FXUS66 KMTR 170955
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
255 AM PDT Sat Oct 17 2026
...New MARINE...
.KEY MESSAGES...
Updated at 1053 PM PDT Fri Oct 16 2026
- Warm temperatures continue Saturday
- Near critical fire weather conditions Saturday across higher
elevations
- Much cooler weather arrives Sunday with rain chances mid-week
&&
.SHORT TERM...
Issued at 1053 PM PDT Fri Oct 16 2026
(Tonight through Sunday)
Clear skies continue this evening, but the offshore winds have
calmed substantially. The SFO-WMC gradient peaked at -13.7 mb
Friday morning and has since weakened to -7.8 mb. 24 hours ago Mt.
St. Helena was reporting NNE winds of 44 mph. Now it's NW at 5.
The 500 mb ridge over northern California is weakening. The 00Z
balloon measured the 500 mb height at 5910m, down from 5930m at
12Z. That's still above the 90th percentile for this time of year,
but the trend will accelerate through the day Saturday as the
Channel Islands cut-off low gets absorbed back into the jet
stream, bringing more zonal flow to the upper levels.
At the surface this means the return of onshore winds and the
start of a cooling trend. Max temperatures Saturday are expected
to be a couple degrees cooler than Friday. It will still be a
warm, sunny day for everyone. The bigger change is Sunday. A new
deep low pressure system approaches from the west. This system
will bring back the marine layer and drop the max temperatures by
another 10-15 degrees. It will also bring another round of wind,
but from the west this time. Our current forecast shows gusts up
to 50 mph through Altamont Pass Sunday night. That's the outlier.
Most places will have gusts in the 15-25 mph range.
&&
.LONG TERM...
Issued at 1053 PM PDT Fri Oct 16 2026
(Sunday night through next Friday)
A brief period of zonal flow aloft gets interrupted on Monday as an
upper-level trough pushes southward along the California coast.
Ahead of the low pressure system, onshore winds increase across the
Bay Area and Central Coast with a deepening marine layer, resulting
in cooling to near seasonal temperatures. The trough becomes a cut-
off low by Tuesday and lingers off the Central Californian coast
before gradually propagating inland through Southern California
Wednesday evening into Thursday. The system's position allows for
the reinforcement of cold air advection across the Bay Area and
Monterey Bay regions through the end of next week.
In terms of precipitation chances, light rain showers are possible
Tuesday through Thursday across the Bay Area and Monterey Bay
regions. The coastline and southernmost extent of the Central Coast
have a 40-60% probability of light rain meanwhile the North, East,
and South Bay have a 20-40% chance. In addition to rain, there is
potential for thunderstorm development during the aforementioned
time period. The three main ingredients required for thunderstorm
development are instability, lift, and moisture. Given the complex
terrain within the region, we can guarantee some degree of
orographic lifting as the system moves inland. Both the European and
GFS ensemble members are hinting at precipitable water values around
the 75th percentile for this time of year, so we can also check
moisture off our thunderstorm ingredient list. Finally, we have
instability. European ensemble members are exhibiting 500 mb
temperatures around -15 to -20 degrees Celsius, which falls below
the 10th percentile for this time of year. Meanwhile, the 850 mb
temperatures are around the seasonal normal of 9 to 12 degrees
Celsius. Atmospheric instability may arise from the mixing of air
masses, particularly when a cold air mass sits on top of a warm air
mass. Since cold air is more dense than warm air, it will sink to
the surface while forcing the ascent of warm air. As warm air rises,
it cools adiabatically and condenses, resulting in latent heat
release, which further warms the surrounding air allowing for
increased vertical mixing. This process is dependent on other
thermodynamic factors, however the structure of our cut-off low
supports an unstable system due to vertical mixing.
Despite all of the above, there is still high uncertainty regarding
the rainfall and thunderstorm potential as it is dependent on the
exact trajectory of the cut-off low and the alignment of various
elements. Be sure to check back on the forecast for updated
information.
&&
.AVIATION...
(06Z TAFS)
Issued at 915 PM PDT Fri Oct 16 2026
No major changes from previous TAF package...still high conf for
VFR through the period with lingering offshore flow and dry air in
the lower levels. Onshore returns Saturday afternoon.
Vicinity of SFO...Shifting NE overnight. Onshore returns tomorrow
afternoon.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Diurnal wind patterns with VFR
conditions.
&&
.MARINE...
(Today through Thursday)
Issued at 255 AM PDT Sat Oct 17 2026
Gentle to moderate northerly breezes this morning, with winds
gradually increasing as a cold front sweeps through this weekend.
Northerly winds will become strong across the northern outer
waters this evening and then hold through Sunday. Moderate to
fresh northwesterly breezes Monday will diminish and back to
become westerly Tuesday. Moderate seas will prevail through
today, but will gradually be building and become rough across the
outer waters tonight through Tuesday. Light rain is expected
Monday into Tuesday.
&&
.FIRE WEATHER...
Issued at 1053 PM PDT Fri Oct 16 2026
While the offshore winds have calmed substantially, hot and dry
conditions will continue through Saturday. Max temperatures will
be in the mid 80s to low 90s with RH dropping into the the teens
for nearly all areas away from the immediate coast. A significant
cool-down arrives Sunday through Monday with light rain possible
mid-week.
&&
.BEACHES...
Issued at 919 PM PDT Sat Oct 16 2026
Temps cool on Saturday, but still pleasant weather. People may
head to beaches to recreate, but should use caution. Hazardous
beach conditions will develop at Pacific Coast beaches this
weekend and likely into next week. Long period (17 seconds)
southerly/southwesterly swell will bring an increased risk for
sneaker waves and strong rip currents with breaking waves of 7 to
12 feet. Remain off of jetties, piers, rocks, and other waterside
infrastructure as waves can sweep across the shoreline without
warning, pulling people into the sea. Never turn your back on the
ocean!
MM/Sarment
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through Sunday evening for CAZ006-505-
509-529-530.
PZ...Small Craft Advisory from 3 AM to 9 AM PDT Sunday for Pt Arena
to Pt Reyes 0-10 nm.
Small Craft Advisory until 9 AM PDT Sunday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Navarrete
AVIATION...MM
MARINE...KR
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Prepared by Boulder-Creek.com Weather at: Sat Oct 17 04:30:03 PDT 2026
From the National Weather Service
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