Technical NOAA Weather Discussion

FXUS66 KMTR 150749
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1249 AM PDT Tue Sep 15 2026

...New SHORT TERM, LONG TERM...

.KEY MESSAGES...
Updated at 1206 PM PDT Mon Sep 14 2026

 - Cooling trend and below normal temperatures through mid-week;
   return to seasonal temperatures late week into weekend

&&

.SHORT TERM...
Issued at 1204 AM PDT Tue Sep 15 2026
(Today and tonight)

Interesting to see some high clouds continue to stream over the
heart of the Bay Area early this morning. In fact, the stream of
clouds have been like a conveyor belt of upper level moisture over
the last 36-48 hrs. Why the conveyor belt like feed? While the
longwave pattern shows broad troughing over the West (including
CA) it also shows a weak upper level disturbance parked well NE 
of the Hawaiian Island. The sub-tropical jet is the connection
between that weak disturbance and CA. Impactful? No. Interesting
to see a feed for a longer period of time? Yes. Now onto the
forecast.

Marine layer struggled to fully form last night due to dry air and
offshore flow. Tonight we're dropping the offshore flow, but dry
air remains. As of midnight stratus still remains patchy, but at
the night moves on there is higher conf that stratus will become
more widespread over the coastal waters and move locally inland.
The limiting factor will be the lingering dry air.  

Monday was a warm up day, but Tuesday will be a transition day in
the other direction. Broad upper trough will get a boast from a
highly amplified jet nosing into AK and the downstream trough 
deepening. Onshore flow will kick a little more today as well. 
Winds will be strongest along the coast, inland valleys, and 
inland gaps/passes (Altamont). Onshore flow and dropping 500mb 
heights will bring a general cooling of 3 to 5 degrees nudging 
much of the region to below normal for this time of year.

Moistening in the low level and falling 500mb heights will once
again promote better stratus development Tuesday night into
Wednesday with the depth increasing of 1000-1500 ft for the marine
layer. 

&&

.LONG TERM...
Issued at 1204 AM PDT Tue Sep 15 2026
(Wednesday through Monday)

How a about a little taste of late fall to begin the Long Term.
The aforementioned amplified jet will to drive another upper low
down the PacNW coast and into NorCal/OR. Latest guidance actually
cuts off the low from the main flow pattern. Not only are we
getting another upper low, but this one will be chilly for this 
time of year with 850mb temps dropping close to 10C, which is an
anomaly of 5 to 7. So what does this mean for overall sensible 
weather? The cooldown in temperatures will continue with highs 
dropping another 3 to 8 degrees. Highs will be mainly 60s coast 
and upper 60s to mid 80s interior. Onshore flow will bring gusty 
conditions again. The marine layer will be a tricky one. Generally
speaking one would expect a deeper marine layer and that's what 
the forecast has Wednesday into Thursday. However, one always has
to watch stratus development as the airmass aloft begins to 
approach the current SST. If they're too close, stratus 
production generally declines. Confidence is too low at this point
to remove stratus, but will monitor nonetheless. Given that the 
stratus mid- week is tricky and not classic, impacts from extreme 
nocturnal drying events are less likely.

The upper level low departs eastward late Thursday into Friday
with shortwave ridging to follow. Subtle warming trend develops 
for next weekend with interior locations solidly returning to the
80s and far interior mid 90s.


&&

.AVIATION...
(06Z TAFS)
Issued at 1007 PM PDT Mon Sep 14 2026

Gentle to moderate onshore breezes continue this evening, becoming 
light overnight. Above the surface, a weak offshore flow continues 
to keep stratus out of most terminals this TAF period but will 
gradually diminish by Tuesday morning. Wednesday night looks more 
promising for widespread stratus due to the absence of an offshore 
flow and the passage of an upper-level trough, which will allow the 
marine layer to deepen. That being said, VFR prevails this evening 
except at HAF where MVFR ceilings have already formed. Medium 
confidence for APC to develop borderline MVFR-IFR ceilings early 
Tuesday morning and scatter out by mid Tuesday morning. Diurnal 
onshore breezes increase Tuesday afternoon into the evening.

Vicinity of SFO...VFR with gentle onshore breezes through the night. 
Low confidence on the development of MVFR ceilings tonight, 
therefore the TAF advertises SCT decks. If ceilings become robust 
overnight, they will likely be short-lived and scatter out by late 
Tuesday morning. Wednesday has a higher chance of developing MVFR 
ceilings, likely after the current TAF cycle. Diurnal onshore 
breezes prevail through the TAF period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Gentle onshore winds continue through the 
night and increase by Tuesday morning. High confidence for IFR 
ceilings to develop around midnight, filling in over MRY first 
followed by SNS. Stratus is expected to scatter out by late Tuesday 
morning resulting in a short period of VFR conditions. An earlier 
onset of MVFR stratus is expected tomorrow evening around sunset. 

&&

.MARINE...
(Tonight through next Sunday)
Issued at 1007 PM PDT Mon Sep 14 2026

Northwesterly breezes will begin to diminish tonight with
hazardous conditions for small craft lingering across the northern
waters at least into Wednesday morning. Moderate to rough seas
will prevail through Tuesday before subsiding to become slight to
moderate Wednesday through Friday.

&&

.FIRE WEATHER...
Issued at 436 AM PDT Mon Sep 14 2026

A few patches of coastal stratus have developed overnight. North
to northeast breezes have developed in the higher terrain. Offshore
winds will diminish by late morning or early afternoon and return
to diurnally breezy onshore winds in the afternoon and evening. A
surface to lower level trough of low pressure will develop along 
the north Central Coast Tuesday through Tuesday night. Surface to
lower level troughing will expand to include the Bay Area Wednesday
through Thursday. This will bring higher humidity with good recoveries
from sea level to lower elevations to around 2500 feet, however above
this elevation especially along the higher ridgetops thermal ridging
and much lower humidity will persist during mid-week.

As such, areas above the marine layer often experience nocturnal 
drying that will likely continue through early this upcoming week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 AM PDT Wednesday for Pt Arena to Pt 
     Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.

     Small Craft Advisory until 3 AM PDT early this morning for Pt 
     Pinos to Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory until 9 AM PDT this morning for Pigeon Pt 
     to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...MM
LONG TERM....MM
AVIATION...Navarrete
MARINE...Sarment

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Prepared by Boulder-Creek.com Weather at: Tue Sep 15 02:30:03 PDT 2026
From the National Weather Service