST Sun Dec 8 2024

Cold overnight temperatures starting tonight with a Frost Advisory
in effect for the southern Salinas Valley 12 AM to 8 AM Monday.
Gusty offshore winds are expected Monday through early Tuesday
across the North Bay Mountains, East Bay Hills, and Santa Cruz
Mountains. Low chance for light rain midweek, higher chance for
accumulating rain Friday into next weekend.

&&

.UPDATE...
Issued at 817 PM PST Sun Dec 8 2024

Temperatures overnight will be coldest in the southern Salinas
Valley where temperatures are forecast drop to as low as 32
degrees F. A Frost Advisory remains in effect from 12 AM to 9 AM
Monday for the aforementioned areas. However, the HRRR is
indicating that temperatures will not drop that low and range from
36-40 degrees F, thus we will continue to monitor temperatures
across this region and adjust the advisory as needed. Haze also
remains in the forecast, but the latest short-range guidance is no
longer indicating widespread haze, yet pockets of lower
visibilities remain possible. In addition, offshore winds in the
higher terrain of the North Bay, East Bay, and the Santa Cruz
Mountains remains likely into Monday morning and should help to
keep the atmosphere mixed. So, much of the forecast depends on how
strong the offshore winds will be and how much the atmosphere
remains mixed. For more, please see the latest short and long term
discussions below.

&&

.SHORT TERM...
(This evening through Monday)
Issued at 158 PM PST Sun Dec 8 2024

Widespread hazy conditions were observed this morning throughout the
Bay Area with several airports reporting visibility reductions as
late as 10 AM this morning. Wondering why this happened? The 4AM
sounding can help us figure it out. At 4AM, the sounding observed a
temperature inversion, also known as "capping inversion", in the
lowest levels of the atmosphere. Under normal conditions the ambient
air temperature cools as you go up, however, when an inversion is
present the ambient air temperature actually warms as you go up. If
you were to take an air parcel from the surface and launch it into
the atmosphere under non-inversion conditions, the parcel will cool
as it rises and will continue rising until the ambient air
temperature is warmer than the air inside the air parcel upon which
the parcel will sink back down to the surface. Under normal
conditions this allows the atmosphere to be relatively well mixed
and can help prevent prolonged periods of hazy conditions at the
surface. Comparatively, if you attempted to lift a surface air
parcel when an inversion is present, the parcel would not be able to
rise as the air above it would already be warmer than the air at the
surface. This prevents the atmosphere from being well mixed and can
keep pollutants trapped at the surface, leading to more widespread
hazy conditions as was observed this morning.

Hazy conditions should improve this afternoon into the evening as
surface temperatures continue to warm, weakening the low level
temperature inversion. Surface winds will also pick up slightly
(peaking between 10 to 15 mph today) which will help to increase
mixing in the lower levels of the atmosphere and help any lingering
patches of haze to dissipate. Unfortunately, model guidance shows
the low level temperature inversion strengthening again overnight
and is expected to persist into tomorrow. This may allow for a
return of hazy conditions overnight through tomorrow morning.
Outside of haze concerns, satellite shows a few high level clouds
moving out of the Monterey/San Benito region with clear skies to
prevail everywhere afterwards. Temperatures will peak in the mid to
upper 60s across the interior while closer to the cost highs will
largely be in the upper 50s to low 60s. With clear skies expected
overnight, low temperatures will drop between 3 to 6 degrees from
last night. For most locations this keeps lows in the low to mid 40s
while portions of the interior North Bay, East Bay, and Central
Coast will cool into the mid to upper 30s. Across the southern
Salinas Valley a Frost Advisory has been issued from 12 AM to 9 AM
Monday for temperatures as low as 32 degrees. The coldest spots will
be in the Fort Hunter Liggett area and around the town of Bradley
where lows will be between 32 to 34 degrees. Spots closer to and
north of King City within the southern Salinas Valley will be
slightly warmer with lows between 36 to 38 degrees expected.

