FXUS66 KMTR 190501
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1001 PM PDT Sun Oct 18 2026

...New AVIATION, MARINE...

.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...
(06Z TAFS)
Issued at 1001 PM PDT Sun Oct 18 2026

MVFR stratus is building into the valleys of the greater SF Bay Area 
and patches around Monterey Bay with further expansion expected 
overnight. 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 Monday evening, when winds diminish and turn 
southerly. Low confidence in the stratus extent and ceilings 
beginning on Monday night owing to potential mixing out of the lower 
layers as the trough approaches. Any rain chances from the incoming 
trough come later on Tuesday, after the TAF period ends.

Vicinity of SFO... MVFR ceilings persist through Monday morning. 
Moderate northwest winds will continue through Monday evening with 
strong gusts expected in the afternoon and evening hours with a more 
southerly component as the winds diminish Monday evening. Low 
confidence for stratus returning to the terminal Monday night.

SFO Bridge Approach... Similar to SFO. 

Monterey Bay Terminals... MVFR ceilings at the terminals will clear 
late Monday morning or early afternoon. Gentle to moderate northwest 
winds prevail through Monday evening, with light southerly flow 
developing Monday night.

&&

.MARINE...
(Tonight through next Saturday)
Issued at 1001 PM PDT Sun Oct 18 2026

Moderate to fresh northwesterly breezes will prevail through
Monday, with hazardous conditions for small craft lingering at the
inner coastal waters south of Pigeon Point through Monday morning.
Winds will diminish and back to a gentle southerly breeze 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.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 AM PDT Monday for Pigeon Pt to Pt 
     Pinos 0-10 nm.

     Small Craft Advisory until 9 AM PDT Monday for Pt Pinos to Pt 
     Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...DialH
MARINE...DialH

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 22:30:03 PDT 2026