FXUS66 KMTR 210004
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
504 PM PDT Sun Sep 20 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 925 PM PDT Sat Sep 19 2026
- Brief cooling through Monday
- More substantial warming trend Tuesday through Thursday
&&
.SHORT TERM...
Issued at 1114 AM PDT Sun Sep 20 2026
(This afternoon through Monday)
A cut-off low at 500 mb has allowed the marine layer to deepen
(currently 2,000 ft) and ushered in a blanket of high clouds
above the eroding stratus. The radar is actually showing pretty
decent elevated returns over the ocean, and I just switched the
scanning strategy into "precip mode". While there is some precip
in the upper atmosphere, the air between these clouds and the
marine layer is very dry. Most, if not all, of these rain drops
will evaporate before reaching the surface. The combination of
these cloud decks and a cooler upper atmosphere is keeping max
temperatures notably cooler than yesterday. The coast will be
stuck in the 60s all day, with inland ares broadly in the 70s this
afternoon. The low will continue to approach from the west,
moving overhead by Monday morning. This will allow the marine
layer to deepen further and brings a good chance for drizzle
Monday morning. Temperatures will be very similar Monday before a
warming trend starts Tuesday. Before we get to that...
Could we get surprised by high based thunderstorms today? Probably
not, but it's worth keeping an eye on. As the cut-off low
approaches from the west, the associated clouds moving in from the
SW look better than expected. Cloud tops are around 30,000 feet,
and there are even some 20-30 dBz radar returns around 15,000'.
Those numbers don't scream thunderstorms at all, but there is an
outside chance that this system could become more energetic as it
approaches the Central Coast over the next few hours. The 18Z
HRRR shows a distinct blob of 500 mb vorticity moving into
Monterey County around 1 PM this afternoon before pushing north.
This HRRR also shows a signal across Monterey and San Benito
County in the lightning flash rate product.
On the other hand, there is virtually no LightningCast activity,
and the reflectivity at -10C and -20C is far too weak to support
the amount of ice particles needed for thunderstorms to develop
at the moment. With ample shear associated with the cut-off low
(45 kts 0-6 km), CAPE is the limiting factor. There is a
conditionally unstable layer with lapse rates around 7 C/km, but
most of the moisture is above that. As the low approaches, the
moisture will lower and that may open the door. Again there is a
very small chance of lightning, but if it does happen, it's
unlikely to be accompanied by any rain. This poses an obvious fire
weather concern as dry lightning can cause new ignitions.
&&
.LONG TERM...
Issued at 1114 AM PDT Sun Sep 20 2026
(Monday night through next Saturday)
The low will continue to slide east, and we should be firmly on
the back side by Tuesday morning. This will allow a low amplitude
ridge to build over the next few days. This pattern change will
result in a warming trend while compressing the marine layer. 850
mb temperatures are expected to climb above 20 C Wednesday -
Thursday, but the trend has been a little cooler since yesterday's
forecast. The widespread moderate HeatRisk is now mostly confined
to Thursday. It used to look like Wednesday and Thursday. While
it will certainly be warm through the middle of the week, if the
recent trend continues we may not need a Heat Advisory.
Heat Advisories are somewhat common in late September, but they
typically come with offshore winds and a longer duration event.
This is a quick warm-up with continued onshore winds and some,
albeit shallow, marine layer surviving along the coast.
The pattern changes again by Friday as another trough approaches
the Pacific Northwest. This will reverse our warming trend, and
more seasonal temperatures are expected through next weekend.
&&
.AVIATION...
(00Z TAFS)
Issued at 504 PM PDT Sun Sep 20 2026
Satellite shows mid to high level clouds and KMUX shows a few
high based showers, otherwise VFR prevails except for areas of
coastal stratus /MVFR-IFR/. Onshore winds will bring stratus and
patchy drizzle /MVFR-IFR-LIFR/ inland tonight and Monday morning.
Conditions improve to MVFR-VFR near the coast Monday afternoon,
VFR prevails inland. Stratus and a few patches of light drizzle
redevelop Monday night and early Tuesday morning.
Vicinity of SFO...VFR and a west wind 10 to 20 knots during the
evening, west wind diminishes to near 10 knots tonight and Monday
morning. MVFR stratus ceiling forecast 04z-19z Monday. Stratus
/MVFR/ redevelops Monday evening and night. West wind 15 knots
Monday afternoon to mid evening.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Stratus /MVFR-IFR-LIFR/ moves inland
tonight. Patchy light drizzle tonight and Monday morning. Conditions
improve to MVFR-VFR by late Monday morning and afternoon. Stratus
/MVFR-IFR/ redevelops Monday night. Onshore winds 5 to 15 knots.
&&
.MARINE...
(Tonight through next Friday)
Issued at 455 PM PDT Sun Sep 20 2026
An upper level area of low pressure will keep pressure gradients
generally light across the southern waters tonight into Monday. As
the low moves inland surface pressure gradients will increase over
the northern outer waters with increasing northwest winds for the
offshore waters from Point Arena southward towards Point Reyes
resulting in building short period seas. Lighter wind and seas
continue across the southern waters through midweek as high
pressure rebuilds.
&&
.FIRE WEATHER...
Issued at 416 AM PDT Sun Sep 20 2026
Near to slightly below normal temperatures, diurnal winds, a deep
marine layer, and a chance for drizzle are on tap for Sunday and
Monday. Warming and drying trend kicks off Tuesday with temperatures
up to 15 degrees above normal by Thursday. Areas of moderate
HeatRisk are expected as a result Wednesday and Thursday. The marine
layer will compress below 1,000 feet during this warm-up with
nocturnal drying expected above it. Diurnal winds will prevail;
however, they will be weaker than the preceding days.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt
to Pt Pinos 0-10 nm.
Small Craft Advisory until 3 AM PDT Monday for Pigeon Pt to Pt
Pinos 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory until 3 PM PDT Tuesday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Canepa
MARINE...RW
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Prepared by Boulder-Creek.com Weather at: Sun Sep 20 20:30:03 PDT 2026
From the National Weather Service
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