FXUS66 KMTR 010700
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1200 AM PDT Thu Oct 1 2026
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 1156 PM PDT Wed Sep 30 2026
- Elevated fire weather conditions across the higher terrain
Thursday
- Moderate to major HeatRisk across the interior through
Wednesday
- Minor coastal flooding for bayshore locations along the San
Francisco Bay, San Pablo Bay, and Monterey Bay Thursday
&&
.SHORT TERM...
Issued at 1156 PM PDT Wed Sep 30 2026
(Thursday and Friday)
The 00Z sounding observed an 850 mb temperature of 21.75 degrees
Celsius and a 500 mb height of 5880 meters; both of these are near
the 90th percentile of 20.7 degrees Celsius and 5884 meters,
respectively. The anomalously warm airmass will yield temperatures
10 degrees above normal with moderate HeatRisk for the interior. An
impressive compressed marine layer will keep coastal locations cool
and lead to reduced visibility during the overnight periods. Another
morning of offshore flow with dry conditions will bring elevated
fire weather conditions to the higher terrain. Gusts of 25-35 mph
are expected with gusts up to 45 mph for the highest and most
exposed peaks such as Mt. St. Helena. Minor coastal flooding for
bayshore locations along the San Francisco Bay, San Pablo Bay, and
Monterey Bay will occur during high tide Thursday afternoon.
Depending on how Thursday's forecast verifies, the Coastal Flood
Advisory may need to be extended through Friday evening, at least
for San Pablo Bay if not all bays.
&&
.LONG TERM...
Issued at 1156 PM PDT Wed Sep 30 2026
(Saturday through Wednesday)
Anomalously high heights, 90th percentile 850 mb temperatures and
500 mb heights, and EFI values of 0.90 or greater for maximum and
minimum temperatures all point to it being hot across the interior
Saturday through Wednesday. Global ensemble clusters are in
agreement that the heat will peak Saturday as upper-level longwave
ridging drifts in from the Eastern Pacific Ocean. An upper-level
shortwave trough will skirt the region Sunday; however, it'll have
little impact as it merely pauses the warming trend. Upper-level
ridging will rebuild across the region from the east Monday through
Wednesday. These subtle changes will have little impact on
temperatures and HeatRisk with temperatures 10 to 20 degrees above
normal and widespread moderate with areas of major HeatRisk expected
across the interior. As is typical for heat events, the greatest
uncertainty is at the coast due to the marine layer. As it stands
right now, a compressed marine layer around 500 feet may be able to
hang on. If it erodes or offshore flow develops in the mornings,
coastal spots will be hotter than currently forecast. In terms of a
Heat Advisory, it will likely be for a majority of the interior; the
soonest it would be in effect would be Saturday with a chance for it
to linger into early-to-mid next week. As a proxy, eastern San
Francisco has a 90% probability of reaching 80 degrees on Saturday,
followed by 70% on Sunday, and 50% Monday through Wednesday.
Temperatures will cool off overnight; however, the weaker than
normal onshore flow will bring less relief than normal.
&&
.AVIATION...
(06Z TAFS)
Issued at 1028 PM PDT Wed Sep 30 2026
LIFR ceilings continue to impact coastal terminals through late
Thursday morning with reductions to visibility becoming more
prevalent overnight. Medium confidence for IFR to borderline MVFR
ceilings to develop over the North Bay and south San Francisco Bay
terminals early Thursday morning. Slightly lower confidence for STS
to develop ceilings as denoted by the TEMPO group given the
combination of a weak push of offshore flow and a compressed marine
layer. Medium to high confidence for SJC and LVK to remain VFR
through the TAF cycle. Onshore winds gradually ease overnight and
increase to ~10-15 kt Thursday morning as stratus mixes out.
Vicinity of SFO...VFR as northwest winds gradually ease to ~5-10 kt
overnight. Higher confidence for borderline MVFR-IFR ceilings to
impact OAK compared to SFO. While a stratus deck is currently
hovering over the San Bruno gap, the wind direction is unfavorable
to allow it to fill over the terminal. Despite this, there is still
a 50-60% chance for this deck to traverse over the terminal.
Northwest breezes increase to around 15 kt Thursday early afternoon.
SFO Bridge Approach...Low confidence for widespread ceilings
overnight. Cloud cover will likely be patchy in nature overnight.
Monterey Bay Terminals...LIFR conditions will continue to impact the
terminals overnight with reductions in visibility due to mist or
fog. Medium confidence on exactly when the drastic visibility
impacts occur. VFR prevails mid Thursday morning into the evening
before IFR ceilings move onshore again.
&&
.MARINE...
(Tonight through next Tuesday)
Issued at 1028 PM PDT Wed Sep 30 2026
Moderate to strong northwest breezes persist across the outer
coastal waters with rough wind driven seas. Due to elevated seas,
maritime conditions remain hazardous to small craft through
Thursday, particularly over the outer waters north of Point Reyes.
Northwest winds become gentle to moderate by late Thursday
morning, allowing seas to steadily abate to moderate by Friday.
Marine conditions improve Friday into the weekend.
&&
.FIRE WEATHER...
Issued at 502 AM PDT Wed Sep 30 2026
A high pressure system will move in from the west and compress the
marine layer to approximately 500 feet. Hot and dry conditions
will prevail through late week and weekend. Expect poor overnight
humidity recoveries, with overnight relative humidities as low as
20-30% above the marine layer.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 5 PM PDT Thursday for CAZ006-506-
508-529-530.
PZ...Small Craft Advisory until 9 AM PDT Thursday for Pt Arena to Pt
Reyes 10-60 NM.
Small Craft Advisory until 3 AM PDT Thursday for Pigeon Pt to Pt
Pinos 10-60 NM.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Navarrete
MARINE...Navarrete
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Prepared by Boulder-Creek.com Weather at: Thu Oct 1 02:30:03 PDT 2026
From the National Weather Service
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