FXUS66 KMTR 181824
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1124 AM PDT Fri Sep 18 2026
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 1057 PM PDT Thu Sep 17 2026
- Warming trend through Saturday
- Brief cooldown Sunday
- Notable warming trend next week
&&
.SHORT TERM...
Issued at 1120 AM PDT Fri Sep 18 2026
(This afternoon through Saturday)
The marine layer compressed to around 1,700 feet this morning. That
trend will continue over the next 24 hours as a short wave ridge
builds over the area. While the W-E gradients are all still weakly
positive (SFO-WMC +1.9mb, SFO-SAC +1.2mb) we did see some NE,
offshore winds this morning. The gradients will continue to drop and
become negative this afternoon, however the onshore sea breeze will
overpower any offshore flow during the day.
In response to the building ridge and compressed marine layer,
Inland temperatures will be roughly 5 degrees warmer today compared
to yesterday, and another 5 degrees warmer Saturday. While less
expansive, the marine layer will continue to keep high temps along
the coast in the 60s with low clouds and possible fog in the
morning. The offshore wind gradient will peak overnight, though
the strength still isn't overly impressive. Still, NE winds with
gusts up to 20-30 mph are possible across the interior North Bay
Mountains late tonight and early Saturday morning.
&&
.LONG TERM...
Issued at 1120 AM PDT Fri Sep 18 2026
(Saturday night through next Thursday)
The pattern remains quite active in the long term forecast, although
the next few features are pretty weak. Saturday's ridge is
quickly replaced by a weak cut-off low approaching from the west
through the day Sunday. This will reverse the warming trend and
allow the marine layer to expand a bit. This low will gradually
slide overhead by Monday morning, giving us a good chance for
drizzle.
By Tuesday another ridge will start to build from the west, kicking
off a warming trend that will last through the rest of the week.
The ECMWF ensemble shows a 90% chance that the 850 mb temperature
gets above 20C on Wednesday with a mean around 23C. 23C is above
the the 90th percentile for this time of year and would bring
inland surface temperatures back into the 90s.
&&
.AVIATION...
(18Z TAFS)
Issued at 1100 AM PDT Fri Sep 18 2026
Lingering low clouds and fog has dissipated from most of the
interior valleys and Central Coast but nw winds still feeding a
fresh batch of stratus over SF Bay. Most terminals VFR the rest of
today into early evening except for SFO/OAK and HAF. Marine layer
compressing with onshore winds so stratus should push inland
early this evening.
Vicinity of SFO...Stubborn broken deck still occurring over SFO
and satellite trends indicate upstream stratus still spreading
inland. For now tafs will keep 21z for clearing time. Confidence
low overall given atypical synoptic pattern. Once clearing occurs
expect low clouds to spread back over the terminal around 08-09z
tonight.
SFO Bridge Approach...Clear skies reported over bridge approach
and that should last through at least 03-05z this evening.
Monterey Bay Terminals...Low clouds have cleared for this morning.
Expect an early return for KMRY, around 02z forecast at this time
with low cigs spreading down the Salinas valley overnight. Shallow
marine layer depth on Saturday suggests clearing by around 18z.
&&
.MARINE...
(Today through Wednesday)
Issued at 429 AM PDT Fri Sep 18 2026
Gentle west to northwest breezes become moderate to fresh by the
afternoon into the weekend. Strong gusts up to 30 kt are possible
across the northern coastal waters, Big Sur coastal jet region,
and the Monterey Bay region this morning through Saturday. Winds
gradually diminish across the southern coastal waters by Saturday
evening, but remain elevated across the north. Slight to moderate
wind driven seas persist through the weekend and steadily build to
rough by early next week.
&&
.FIRE WEATHER...
Issued at 429 AM PDT Fri Sep 18 2026
The marine layer is expected to begin redeveloping itself through
the day as a ridge develops over the state. By the evening hours the
marine layer should be around 1000-1500 feet, along with attendant
impacts of stratus development below the marine layer inversion and
poor overnight humidity recoveries within and above the thermal
belts. As the ridge develops, weak offshore flow is possible
Saturday morning, particularly across the higher elevations of the
North and East Bay.
DialH
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 AM PDT
Saturday for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos
to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Sunday
for Pt Arena to Pt Reyes 0-10 nm.
Small Craft Advisory from 3 PM Saturday to 3 AM PDT Sunday for
Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras
Blancas 0-10 nm.
Small Craft Advisory from 9 PM this evening to 3 AM PDT Sunday
for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 10-
60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...RW
MARINE...RW
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Prepared by Boulder-Creek.com Weather at: Fri Sep 18 12:30:03 PDT 2026
From the National Weather Service
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