FXUS66 KMTR 171124
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
424 AM PDT Fri Jul 17 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1226 PM PDT Thu Jul 16 2026
- The marine layer returns today
- Cooler and more humid conditions through the weekend
&&
.SHORT TERM...
Issued at 1250 AM PDT Fri Jul 17 2026
(Today and tonight)
The marine layer is back! That doesn't necessarily mean it's
filled with clouds yet, but the latest weather balloon confirmed a
1,200 feet deep layer of cool, moist air below a strong
temperature inversion and much drier air aloft. In fact if you
were to hike Loma Prieta right now, the trail head temperature
and relative humidity is 55F/95%, while the summit is 71F/11%.
Stratus is forming in this shallow cool layer, currently extending
from San Francisco to the Santa Cruz county line, with separate
patches over Marin, the East Bay and the Monterey Peninsula. This
coverage will expand through the pre-dawn hours with good
radiational cooling and moisture transport from the ocean.
Buoy observations can tell us a lot about marine layer stratus and
fog potential. We're looking for 4 indicators. First we want to see
the air temperature slightly higher than the SST. This indicates
a stable inversion as there is a huge source of cooling available
for the air temperature to drop towards the SST. Second is for the
dew point to be higher than the SST. This one might not seem as
obvious as the other factors, so let's linger on it for a minute.
The dew point is one way to measure the amount of water vapor
(H2O in gas form) in the air, and specifically tells us how cold
the temperature has to drop for condensation (liquid H2O, dew,
fog, clouds) to form. At night the air temperature is almost
always going to trend towards the SST over the water. So if the
SST is cooler than the dew point, the air temperature will drop
towards the SST, hitting the dew point on the way down. The only
way to continue cooling below the dew point is to first lower the
dew point via condensation. The third indicator is a small dew
point depression. That's simply the air temperature minus the dew
point. If they are within 2 or 3 degrees, it doesn't take much
cooling for the air to become saturated. The final indicator is
moderate wind speeds. We want enough to advect the air over the
cooler water, but not too much to mix in the drier air above the
marine layer.
So let's look at the buoys using our 4 indicators:
1) Air temp > SST
2) Dew point > SST
3) Dew point depression < 3F
4) Winds 10-25 kts
Monterey (46042) is reporting an air temp of 57.2F, SST of 54.7F,
dew point of 54.1F, and sustained winds of 20 knots. So that hits
indicator 1 and 4, but not quite 2 or 3. That's still pretty good
since the dew point is only 1/2 degree below the SST and the dew
point depression is just outside the threshold at 3.1 F. Let' s look
at another example. Half Moon Bay (46012) has an air temp of 57.4F,
dew point of 54.5F, SST of 57.7F, and winds of 15 knots. With a
higher SST, that only hits indicator 3 and 4. Based on these
observations the stratus deck should continue to expand, but it will
take some time.
With the reforming marine layer, temperatures will be noticeably
cooler today than recent memory. The coast will be stuck in the 60s.
Inland and high elevations area will remain warm and dry, but they
too will get some relief from the cooler onshore wind.
&&
.LONG TERM...
Issued at 1250 AM PDT Fri Jul 17 2026
(Saturday through Thursday)
The pattern is now pretty stable. At 500 mb there is a trough off
the coast and a strong ridge over the central US. There is really no
indication that will change anytime soon. Everything will be pretty
standard for this time of year with marine layer clouds, onshore
winds, and near normal temperatures. The one interesting part of the
forecast continues to be Tropical Storm Elida. This system will soon
become a hurricane in the Eastern Pacific and move NNE into open
water through the weekend. While it will certainly fall apart after
hitting the much cooler waters north of 25N, the residual moisture
is very likely to get absorbed in our persistent mid-level SW flow
and transported over the Bay Area. Expect to see some mid to high
level clouds Sunday - Tuesday. Despite the moisture, it still
doesn't look favorable for any thunderstorm activity. The GFS
sounding over SFO shows a max 700-500 mb lapse rate of 7.2 and 0
MUCAPE. Even with the moisture, that's not going to cut it for
convection. These will more likely be harmless stratiform clouds
that make for a colorful sunset. We would need an upper level cold
system to move in from the NW at the same time as this moisture
arrived to trigger a major thunderstorm event. This won't be the
last storm of the season, though. El Nino has a strong positive
impact of Eastern Pacific hurricane activity and models suggest
there will be a conga line of tropical cyclones over the next
couple weeks.
&&
.AVIATION...
(12Z TAFS)
Issued at 424 AM PDT Fri Jul 17 2026
Currently a mixed bag of IFR-VFR. Patchy stratus continues to expand
with low confidence on it reaching the interior terminals of LVK,
SJC, and STS this morning. The 1,200 foot deep marine layer will
deepen tonight to near 1,500 feet, allowing for the inland intrusion
of stratus. Moderate to high confidence on all terminals getting an
MVFR ceiling tonight. Coastal terminals and STS may be locally lower
at IFR. Diurnal winds will prevail.
Vicinity of SFO...Currently MVFR with westerly flow. Conditions will
begin to improve by 16Z with high confidence in VFR by the
afternoon. High confidence on the return of the MVFR ceiling
tonight.
SFO Bridge Approach...Low clouds below FL045 will remain confined to
OAK and SFO and northward this morning. Tonight, a deeper marine
layer will allow the San Mateo Bridge Approach to fill in.
Monterey Bay Terminals...Currently IFR with southwesterly flow at
MRY and IFR with northwesterly flow at SNS. Stratus will clear out
late morning (SNS) to early afternoon (MRY) and remain close to the
coast today. High confidence in ceilings on the cusp of
IFR/MVFR returning to the terminals tonight.
&&
.MARINE...
(Today through Wednesday)
Issued at 424 AM PDT Fri Jul 17 2026
Moderate to strong northwesterly breezes will bring hazardous
conditions for small craft to the inner waters, outer waters, and
Monterey Bay. Moderate seas will prevail today with rough seas in
the outer waters. Conditions improve Saturday as seas abate and
northwesterly breezes diminish.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
evening for Mry Bay.
Small Craft Advisory until 3 AM PDT Saturday for Pigeon Pt to Pt
Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt
Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt
Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Sarment
MARINE...Sarment
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Prepared by Boulder-Creek.com Weather at: Fri Jul 17 04:30:04 PDT 2026
From the National Weather Service
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