FXUS66 KMTR 271223
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
523 AM PDT Sun Sep 27 2026
...New AVIATION...
.KEY MESSAGES...
Updated at 1204 PM PDT Sat Sep 26 2026
- Cooler conditions through Sunday
- Warmer temperatures return Tuesday through next week
&&
.SHORT TERM...
Issued at 1159 PM PDT Sat Sep 26 2026
(Tonight through Monday)
A 500 mb open wave trough is approaching the Pacific Northwest
and far northern California. The 500 mb trough is forecast to
become a closed off 500 mb low Sunday over northern California
and continue moving southeastward reaching southern CA/NV later
Monday and Monday night. The polar jet and subtropical jet are
working in unison helping to drive this low forward. The polar jet
is digging the trough southeastward and the subtropical jet is
bringing winds to the base of the trough which will move the
trough eastward. From ~ 700 mb to 500 mb there is a decent cold
pool of air associated with the low, less than 10th percentile for
late Sep Oakland upper air climatology. However, humidity levels
appear mostly unorganized in the 700 to 500 mb layer and there is
minimal potential instability while the low moves over our area
Sunday through Sunday night. If development is just right there
may be a few spots where mid and high clouds produce virga. The
anticyclonic curvature of the polar jet will bring subsidence and
lower 700-500 mb layer humidity by later Sunday; e.g. recent HRRR
output show mid and high level clouds departing to our east by
late morning or afternoon Sunday.
Closer to ground level, from the surface to lower levels cool air
advection has already begun, which is helping clouds to develop.
There is a slight chance that either light drizzle or very light
rain develops within the boundary layer cooling along the coast
and inland from San Francisco to the East Bay and areas southeast
to the Monterey Peninsula to Point Sur through Sunday morning.
This is a complex pattern since the aforementioned 500 mb low has
detached from the surface low far to our northwest, however there
is still a channel of cooler air arriving on northwest winds along
the coastline and inland. The probability of precipitation is
low, a slight chance. At this time, measurable precipitation is
not in the grids for tonight and Sunday morning. From a fire
weather perspective, we've seen observations of nocturnal drying
at higher elevations Saturday evening, with substantially lower
dewpoint temps and RHs in the single digits. The models are having
a lot of difficulty trying to capture this complexity; all of the
above described is a good example as to just how complex e.g.
Sep-Oct weather can be in our cwa.
With the combined cooling, various cloud layers and slight chance
for light drizzle/light rain Sunday morning, daytime highs Sunday
will be below late Sep normals with highs mainly in the 60s and 70s
to a few areas in the lower 80s in the southern interior. Warmer
temperatures revisit our forecast area Monday with 500 mb ridging
returning in the wake of the 500 mb low. Lower 80s inland for daytime
highs will be more prevalent, closer to the coast and bays 70s to
80 and a mix of mid 60s to 70s at the coastline.
&&
.LONG TERM...
Issued at 1159 PM PDT Sat Sep 26 2026
(Monday night through next Saturday)
500 mb ridging as well as surface high pressure will build in over
far northern California, steepening the ACV-SFO pressure gradient
to 5 mb to 7 mb Monday night through Tuesday. Warmer lower level
temperatures will connect with daytime surface heating and result
in warmer to hotter temperatures, with 90s more prevalent inland
Tuesday. The strengthening ACV-SFO will produce offshore winds,
how strong these winds get will determine how warm/hot it gets at
the coastline beginning Tuesday. Similarly, Wednesday will have
offshore wind potential too. Downsloping and drying will increase
fire weather concerns, please see more in fire weather discussion.
Going back to how the polar jet and subtropical jet streams are
working in unison in the nearest term, this appears to be true as
well through next week based on recent ECMWF forecasts. The pattern
is progressive, the westerlies will help nudge next week's 500 mb
ridge forward, somewhat limiting the strength and potentially the
duration of offshore winds. Aside from the early Sep quick hot spell
with the retrogressive subtropical ridge and strong compressional
warming, and minor offshore winds here and there, so far Sep has
featured minimal offshore wind flow.
&&
.AVIATION...
(12Z TAFS)
Issued at 523 AM PDT Sun Sep 27 2026
Satellite imagery this morning shows high clouds over the Bay Area
and Central Coast, which makes it hard to see stratus development
underneath it. The exception to this would be the interior Central
Coast, where you can see the stratus slinking down the Salinas
Valley. MVFR cigs are expected with any additional stratus develop,
likely clearing by mid to late morning for most sites. As the upper
low moves across the region, the marine layer is expected to be
around 1000-1500ft. We may see stratus return for sites around SF
Bay and Monterey Bay, whereas offshore flow might keep stratus at
bay for the North Bay.
Vicinity of SFO...MVFR cigs should stick around through mid morning
to late morning with onshore flow gradually increasing. Guidance
shows an uptick in winds late this afternoon and into the evening
hours, where gusts to 20-25kt look possible. Winds look to weaken
late into the evening and early overnight hours, leading to the next
forecast challenge: stratus. Most models tend to show MVFR cigs
returning between 9-12Z, though some show it trying to onset
earlier. Opted to keep the previous trend with the stratus being
shorter lived into Monday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...No major changes to the forecast this
morning as MVFR/IFR cigs hover around the Bay. Look for these
conditions to continue through mid to late morning, with VFR
conditions returning for the afternoon. Cigs return potentially as
early as 0Z to the taf sites, and should be socked for the rest of
the TAF cycle.
&&
.MARINE...
(Today through Friday)
Issued at 256 AM PDT Sun Sep 27 2026
Moderate north winds will persist today, becoming fresh to strong
as we head into Monday. These winds hold through midweek, with
winds becoming moderate to fresh by the end of the work week. Wind
gusts will be strong to near gale force at times, with the
strongest gusts occuring over the northern outer coastal waters
and from Monterey Bay south to Big Sur. Tropical activity will
produce long period southerly swell by the middle of the week.
&&
.FIRE WEATHER...
Issued at 420 AM PDT Sat Sep 26 2026
Our diffuse cold front continues to slowly move southward through
Southern California. In its wake, some areas have benefited from
the cooler and slightly more "moist" conditions this morning. The
exception, however, is across the higher terrain of the Santa
Cruz Mountains as well as the Santa Lucia and Diablo Ranges where
RH recovery remains poor (20-40%). Afternoon humidity will remain
under 40%, largely for areas away from the coast. A push of
onshore flow is expected later this evening into Sunday morning
resulting in the marine layer deepening to around 2,000 feet ahead
of an upper trough. This moisture push should diminish the areal
coverage of poor RH recovery Sunday and Monday morning. A few
sprinkles cannot be ruled out along the highest peaks associated
with this upper low Sunday AM. Dry and modest offshore breezes
will be a possibility Tuesday and into Wednesday, with the
potential for wind gusts above 35 mph being greatest across the
higher terrain of the North Bay in the wake of this upper low.
Farther south across the Central Coast, the primary impact from
this low will be a return to widespread poor RH recovery with some
locales across the Santa Lucia Range struggling to see max RH
above 30% Wednesday morning.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 AM this morning to 3 AM PDT Monday
for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pt Arena to Pt
Reyes 0-10 nm.
Small Craft Advisory until 2 AM PDT Tuesday for Pt Arena to Pt
Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory from 9 PM this evening to 9 AM PDT Monday
for Pigeon Pt to Pt Pinos 10-60 NM.
&&
$$
SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...KR
MARINE...KR
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Prepared by Boulder-Creek.com Weather at: Sun Sep 27 06:30:04 PDT 2026
From the National Weather Service
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