FXUS66 KMTR 211130
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
430 AM PDT Mon Sep 21 2026
...New AVIATION, MARINE, FIRE WEATHER...
.KEY MESSAGES...
Updated at 1201 AM PDT Mon Sep 21 2026
- Cool conditions today
- Warming trend resumes Tuesday, cooler again Friday into the
weekend
&&
.SHORT TERM...
Issued at 1201 AM PDT Mon Sep 21 2026
(Today and tonight)
The local weather pattern remains dominated by a cut-off low with a
broad rotation centered to the west of Monterey and San Luis Obispo
Counties. The satellite imagery reveals scattered to broken high
clouds associated with the low pressure system, some low clouds
developing across the inland valleys, and some convective activity
south of the Monterey County coast that has been potent enough to
show up on the radar returns. One of those cells even lit up
LightningCast (long story short, a satellite based algorithm for
lightning probability) with a 50% probability of lightning but fell
apart before reaching the southern Monterey County shore. However,
most of them haven't corresponded to any rainfall at the surface, as
the 00Z (5 PM PDT) sounding from Oakland revealed that the lower
layers remain very dry. But could we still see a high based
thunderstorm through the night? It's unlikely, even if the HRRR
continues to hold on to K-Index values up to 22C, which would
correspond to isolated thunderstorms, and some lightning flash
signals near the Santa Lucia range around 4 AM to 8 AM. CAPE values
are still the limiting factor; the 00Z OAK sounding continued to
show a conditionally unstable layer with lapse rates around 8.5
C/km, but this layer was still below the moist air. This is
supported by the reflectivity values at -10 C and -20 C, which
remain below the values that would be necessary to support the ice
particles that would be necessary for thunderstorm development. We
will continue to monitor the cells as they develop, because while
the risk of lightning is very small, the chance that it would be
accompanied by wetting rains (at least 0.1" of rain) are zero, thus
posing an obvious concern for dry lightning-induced fire starts.
As the low moves onshore later this morning and passes through to
our south, cool conditions will continue with highs ranging from the
middle 70s to lower 80s in the inland valleys, the middle 60s to the
middle 70s near the Bays, and the upper 50s to middle 60s along the
Pacific coast. Typical diurnal onshore winds develop this afternoon
and evening with gusts reaching 20 to 30 miles per hour through gaps
and passes, and within the northern Salinas Valley.
&&
.LONG TERM...
Issued at 1201 AM PDT Mon Sep 21 2026
(Tuesday through Sunday)
The cut-off low will continue to move to the east and will be out of
our area by Tuesday morning, degrading into a shortwave trough as it
moves into the Great Basin. This will allow more zonal flow to
develop over the region and start a warming trend through Wednesday
and Thursday. The word "ridge" might be a little strong for what the
models are expecting, but there is definitely a northward bump in
the flow that develops by Wednesday. Temperatures on Wednesday and
Thursday will range from the middle 80s to middle 90s in the inland
valleys to the middle 70s to middle 80s near the Bays, and the
middle 60s to lower 70s along the Pacific coast. There are some
pockets of Moderate HeatRisk (moderate risk for heat-related illness
among sensitive populations) possible on those two days in favored
interior locations, but widespread heat impacts are not expected. As
the previous forecaster noted, Heat Advisories in late September
typically happen with offshore flows and longer duration events.
By Friday, another trough arrives from the eastern Pacific and
reverses the warming trend, with temperatures closer to seasonal
averages into the weekend. Longer-range outlooks show a slight lean
towards temperatures warmer than seasonal averages and precipitation
totals above seasonal averages for the last few days of September
into the first few days of October.
&&
.AVIATION...
(12Z TAFS)
Issued at 423 AM PDT Mon Sep 21 2026
Currently LIFR-MVFR. An upper-level low is bringing high clouds and
elevated rain showers with a low probability for any thunderstorms
to our area. Stratus on the cusp of IFR/MVFR will persist through
late-morning before retreating to the coast. Reasonable worst case
scenario is bayshore (OAK and SFO) and coastal (HAF and MRY)
terminals remain MVFR all day due to an increase in onshore flow
this afternoon as stratus would be retreating. Drizzle will be
possible through the morning, especially at HAF and MRY, which would
lead to further reduced ceilings/visibilities. Diurnal winds will
prevail.
Vicinity of SFO...Currently MVFR with westerly flow. Moderate
confidence in the overall forecast and 12Z TAF. Reasonable worst
case scenario is MVFR stratus lingers through the TAF period with
increasing onshore winds this afternoon. The best chances for VFR
will be this afternoon (19Z-00Z) with an early return of MVFR
stratus likely. Diurnal winds will prevail.
SFO Bridge Approach...The San Mateo Bridge Approach will likely be
MVFR through at least late-morning, improving before the terminal.
Monterey Bay Terminals...Currently MVFR with southwesterly flow at
MRY and VFR with westerly flow at SNS. High confidence in MVFR
stratus through late-morning. As it retreats to the coast this
afternoon, MRY will likely remain MVFR with an increase in onshore
flow. Nonetheless, another early return of IFR-MVFR stratus is on
tap tonight. Diurnal winds will prevail.
&&
.MARINE...
(Today through Saturday)
Issued at 423 AM PDT Mon Sep 21 2026
Moderate to fresh northwesterly breezes will prevail Monday.
Locally strong breezes across the northern waters will generate
hazardous conditions for small craft. Northwesterly breezes
increase Tuesday to become fresh to strong, bringing hazardous
conditions for small craft across the coastal waters. Conditions
begin to improve Wednesday with strong gusts likely through
Friday. Moderate seas Monday will build to become rough Tuesday
and Wednesday before subsiding to become moderate Thursday.
&&
.FIRE WEATHER...
Issued at 423 AM PDT Mon Sep 21 2026
An upper-level low will support near to slightly below normal
temperatures, increased diurnal winds, a deep marine layer, and a
chance for drizzle today. It is also bringing elevated rain
showers, fortunately there is a low probability for thunderstorms
as little to no rainfall is expected to reach the ground. Warming
and drying trend kicks off Tuesday with temperatures up to 10
degrees above normal and areas of moderate HeatRisk Wednesday and
Thursday. The marine layer will compress below 1,000 feet during
this warm-up with nocturnal drying expected above it.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 PM PDT
Tuesday for Pt Arena to Pt Reyes 0-10 nm.
Small Craft Advisory until 3 PM PDT Tuesday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Sarment
MARINE...Sarment
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Prepared by Boulder-Creek.com Weather at: Mon Sep 21 06:30:04 PDT 2026
From the National Weather Service
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