FXUS66 KMTR 201912
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1212 PM PDT Sun Sep 20 2026
...New SHORT TERM, LONG TERM...
.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...
(18Z TAFS)
Issued at 1100 AM PDT Sun Sep 20 2026
High clouds are slowing the burn off of low cigs so far this
morning but clearing of low clouds still forecast at most
terminals away from the coast by 19z. As low clouds burn back it
will be replaced by extensive high clouds passing over the region
in association with upper low offshore. This low will keep the
marine layer deep with low cigs spreading inland early this
evening and lasting at least through 18-19z Monday.
Vicinity of SFO...Low cigs forecast to mix out around 19z with
extensive high clouds and west winds around 15 kt this
afternoon/evening. Low cigs spread back inland around 04z lasting
overnight through 19z Monday.
SFO Bridge Approach...Lingering haze and cigs at least through
19-20z this morning before improving conditions later this
afternoon and early evening.
Monterey Bay Terminals...Low cigs may not clear for KMRY today or
only intermittently from 20-23z this afternoon. KSNS to clear by
19z but mid and high clouds linger through the day. Low cigs to
spread back inland early this evening and linger through 18-19z
Monday.
&&
.MARINE...
(Today through Friday)
Issued at 416 AM PDT Sun Sep 20 2026
Fresh to strong northerly breezes will create hazardous conditions
for small craft across most of the coastal waters through Monday
morning. Moderate to fresh northerly breezes will prevail Tuesday
through Thursday with locally strong breezes across the northern
outer waters. Slight to moderate seas will prevail through Monday,
building to become moderate to rough Tuesday through Thursday.
&&
.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 from 3 PM this afternoon to 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 9 AM PDT Monday for 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: Sun Sep 20 12:30:03 PDT 2026
From the National Weather Service
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