FXUS66 KMTR 190442
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
942 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

&&

.UPDATE...
Issued at 915 PM PDT Tue Aug 18 2026

Forecast remains on track this evening with inland stratus push
not quite as extensive as it was at this time last night.
Temperatures above 1000ft are generally running 5-10 degrees
warmer than 24hrs ago, with little change below 1000ft. Overnight
recoveries at higher altitudes will be poor to very poor,
especially for the East Bay Hills and the Santa Lucias, which
includes the area affected by the Timber Fire. Fortunately, winds
continue to be light.

&&

.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...
(06Z TAFS)
Issued at 942 PM PDT Tue Aug 18 2026

Currently a mixed bag of LIFR-VFR. Prevailing persistence forecast 
tonight with little change in the pattern or marine layer. As such, 
IFR ceilings are forecast to impact all terminals tonight. If the 
marine layer compresses, lower chance of it reaching LVK or SJC. VFR 
expected for most by tomorrow afternoon with low stratus sticking 
close to the coast. HAF will likely remain sub-VFR through the TAF 
period with a low chance for ceilings to hang on at MRY and SNS 
through the day. Diurnal winds will prevail.

Vicinity of SFO...Currently IFR with northwesterly flow. High 
confidence in VFR prevailing by tomorrow afternoon. Diurnal winds 
will prevail. Lower (40%) confidence in the return of an IFR ceiling 
tomorrow night.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently LIFR with southwesterly flow at 
MRY and IFR with northwesterly flow at SNS. SNS is expected to 
further deteriorate to LIFR with LIFR-IFR conditions expected 
through tomorrow morning at both terminals. Low stratus will stick 
to the coast during the afternoon with another early return tomorrow 
evening. Diurnal winds will prevail.

&&

.MARINE...
(Tonight through next Monday)
Issued at 942 PM PDT Tue Aug 18 2026

Moderate to fresh northwesterly breezes will prevail through
Saturday. Locally strong northwesterly breezes across the northern
outer waters and along the coastal jet regions will bring
hazardous  conditions for small craft into Thursday. Rough seas in
the outer  waters will abate Thursday with moderate seas
prevailing by  Friday.

&&

.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...Sarment

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Prepared by Boulder-Creek.com Weather at: Tue Aug 18 22:30:03 PDT 2026