FXUS66 KMTR 182323
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
423 PM PDT Tue Aug 18 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 218 PM PDT Tue Aug 18 2026
- Modestly compressed marine layer through midweek
- Near normal temperatures through Wednesday followed by a slight
cooling
- Elevations above about 1000 feet will remain warm overnight
&&
.SHORT TERM...
Issued at 218 PM PDT Tue Aug 18 2026 (This evening through
Wednesday night)
A 500hPa of high pressure anchored over the desert southwest
continues to nose into our service area today yielding near normal
temperatures for most areas. The exception are area ridges where
temps are running a few degrees above this time yesterday. The
more interesting part is the relative humidity. RH values at
elevation are anywhere from 5 to 20 percent below this time
yesterday. For example, the Hastings RAWS (Santa Lucia Range -
Monterey County) has an RH of 14% as of 1:11 PM today. At the same
time on Monday the RH was 38%. That's a 24% drop in RH in 24
hours. This is to state that the upper ridge has compressed the
marine layer which is allowing for significant drying at
elevation. These patterns continue into the night as well and can
drop RH into single digits during nocturnal drying events. As the
ridge pushes in a touch more tonight expect slightly warmer temps
and drier conditions above the marine layer. While the NBM seems
to handling the bulk of the near term well, areas above about a
1000 feet were warmed and dried slightly using a blend of NBM and
short range high resolution models such as HRRR, RAP, and NAM. For
our populated areas conditions tonight through Wednesday night
will be a near copy of the previous 24 hours. The coastal areas
will see some low clouds and some fog. There may even be some
light drizzle in areas like Half Moon Bay, Santa Cruz, and Santa
Rosa in pre-sunrise AM hours of Wednesday.
&&
.LONG TERM...
Issued at 218 PM PDT Tue Aug 18 2026 (Thursday through next
Monday)
An upper level area of low pressure has been quasi-stationary over
the NE Pacific for the last few day. By the time we get into late
week, the upper low is expected to transit Eastward into the
Pacific Northwest, pulling an upper trough into the Bay Area.
Looking at the WPC Clusters, confidence is high this trough will
impact the Bay Area and Central Coast with an onshore push of
marine air and a deepening marine layer. This will cause clouds to
hang on a little longer in the morning hours and redevelop a
little earlier in the late afternoon. The deepening marine layer
will also bring higher humidity to areas of elevation. However,
surface winds will also kick in with breezy conditions at the
coast, in coastal gaps like the Petaluma gap, and over East Bay
saddles like Altamont Pass and the Delta. This cuts two ways. One
being higher moisture content is good in helping to suppress fire
ignitions. Two being the breezy conditions on fully cured fine
fuels. If something gets going into the lighter to moderate fuels,
it could run with the onshore winds. From a purely sensible
weather standpoint, it will simply be pleasant weather all week.
The final piece the address in the medium term is mid level
moisture advecting over the area late in the week. Model are all
over the map with this potential. While the probability is there,
confidence remains low as to where the highest PWAT will advect
towards. Whenever we see a pattern like this the first thought is
high based convection. Looking out as far as possible with higher
resolution models, moisture and instability do look to be present,
in some current model runs. The limiting factor is cooling aloft.
Like we have seen in other similar situations this summer, the
lacking ingredient is cooling aloft. This would really turn over
the mid layers and could provide deeper convection. However, at
this time cooling aloft is not in the cards. As it stands right
now we have moisture spreading in Thursday night into Friday (and
beyond) with marginally supportive lapse rates during times of
highest moisture advection. Even if the vertical development
could get going, tapping into the zone to exchange charges and
produce lightning has less than a 1% chance at this time. I just
don't see it happening. That said, we'll need to watch it over the
coming days as we get more data. The most we would likely see are
some high based convective showers that produce little to no
measurable precipitation at the ground. The only other item to
watch for during any convective shower activity would be locally
gusty winds, especially in the mountains where it will be closer
to the base of the storms.
&&
.AVIATION...
(00Z TAFS)
Issued at 422 PM PDT Tue Aug 18 2026
Currently VFR at all terminals except for HAF which is IFR. Moderate
to high confidence in a persistence forecast prevailing with the
return of IFR ceilings for most terminals. If the marine layer
compresses further, then lower confidence in ceilings reaching LVK
and SJC with lower ceilings and visibilities elsewhere. Diurnal
winds will prevail.
Vicinity of SFO...Currently VFR with gusty northwesterly flow.
Moderate (60%) confidence in the return of an IFR ceiling tonight.
Diurnal winds will prevail.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Currently VFR with westerly flow at both
terminals. High confidence in IFR ceilings returning tonight with
periods of LIFR conditions. Diurnal winds will prevail.
&&
.MARINE...
(Tonight through next Sunday)
Issued at 422 PM PDT Tue Aug 18 2026
A fresh to strong northwest breeze will develop across the outer
waters through the night and extend into the inner waters on
Wednesday. Winds then remain elevated through the end of the
forecast period. Locally stronger winds are possible in the
coastal jet regions north of Point Reyes and along the Big Sur
coast. Seas build to 7 to 9 feet tonight and into Wednesday before
subsiding by the weekend.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM Wednesday to 3 AM PDT Thursday
for Pt Arena to Pt Reyes 0-10 nm-Pt Pinos to Pt Piedras
Blancas 0-10 nm.
Small Craft Advisory until 3 AM PDT Thursday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...BFG
LONG TERM....BFG
AVIATION...Sarment
MARINE...RGass
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Prepared by Boulder-Creek.com Weather at: Tue Aug 18 20:30:04 PDT 2026
From the National Weather Service
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