112
FXUS66 KMTR 180039
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
539 PM PDT Sat Aug 17 2024

...New AVIATION...

.SYNOPSIS...
Issued at 1102 AM PDT Fri Aug 16 2024

Seasonal to slightly cooler than normal temperatures will continue
through the weekend into next week. Increased coverage of early
morning marine layer stratus, fog and drizzle is likely this
weekend. Less than 10% chance for thunder on Saturday.

&&

.SHORT TERM...
(This evening through Sunday)
Issued at 259 PM PDT Sat Aug 17 2024

Upper-level troughing continues to be the name of the game. Said
upper-level trough is responsible for a non-zero (<10%) chance of
thunderstorms today across the North Bay in addition to gusty
conditions through gaps and passes. While our region is not
experiencing critical fire weather conditions, it is imperative to
remain vigilant when it comes to exercising fire weather safety as
grasses remain dry and winds will easily spread fire. Temperatures
will be near normal with tomorrow being slightly warmer than today.
Stratus is expected to fill valleys and low-lying areas tonight,
receding back to the coastline by tomorrow afternoon.

&&

.LONG TERM...
(Sunday night through next Friday)
Issued at 259 PM PDT Sat Aug 17 2024

As the upper-level low fills, temperatures will continue to warm up
through Tuesday. A brief, gradual cooldown will take place Wednesday
through Friday as troughing is expected to take shape. Uncertainty
with the forecast primarily lies in the amplitude of the trough -
will it be a cutoff low or will it be an open wave? If it is a
cutoff low, it will linger longer, the North Bay will have better
chances for measurable precipitation, and onshore winds will
noticeably increase. If it is an open wave, it will move through
quicker with the better chances of precipitation confined to the
north. Either way, any kind of troughing offshore will promote
moist, onshore flow, which is a good thing.

&&

.AVIATION...
(00Z TAFS)
Issued at 539 PM PDT Sat Aug 17 2024

It`s a complex pattern resulting in largely variable weather
conditions this afternoon, it`s VFR inland while it`s a mix of
MVFR and localized IFR (including fog, mist) along the immediate
coast. Higher dewpoint air i.e. higher water vapor has arrived
across much of the forecast area, surface dewpoint temperatures
are in the lower to mid 60s. Additionally a layer of cool air
advection aloft with the offshore trough of low pressure has moved
in mostly between 925 mb (~2500 feet) and 850 mb (~5000 feet)
resulting in low clouds with a mix of stratus/stratocumulus clouds
and periodic light drizzle/light rain along the coast. SFO
Airport reported a trace just after 4 pm for example. For the 00z
TAFs, dry weather is forecast specifically at the terminals

Cool air advection aloft has weakened and deepened the marine
layer temperature inversion per recent profiler data at Bodega
Bay, Fort Ord and Point Sur. A loss of northwest winds and upwelling,
including a still steep August sun angle and heat input into sea
surface temperatures has also reduced the lower level temperature
inversion to sea surface temp difference. The amount of stratus
returning tonight including the development of IFR will rest
largely on clear sky aloft for radiative cooling and if there`s
any additional cool air advection. The 00z TAFs carry a combination
of MVFR-IFR ceilings for tonight and Sunday morning, diurnal surface
warming should help lift ceilings most locations to MVFR-VFR
Sunday afternoon.

Vicinity of SFO...North-south band of low clouds /MVFR/ with
higher dewpoint air is adhering to the SF Peninsula for the
evening. Post sunset cooling will be a combination of residual
cool air advection and radiative cooling supporting a continuation
of ceiling at the terminal for the overnight hours and Sunday
morning. The RAP model is forecasting IFR during the evening,
kept in inherited 18z TAF IFR ceiling forecast for 06z this
evening. KHAF has reported a low ceiling & visibility in fog,
thus LIFR-IFR is there on the other side of SF Peninsula. IFR
06z-16z Sunday then improving to MVFR-VFR thereafter for the
remainder of the morning and afternoon. West wind near 12 knots
decreasing to 5 to 10 knots tonight and Sunday morning, increasing
to near 15 knots Sunday afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Same as the Bay Area, largely variable
weather conditions here this afternoon. Low clouds moving in over
KWVI with IFR ceiling reported, KSNS and KMRY holding onto VFR for
the time being. Low clouds will initially run into late afternoon
diurnal surface warming and vertical mixing with surface temperatures
in the lower 70s over the southern Monterey Bay. With the weakened
marine layer temperature inversion on recent Fort Ord profiler data,
00z TAFs currently advertise tempo MVFR ceilings, however may need
to amend for IFR if the low ceilings over the northern Monterey Bay
hold together as the ceilings move south and east. Ceilings mixing
out by late Sunday morning with generally MVFR-VFR conditions during
the afternoon. Onshore winds 5 to 15 knots during the period.

&&

.MARINE...
(Tonight through next Thursday)
Issued at 259 PM PDT Sat Aug 17 2024

Light to gentle breezes are anticipated across most of the marine
zones tonight. The exception will be across the northern San
Francisco and San Pablo Bays where intermittent moderate breezes
are forecast. Primary swell will continue to roll in from the west
and northwest with wave heights ranging between 2 and 6 feet.
Fresh to moderate breezes are expected Sunday afternoon into next
week, especially near and south of Point Sur. In response, seas
will increase to 6 to 8 feet across southern zones and low-end
marine headlines may be needed Sunday afternoon and into early
next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Canepa
MARINE...Bain

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