FXUS66 KMTR 041139
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
439 AM PDT Fri Sep 4 2026

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 1151 PM PDT Thu Sep 3 2026

 - Coastal Flood Advisory in effect around high tide bayside 
   Marin County Friday afternoon and evening

 - Hazardous beach conditions at Pacific Coast beaches Friday
   through Monday

 - Below normal temperatures through the weekend, a gradual  
   warming and drying trend Monday through Wednesday

&&

.SHORT TERM...
Issued at 103 AM PDT Fri Sep 4 2026
(Today and tonight)

A 500 mb low pressure system (geopotential height 555 decameters,
much below normal for early September) will move onshore over Oregon
today. Over our forecast area 500 mb heights are in the 570 decameters
range which is near the 10th percentile for early September. If it
was any time in the cool season 570s would not be a big deal or 
impressive. However, for early September much below normal heights
over our area are due to chilly temps in the air column above per
inspection of the 925 mb, 850 mb, 700 mb temps on the Oakland air
upper sounding. These readings are near or less than the 10th 
percentile as well, i.e. at the lower end (cool temps) but within 
Oakland upper air long term climatology for early September since 
1948. The current weather pattern is a great example of water 
vapor rich air (precipitable water was 1.35", near the early Sep 
max moving average) and cool air within a trough of low pressure 
resulting in areas of measurable precipitation, in early September
(normally the third driest month behind July and August) when 
it's typically warmer/hot and dry when e.g. Sep-Oct favor offshore
winds. The larger scale northern hemispheric pattern continues to 
be active with ~ 6 long wave troughs, a signature of rather strong
jet stream winds, wind shear and unsteady flow. The August Pacific
Decadal Oscillation remained negative. The North Pacific high and
its clockwise wind flow remained rather robust. A strong equator 
to pole temperature gradient continues, and not to mention three 
hurricanes at the same time in the eastern Pacific, a powerful 
show of thermodynamics (flow of heat) from sea surface to the 
highest thunderstorm cloud tops. Just like a cup of hot coffee, an
open system where heat and mass are free to flow, 100% whenever 
there is an imbalance heat and mass will flow.

Back to our area, within the aforementioned long wave trough a 
much smaller scale cool frontal boundary with areas of light 
measurable rain will continue to move southeastward over the 
forecast area today. Early September rainfall has reached a few 
records, we plan to send a record event report on this overnight. 
Dry weather revisits the area today in the post frontal cooler than 
normal air mass for early September. Forecast daytime highs will 
vary from the 60s coastside/bayside to the 70s/80 inland.

&&

.LONG TERM...
Issued at 103 AM PDT Fri Sep 4 2026
(Saturday through next Thursday)

Jet stream winds and additional shortwave energy will generally 
reach the base of the 500 mb low pressure system over the west. 
This will provide forward motion to the low, nudging the southern
U.S. high forward through early to mid next week. The redevelopment
of 500 mb troughing over the eastern Pacific during the middle of
next week will provide a small chance of entraining tropical moisture.
This is a long time from now, it's a low confidence forecast, but
bears monitoring.

&&

.AVIATION...
(12Z TAFS)
Issued at 424 AM PDT Fri Sep 4 2026

A weak cold front is exiting the southern Bay Area and moving
through the Central Coast this morning. Showers remain in the
vicinity of SJC and LVK this morning but are expected to progress
eastward over the next few hours. Patchy stratus has developed
this morning after high clouds associated with the cold front
cleared out. Stratus is more widespread across the North Bay where
confidence is higher in MVFR to LIFR conditions prevailing.
Comparatively, stratus is patchier over the Bay Area which is 
decreasing confidence as to how long stratus will linger this 
morning. Included a tempo for patchy stratus at most sites this 
morning where confidence was lower through 16Z. Breezy onshore
winds return during the afternoon before winds ease overnight.
Stratus is likely to return tonight but it will be later than
normal. The forecast currently leans on later arrivals between
09-12Z with the marine layer remaining mixed out through Saturday.
Initial guidance suggests the marine layer will try to redevelop
around 2000 ft by late this weekend so we may see more traditional
stratus patterns return by then.

Vicinity of SFO...Stratus has cleared in the vicinity of SFO and
the SF Bay with confidence low that stratus will successfully
redevelop this morning. VFR conditions prevail this
afternoon/evening with scattered clouds around 5000 feet moving
in. Guidance is mixed as to if stratus will reach SFO tonight. For
now, leaned on the side of VFR conditions continuing given the
marine layer being mixed out/struggling to redevelop. If stratus
does develop it will likely be later in the night (approx. after
08Z).

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...The cold front is making its way through
the Central Coast with MVFR CIGs prevailing at both sites. As the
front moves southwards, ceilings are expected to lower slightly
and be more on the IFR-MVFR border. Stratus will clear by late
morning with VFR persisting into the evening before stratus
returns late tonight. Breezy onshore winds return during the
afternoon and evening before winds decrease overnight.

&&

.MARINE...
(Today through Wednesday)
Issued at 424 AM PDT Fri Sep 4 2026

A gentle to moderate west to northwesterly breeze will continue
through the weekend with diurnal fresh to strong gusts across the
Monterey Bay, the Big Sur Coast, and near Point Reyes. Occasional
fresh to strong gusts are possible within the San Pablo and San
Francisco Bays. More widespread fresh to strong gusts return early
next week with hazardous conditions for small craft likely before
winds diminish mid to late week. Long period southerly swell
associated with tropical activity well to our south will arrive
today and linger into next week. Unsettled weather is possible mid
to late week as tropical moisture tracks northwards in the
Eastern Pacific.

&&

.BEACHES...
Issued at 1010 PM PDT Wed Sep 2 2026

A Beach Hazards Statement is in effect Friday-Monday as the swell 
from both Hurricane Marie and Hurricane Karina reaches the coast.
As these hurricane whip up the ocean, the waves travel over 1,000
miles in all directions, gradually flattening out to low 
amplitude, but long period swell. These waves aren't very 
intimidating if you're on a boat in the open ocean, but the long 
period allows them to push much farther up the beach. Long period 
swell always increases the chance for sneaker waves, but that's 
particularly true this weekend since there are 2 different sets of
southerly swell, and their energy will periodically combine. In
addition to sneaker waves, expect strong rip currents and 
breaking waves up to 10 ft. As this is a holiday weekend and the 
weather is otherwise pretty nice, the exposure to the risk is 
high. Avoid the water if you're not an expert swimmer. If caught 
in a rip current, swim parallel to shore until you stop feeling 
the pull. Even on the beach, be prepared for sneaker waves this 
weekend and never turn your back on the ocean.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through Monday evening for CAZ006-505-
     509-529-530.

     Coastal Flood Advisory from 4 PM this afternoon to 9 PM PDT this 
     evening for CAZ506.

PZ...Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this 
     evening for Mry Bay.

&&

$$

SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...Kennedy
MARINE...Kennedy

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Prepared by Boulder-Creek.com Weather at: Fri Sep 4 08:30:02 PDT 2026