FXUS66 KMTR 182045
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
145 PM PDT Sun Oct 18 2026
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 143 AM PDT Sun Oct 18 2026
- Hazardous beach conditions continue through tonight
- Cooling trend begins today and persists into the work week
- Light rain showers possible mid-week
&&
.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...
(18Z TAFS)
Issued at 1020 AM PDT Sun Oct 18 2026
Currently IFR-VFR. The marine layer was observed around 500 feet
this morning and is expected to deepen through the TAF period.
Stratus is off the coast with a feed through the Golden Gate Gap.
With the deepening of the marine layer, moderate confidence in MVFR
conditions returning to all terminals tonight as it's able to
penetrate farther inland. Onshore winds will prevail.
Vicinity of SFO...Currently VFR with northwesterly flow. Moderate
confidence in an MVFR ceiling developing at the terminal tonight,
likely early. The San Bruno Gap has been able to hold off stratus so
far, but if the marine layer deepens to around 1,000 feet, there
will be nothing stopping IFR/MVFR stratus from reaching the terminal
earlier. Westerly/Northwesterly winds will prevail, becoming gusty
in the afternoon and evening.
SFO Bridge Approach...The San Mateo Bridge Approach will likely have
clouds at or below FL070 from 04Z to 17Z.
Monterey Bay Terminals...Currently VFR and calm at both terminals.
Moderate confidence on MVFR conditions returning to the terminals
tonight. Diurnal winds will prevail.
&&
.MARINE...
(Today through Friday)
Issued at 1020 AM PDT Sun Oct 18 2026
Moderate to fresh northwesterly breezes will prevail through
Monday, bringing widespread hazardous conditions across 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 Pt Arena to
Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.
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 Pinos
to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.
&&
$$
SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...Sarment
MARINE...Sarment
Visit us at www.weather.gov/sanfrancisco
Follow us on Facebook, X, and YouTube at:
www.facebook.com/nwsbayarea
x.com/nwsbayarea
www.youtube.com/nwsbayarea
Prepared by Boulder-Creek.com Weather at: Sun Oct 18 14:30:03 PDT 2026
From the National Weather Service
|