NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 010546

Area Forecast Discussion
National Weather Service San Francisco CA
1046 PM PDT Sat Sep 30 2023

...New AVIATION...

Issued at 120 PM PDT Sat Sep 30 2023

Lingering afternoon/evening showers in the North Bay after a cold
front passed through the Bay Area this morning. Warming trend
with dry weather next week.


Issued at 741 PM PDT Sat Sep 30 2023

Our radar still is in "precip" mode as a few spots of showers
move southward over the North Bay. The latest observations show that
Sonoma, Marin, and Napa counties received the most rainfall in the
last 12 hours with reports of 0.15-0.20 inches total in some areas.
Our GLM and local spotter reported thunder associated with the rain
band east of Cloverdale, but otherwise, lightening/thunder activity
has been insignificant. Temperatures are on the colder side compared
to yesterday at this time with temperatures near 2-5 degrees colder.
Expect min temperatures tonight to lower between the mid 40s to
mid 50s across the region. Forecast is still on track for the high
pressure building over our region bringing some warmer and drier
conditions on Sunday.


(This evening through Sunday)
Issued at 120 PM PDT Sat Sep 30 2023

The cold front passed through the Bay Area and central coast early
this morning, bringing scattered light to moderate rain showers,
particularly south of San Francisco. Most populated areas
recorded less than 0.1", which 0.25" or more in some coastal
mountains of Santa Cruz and Monterey Counties. While the surface
front has passed, the supporting upper level low is still bringing
instability to the North Bay. Scattered showers are observed on
radar moving north to south across Napa and Sonoma Counties, and
will soon reach Marin. These showers look quite robust, though
they haven`t produced any lightning. Cloud tops are not deep
enough into the freezing layer to generate the conditions
necessary for thunderstorms, but some strikes are possible as the
afternoon progresses. Shower activity is expected to decrease this
evening as daytime heating subsides. Sunday is expected to be
calmer, drier, and warmer, with no threat of rain.


(Sunday night through next Friday)
Issued at 120 PM PDT Sat Sep 30 2023

By Sunday night the upper-level low will be over the Great Basin,
well east of our area. This will support moderate NNE wind, which
will keep the forecast dry and initiate a warming trend through
the week.

As the low pressure continues to march East, high pressure will
encroach from the Pacific. The gradient between these two systems
will support moderate NNE flow Sun afternoon. By Mon, the Bay
Area will enter the high pressure portion of an omega block in the
jet stream. This pattern will slow down approaching weather
systems and should keep the forecast dry through the week. The big
story is a warming trend through Fri. Despite temperatures being
well below average today, coastal locations will see daytime highs
in the 70s (roughly 5 degrees above normal), while some inland
locations end up in the 90s (roughly 10 degrees above normal) from
Wed-Fri. Early October is typically one of the warmest periods of
the year, so anything above normal will be noticeable. The longer
nights and drier airmass will allow for good nighttime cooling so
no heat products are expected. This general pattern should last
through the weekend. The pattern will dry the fuels and increase
fire weather concerns as we head into the historical peak month
for fire weather concerns for the Bay Area.


(06Z TAFS)
Issued at 1045 PM PDT Sat Sep 30 2023

Chances of rain showers across the North Bay associated with the
frontal passage earlier today diminishing into the overnight hours.
VFR conditions prevail throughout the North and South Bay. North Bay
conditions are expected to remain VFR throughout the TAF period with
moderate confidence of few to scattered clouds in the lower levels.
Low confidence that ceilings will form in the North Bay but
guidance suggests that if ceilings do form cloud bases will be
above 3000ft. South Bay VFR conditions will transition to MVFR
conditions temporarily overnight before returning to VFR
conditions in the early morning. MVFR conditions along the Central
Coast will remain through the overnight hours and into the early

Vicinity of SFO...VFR conditions overnight with relatively weak
onshore flow before becoming breezier during the day with stronger
gusts up to 22kts possible. Moderate confidence that conditions will
remain VFR overnight and through the rest of the TAF period.
Guidance suggests that if a ceiling does try to form it would likely
be very late overnight/into the early morning but even if a ceiling
does try to form cloud bases are expected to remain above 3000ft.

SFO Bridge Approach...Similar to SFO

Monterey Bay Terminals...MVFR conditions present at both KMRY and
KSNS are expected to prevail overnight and into the midmorning.
Moderate confidence that conditions may temporarily become IFR in
the early morning hours with a decrease in visibility and BR
possible. Conditions are expected to improve by midmorning with
clouds expected to begin clearing out between 17-19Z, returning to
VFR. Winds are expected to be slightly breezy in the afternoon with
primarily onshore flow from the west before slowing down later in
the evening.

(Tonight through next Thursday)
Issued at 741 PM PDT Sat Sep 30 2023

Fresh to strong NW winds throughout the area with near gale force
gusts north of Point Reyes. Moderate to rough seas are on a
building trend through the day. Sunday will be a transition day as
high pressure becomes the dominant feature where winds will
decrease and seas abate creating more favorable conditions
expected through the week.


PZ...Small Craft Advisory until 3 AM PDT Sunday for PZZ540-545-575.

     Gale Warning until 3 PM PDT Sunday for PZZ570.



LONG TERM....Flynn

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NWS MTR Office Area Forecast Discussion