FXUS66 KMTR 190254
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
754 PM PDT Sun Oct 18 2026
...New UPDATE...
.KEY MESSAGES...
Updated at 753 PM PDT Sun Oct 18 2026
- Cooling trend continues through the end of the week
- Beneficial light rain showers expected mid-week
- Thunderstorms possible mid-week across southern Central Coast
&&
.UPDATE...
Issued at 753 PM PDT Sun Oct 18 2026
The cooler weather has arrived! Across the Bay Area and Central
Coast, temperatures dropped 10-20 degrees within the last 24 hours
with some spots experiencing up to a 30 degree drop. Accompanied
with the lower temperatures is an increase in humidity. The marine
layer has reformed with a current depth around 1500-2000 feet.
Moderate, gusty onshore winds will help push stratus further inland
tonight. So, enjoy the stratus while you can because it won't last
long! Tomorrow, temperatures are expected to drop another 10-20
degrees with gusts around 20 mph. Breezier locations may experience
gusts up to 30-35 mph which gradually diminish Monday evening.
&&
.SHORT TERM...
Issued at 145 PM PDT Sun Oct 18 2026
(This evening through Monday night)
24 hour trends are cooler across much of the forecast area, due to
cool air advection from the surface up through the lower levels of
the atmosphere. Daytime highs today will peak close to October 18th
normal highs. A pattern shift to long wave troughing is developing
and the trough will usher in below to much below mid-late October
normal temperatures during the upcoming week. Wet weather will
accompany the trough by mid-week.
&&
.LONG TERM...
Issued at 145 PM PDT Sun Oct 18 2026
(Tuesday through next Saturday)
The northern hemisphere long wave trough pattern contracted
somewhat after late September 2026 (interestingly e.g. after the
persistent far downstream Nor'Easter). Just what storms do, the
north-south meridional temperature/energy exchanges temporarily
reduced the meridional temperature gradient resulting in some
contraction that increased the chances we'd have long wave ridging
(and warm/hot spells) which we had at times in October. Warm/hot
spells are most frequent here in Sep-Oct after a long dry summer,
still relatively high sun angle and especially when coupled to
(dry, downsloping) offshore winds.
As mentioned a pattern shift to long wave troughing is taking
place, not so much because of an expansion in long wave troughs
rather the result of a now progressive zonal pattern entering the
entire western CONUS. Strong polar jet stream winds on the back
side of the trough will drive the trough south-southeastward from
the Gulf of Alaska temporarily cutting off a 500 mb low ~ a few
hundred miles to our west-southwest from late Monday night to
Wednesday night. During this time the subtropical jet stream will
merge with the polar jet stream near the base of the newly
forming 500 mb cut-off low. The cold core low will dig deep
passing over higher sea surface temperatures in contrast to the
cold core low, steepening temperature lapse rates and instability
and water vapor availability will cause showers and thunderstorms
to form. The greatest instability will be over the coastal and
offshore waters while the low approaches land during mid-week
it'll encounter marginal diurnal surface instability and much less
if any land surface based instability at night. Precipitable
water is forecast to reach near the 90th percentile during the mid
week timeframe; cold air within the low will help result in
measurable rain and any thunderstorm development will be "wet"
rain producers (vs "dry" thunderstorms). As of current time, the
southern half of the forecast area stands the best chance of
highest measurable rain with lesser amounts to the north.
Forecast rainfall amounts for Tuesday night through Thursday:
0.10" to 0.25" in the North Bay, 0.20" to 0.50" across the rest of
the Bay Area and 0.33" to 1" across Monterey and San Benito
counties. Southerly breezes will prevail, but the low will be
wrapping cold air around from the south as well and daytime highs
are forecast to remain below normal during the period. Far inland
highs will struggle to reach 70F in rain cooled air Wednesday and
Thursday.
For late in the week dry weather will return along with an increase
in daytime highs reaching back to the 70s inland Friday and Saturday
with 60s closest to the coastline.
&&
.AVIATION...
(00Z TAFS)
Issued at 415 PM PDT Sun Oct 18 2026
Patchy MVFR-IFR stratus persists at the immediate coast with the
rest of the region remaining VFR into the evening hours. MVFR
ceilings are expected to develop across the interior overnight with
some patchy ceilings within the central SF Bay Area. Clearing times
Monday morning and afternoon will be rather late owing to an
incoming trough that will lower temperatures and expand the marine
layer. Gentle to moderate onshore winds will persist through the end
of the 24-hour TAF period.
Vicinity of SFO... VFR conditions through the evening with MVFR
ceilings through Monday morning. Moderate northwest winds will
continue through Monday evening with gusty winds expected in the
afternoon and evening hours. Low confidence for stratus returning to
the terminal Monday night before the end of the TAF period, higher
confidence afterwards.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... VFR for the next few hours with MVFR-IFR
conditions developing tonight. Stratus will clear late Monday
morning or early afternoon. Gentle to moderate northwest winds
prevail through the TAF period.
&&
.MARINE...
(Tonight through next Friday)
Issued at 415 PM PDT Sun Oct 18 2026
Moderate to fresh northwesterly breezes will prevail through
Monday, bringing hazardous conditions for small craft to the
coastal waters Sunday afternoon. Winds will diminish and back to
become southerly Tuesday and Wednesday before returning to
moderate to fresh northerly breezes Thursday. Rough seas across
the outer waters will subside to become moderate Monday,
rebuilding to become rough Tuesday. Light rain showers are
expected Tuesday through Thursday.
&&
.BEACHES...
Issued at 1020 AM PDT Sun Oct 18 2026
Hazardous beach conditions will be present at Pacific Coast beaches
Sunday. 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 until 9 PM PDT this evening for CAZ006-
505-509-529-530.
PZ...Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt
to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena
to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos
to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.
&&
$$
SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...DialH
MARINE...Sarment
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Prepared by Boulder-Creek.com Weather at: Sun Oct 18 20:30:03 PDT 2026
From the National Weather Service
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