St Helena, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

243
FXUS66 KMTR 200651
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1151 PM PDT Wed Aug 19 2026

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

.KEY MESSAGES...
Updated at 1147 PM PDT Wed Aug 19 2026

 - Gradual cooling trend today into the beginning of the weekend

 - Elevations above about 1000 feet will have warm temperatures
   and RH values in the teens early this morning into the
   afternoon

&&

.SHORT TERM...
Issued at 1147 PM PDT Wed Aug 19 2026
(Today and tonight)

We`ve been in the middle of a subtle back and forth between
troughing over the EPAC and ridging over the desert southwest the
past week or so. The net result has been only small changes in our
day-to-day ambient weather conditions. However, the influence of
troughing nudging back to the east toward our coastline today, will
result in the beginning of gradual cooling, that will continue into
the weekend. Improved lower boundary layer onshore flow will help
bring temperatures around 5-10 degrees below normal away from the
coast, while still remaining above normal in southern Monterey
County. The depth of the marine layer influence will increase as
well today helping to further support cooler temperatures. The more
mild overnights we`ve been seeing will moderate closer to normal
too, leading to stratus pushing farther inland after sunset into
early Friday morning.

&&

.LONG TERM...
Issued at 1147 PM PDT Wed Aug 19 2026
(Friday through Wednesday)

The gradual cooling trend into the beginning of the weekend levels
off by Sunday into the beginning of next week with only minor,
likely unnoticeable changes through the remainder of the extended
forecast. The strong dome of high pressure over the desert
southwest shows hints of shifting back to the west by the end of
next week, which would bring warmer temperatures, however there is
still no signal for a significant pattern change. Tropical wave
activity is still expected to steer well south and west of our
area with little to no impacts on our forecast attm.

&&

.AVIATION...
(06Z TAFS)
Issued at 935 PM PDT Wed Aug 19 2026

Currently LIFR-VFR. Persistence forecast on tap with moderate to
high confidence in IFR ceilings returning to all terminals tonight.
Further deterioration to LIFR is expected at coastal terminals and
STS. If the marine layer undergoes compression, conditions will
further deteriorate with less inland extent expected. VFR is
expected by the afternoon (except for HAF) with IFR stratus
sticking close to the coast. Diurnal winds will prevail.

Vicinity of SFO...Currently VFR with westerly flow. High confidence
in an IFR ceiling returning tonight with VFR by late-morning.
Diurnal winds will prevail.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently IFR with westerly flow at MRY and
IFR with northwesterly flow at SNS. High confidence in conditions
deteriorating to LIFR. At least a brief period of VFR is expected by
tomorrow afternoon. Diurnal winds will prevail.

&&

.MARINE...
(Tonight through next Tuesday)
Issued at 935 PM PDT Wed Aug 19 2026

Moderate to fresh northwesterly breezes will prevail through
Friday, increasing to become fresh to strong over the weekend.
Hazardous conditions for small craft are expected across the outer
waters and along the coastal jet regions into Friday. Rough seas
in the outer waters will slowly abate with moderate seas
prevailing by Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 AM PDT Friday for 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.

     Small Craft Advisory from 3 PM to 9 PM PDT Thursday for Pigeon
     Pt to Pt Pinos 0-10 nm.

&&

$$

SHORT TERM...JM
LONG TERM....JM
AVIATION...Sarment
MARINE...Sarment

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