FXUS66 KMTR 040805
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
103 AM PDT Fri Sep 4 2026
...New SHORT TERM, LONG TERM...
.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...
(06Z TAFS)
Issued at 1014 PM PDT Thu Sep 3 2026
The front is working its way through the Bay Area and into the
Central Coast, with light rain being reported at the terminals
around San Francisco. North Bay counties have dried out, leaving
behind it clearing skies and mainly VFR conditions. The question for
tonight will be, do we continue to clear or will be get some pockets
of sunrise stratus and fog after the front passes? The latest
guidance leans towards the latter with stratus and fog developing
around sunrise and eroding a few hours after that. VFR conditions
then look to prevail through the forecast period. Medium
confidence in the forecast.
Vicinity of SFO...The front is here while writing the AFD and light
rain is being observed around the San Francisco Bay terminals along
with MVFR cigs. Expect these conditions to linger through the next
few hours, with rain chances diminishing as the front moves south.
The question for these KSFO and KOAK will be, does the stratus last.
Guidance seems to be the most bullish here with cigs flirting around
SF Bay. Opted keep the pessimistic forecast in, though it might be
more of a TEMPO situation or just outside of the range rings.
Conditions for sure look to improve around and after sunrise, with
VFR conditions prevailing during the day. Onshore winds pick up mid
to late afternoon, with sustained speeds around 12-15kt and gusts up
to 22-25kt through sunset.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Low clouds are lingering around Monterey
Bay, with high clouds passing overhead as the front arrives. Lower
clouds should make it to KMRY and KSNS later tonight, with perhaps
even some VCSH or -SHRA for a few hours seeing how well the cold
front has held up. MVFR to potentially IFR cigs will be possible as
the front passes through, with conditions gradually improving behind
it.
&&
.MARINE...
(Tonight through next Wednesday)
Issued at 1014 PM PDT Thu Sep 3 2026
Rain chances will continue to diminish tonight, with gentle to
moderate northwesterly breezes expected in wake of the frontal
passage. Diurnal fresh to strong gusts return within the Monterey
Bay, the Big Sur Coast, and near Point Reyes Friday into the
weekend. Widespread fresh to strong gusts return by early next
week across the coastal waters, decreasing to moderate to fresh
breezes by midweek. Long period southerly swell associated with
tropical activity well to our south will reach the coastal waters
Friday and continue into early next week.
&&
.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 from 6 AM PDT Friday through Monday
evening for CAZ006-505-509-529-530.
PZ...None.
&&
$$
SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...KR
MARINE...KR
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Prepared by Boulder-Creek.com Weather at: Fri Sep 4 04:30:03 PDT 2026
From the National Weather Service
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