FXUS66 KMTR 210701
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1201 AM PDT Mon Sep 21 2026

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

.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...
(06Z TAFS)
Issued at 934 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...Stratus /MVFR/ and a west wind near 10 knots 
tonight and Monday morning. MVFR stratus ceiling forecast until 
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/ including 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 Saturday)
Issued at 901 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 3 AM PDT early this morning 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...DialH
LONG TERM....DialH
AVIATION...Canepa
MARINE...Canepa

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Prepared by Boulder-Creek.com Weather at: Mon Sep 21 02:30:03 PDT 2026