&&

.LONG TERM...
(Monday night through next Saturday)
Issued at 158 PM PST Sun Dec 8 2024

As we head into Monday, an upper level trough will deepen and dig
into the Intermountain West while upper level ridging and high
pressure builds back in over the West Coast. This will result in a
setup conducive for gusty offshore (north to northeast) winds across
our CWA with the interior North Bay Mountains, East Bay Hills, and
Santa Cruz Mountains seeing the strongest gusts. Gusts between 30 to
40 mph are expected across the interior North Bay Mountains,
particularly in Napa County along the borders of Lake and Yolo
Counties. Gusts between 20 to 30 mph are expected across the East
Bay Hills and Santa Cruz Mountains. While gusts will be stronger in
the higher elevations, residents of the North Bay, East Bay, and
near the Santa Cruz Mountains should secure loose outdoors objects
in the case of isolated stronger gusts mixing down to the
valleys/lower elevations. Dry conditions continue across the
interior Central Coast but widespread fire weather concerns are not
expected.

Highs will peak in the mid to upper 60s on Monday before a gradual
cooling trend begins Tuesday with highs peaking in the low to mid
60s through Thursday. Friday into the weekend, highs will drop into
the upper 50s to low 60s as our next potential rain system arrives.
Widespread cold temperatures in the low to upper 30s are expected
Monday morning through Wednesday morning. Confidence continues to
increase that a widespread Frost Advisory will be needed across the
interior regions Tuesday morning which is shaping up to be the
coldest day of the week. We will start to see morning temperatures
warm into the upper 30s to low 40s Thursday into the weekend for the
interior, excluding the North Bay Valleys and southern interior
Monterey County where lows will remain in the mid to upper 30s.
Early risers should remember to check the temperature before heading
outside this week and bundle up as needed.

Guidance continues to indicate the pattern will become more active
mid to late next week with a shortwave trough arriving midweek and a
stronger low pressure system arriving Friday into the weekend.
Precipitation is not expected with the initial shortwave trough
(Wednesday into Thursday) but guidance continues to indicate a low
(15-20%) chance of drizzle along the coastline. The low pressure
system arriving Friday into next weekend continues to be the more
impressive potential rain maker but totals have continued to
fluctuate compared to last night and yesterday. The overall trend is
still the same with the highest totals (around half an inch to an
inch) in the North Bay and generally less than half an inch across
the rest of the Bay Area and Central Coast. Totals are likely to
continue fluctuating as we get closer to Friday so continue to stay
tuned and ready for the latest forecasts!

&&

.AVIATION...
(06Z TAFS)
Issued at 914 PM PST Sun Dec 8 2024

Mostly VFR conditions with clear skies over the terminals but KSNS
is experience MVFR visibility due to haze. Although the sounding
shows that the temperature inversion is not as strong as the
previous nights, there is still a chance that low visibility may
develop over the terminals overnight, but confidence is low at this
moment. North Bay terminals may see some fog development overnight
but confidence is low. VFR is expected to return by 17-18Z, if
conditions become poor overnight. Overall winds will remain light to
moderate through the TAF period.

Vicinity of SFO...VFR through the TAF period with a chance for MVFR
conditions overnight. Confidence is low, but if haze develops over
the terminal, expect as low as 4sm near 09-10Z and lasting for a few
hours. High confidence that VFR will prevail after 18Z through the
remainder of the TAF period. Winds will remain light through the TAF
period as well.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR/MVFR with lower visibility occuring
over KSNS. There is some uncertainty on when the haze will lift and
mix out, as some model suggest as late as Monday afternoon. Higher
confidence that conditions should improve near 18Z, when winds start
to increase.

&&

.MARINE...
(Tonight through next Saturday)
Issued at 914 PM PST Sun Dec 8 2024

Northerly winds will continue to decrease tonight into Monday.
This will allow for seas to abate as well by mid- Monday. For the
rest of the week, look for light, shifting wind and a moderate NW
swell. As the low pressure system moves over our area by late
week, expect a chance for rain, increased winds, and building seas
going into the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Frost Advisory until 9 AM PST Monday for CAZ516.

PZ...Small Craft Advisory until 3 PM PST Monday for Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...SO
MARINE...SO

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NWS MTR Office Area Forecast Discussion