FXUS66 KMTR 171704
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1004 AM PDT Sat Oct 17 2026

...New UPDATE, AVIATION, MARINE, BEACHES...

.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

&&

.UPDATE...
Issued at 1002 AM PDT Sat Oct 17 2026

The forecast largely remains on track this morning with no updates 
anticipated at this time. That said, KMRY (Monterey Regional Airport) 
is already 84 deg F as weak downslope winds resulted in rapid 
warming this morning. Thus, coastal locations may very well be 
warmer than currently forecast before onshore winds return by late 
morning and into the afternoon. 

RGass

&&

.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...
(18Z TAFS)
Issued at 1002 AM PDT Sat Oct 17 2026

Currently VFR. Moderate confidence on LIFR conditions returning 
to the coastal terminals of HAF, MRY, and SNS tonight. The 
atmosphere will need to undergo a lot of change over the next 18 
hours for this to happen coming off of hot and dry conditions with
no marine layer. Upstream observations show some stratus and fog,
but a pattern change with an increase in wind and mixing will 
likely make it more difficult for the marine layer to reform. 
Diurnal winds will prevail. Haze will reduce slant range 
visibilities.

Vicinity of SFO...Currently VFR with northerly flow. High 
confidence in VFR through the TAF period. Northerly flow will back
through the morning to become westerly with the sea breeze this 
afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR with southwesterly flow at
MRY and VFR with southeasterly flow at SNS. Moderate confidence 
on LIFR conditions returning to the terminals tonight. Diurnal 
winds will prevail. 

&&

.MARINE...
(Today through Thursday)
Issued at 1002 AM PDT Sat Oct 17 2026

Moderate to fresh northwesterly breezes will prevail through
Monday with strong breezes across the northern outer waters.
Widespread hazardous conditions will spread across the coastal
waters as a result Sunday afternoon, likely continuing into
Monday. Moderate seas will prevail across the inner waters with
rough seas building across the outer waters Saturday through
Monday, rebuilding again Tuesday. Light rain is expected Monday
night through Wednesday.

&&

.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.

KR

&&

.BEACHES...
Issued at 1002 AM PDT Sat Oct 17 2026

Hazardous beach conditions will be present at Pacific Coast beaches 
through Sunday and likely into next week. Long period 
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 jetties, piers, rocks, and other waterside 
infrastructure as waves can sweep across the shoreline without 
warning, pulling people into the sea. Remain out of the water to 
avoid hazardous surf with strong rip currents and never turn your 
back on the ocean!

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 PM PDT Sunday for Pt Arena 
     to Pt Reyes 0-10 nm.

     Small Craft Advisory from 3 PM to 9 PM PDT Sunday for Pigeon Pt 
     to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos 
     to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 9 PM PDT Sunday for Pt Arena to Pt 
     Reyes 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Navarrete
AVIATION...Sarment
MARINE...Sarment

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Prepared by Boulder-Creek.com Weather at: Sat Oct 17 10:30:03 PDT 2026