Rheem Valley, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

774
FXUS66 KMTR 170437
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
937 PM PDT Wed Sep 16 2026

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 110 PM PDT Wed Sep 16 2026

 - Cooler temperatures through Thursday

 - Warming trend Friday into the weekend

&&

.UPDATE...
Issued at 902 PM PDT Wed Sep 16 2026

The advertised cooldown for today definitely verified. Not only
did highs remain well below seasonal averages they also came in
under forecast values as well. Max temps reached the 60s along the
coast and 70s inland with a handful of 80s well interior. Cold
upper low coming down the coast, strong onshore flow, and
lingering stratus clouds contributed to the cooler temps.

For this evening, stratus is already making an appearance across
the region. Expect coverage to increase overnight. That being
said, this is not a "classic" marine layer setup thanks to the
nearby upper low. Definitely some strato-cu vibes as inland clouds
are popping off over terrain features. No update is needed to the
forecast this evening.

Thursday AM is still on track to start off cool, damp, and cloudy
with drizzle.

&&

.SHORT TERM...
Issued at 110 PM PDT Wed Sep 16 2026
(This evening through Thursday night)

The marine layer was measured to be 2,400 feet deep by the 12Z
sounding from Oakland. Surface observations show it`s closer to
3,000 feet deep along the Central Coast, where all of Santa Cruz
County remains under overcast skies. The NW wind direction created
some downslope dry air that managed to keep Silicon Valley clear,
but that was the exception. The morning stratus reached inland to
Healdsburg, Dublin, and Paso Robles. As a cut-off low over Northern
California continues to drift south, the marine layer will expand
over the next 24 hours. The deeper cloud layer will be enhanced by
some 700 mb vorticity and typical orographic lift to support some
drizzle overnight through Thursday morning. The signal is better
over the Bay Area than the Central Coast, but all of the typical
drizzle locations have a chance. In fact, all 50 of the ECMWF
ensemble members have some precipitation reaching SFO Thursday
morning. This is still drizzle though, so the accumulation will
range from a trace to a few hundredths before the atmosphere mixes
in some drier air by midday.

In addition to drizzle, the cut-off low will bring some strong
onshore winds this evening. The SJSU 2km WRF ensemble shows high
confidence in a onshore 8-10 mb SFO-WMC gradient developing quickly
this afternoon. This is a result of northerly winds west of the low
cooling the coast, while warm southerly winds east of the cut-off
support thermal exhaust and surface low pressure over northern
Nevada. The RRFS shows gusts up to 40 mph this evening in favored
gaps such as Pacheco Pass, Altamont Pass, and the Carquinez
Straight. Gusts up to 25 mph are likely in both the San Francisco
and Monterey Bays later today as well. These winds will peak in the
early evening, but continue well into the overnight hours. The
strongest winds are not widespread enough to warrant a wind
advisory, but expect to feel them while driving through the areas
mentioned.

Outside of drizzle and wind, the cut-off low will bring cool, humid
conditions to the entire Bay Area. Coastal temperatures will be in
the 60s along the coast and 70s inland. Some of the warmer places
will reach the upper 70s, but that`s still roughly 10 degrees cooler
than normal for this time of year. We`ll get a repeat of these cool
temperatures on Thursday before a slight warming trend kicks off
Friday.

&&

.LONG TERM...
Issued at 110 PM PDT Wed Sep 16 2026
(Friday through next Tuesday)

The cut-off low will have weakened and start to be reabsorbed into
the jet stream by Friday. This will open the door for a short wave
ridge to briefly build through Saturday. While short lived, this
feature will cause the marine layer to compress and temperatures to
return back to normal. We may even see some weak offshore winds at
this time. The SFO-WMC gradient is expected to flip negative by
Saturday morning, but there is no indication it will be very strong.
By Sunday a much weaker cut-off low will move in from the west. This
feature will likely keep conditions pretty consistent and relatively
benign through mid week. The 850 mb temperature is expected to hover
around 20C early next week, which is around the 75th percentile for
this time of year. By mid week the uncertainty balloons. Since the
pattern depends on multiple, hard to track cut-off lows, chaos
theory goes into full swing quickly. Ensembles shows an equal chance
of 850 temps reaching 5C or 25C next Thursday, which literally spans
the entire range of historical possibilities for this time of year.
While I like to look for signs of early rain or strong offshore
winds this time of year, with the high uncertainty it`s not really

&&

.AVIATION...
(06Z TAFS)
Issued at 933 PM PDT Wed Sep 16 2026

Currently a mixed bag of MVFR-VFR. Transient TAF period on tap
thanks to an upper-level low to our north that is mixing up the
environment. As a result, ceilings on the cusp of MVFR/low-end VFR
are expected through tomorrow morning for most terminals. Drizzle
will lower ceilings and visibilities; the best chances are in the
Bay Area with chances diminishing southward. Diurnal winds will
prevail.

Vicinity of SFO...Currently VFR with southwesterly flow. Transient
TAF period on tap with clouds on the cusp of MVFR/VFR expected
through tomorrow morning. Drizzle will be possible overnight,
resulting in a lower ceiling/visibility. Diurnal winds will prevail.

SFO Bridge Approach...The San Mateo Bridge Approach will likely
not be as impacted as SFO; however, clouds at or below FL055
should be expected in the vicinity through tomorrow morning with a
chance that they linger into the afternoon.

Monterey Bay Terminals...Currently VFR with southerly flow at MRY
and VFR with southwesterly flow at SNS. Once stratus reaches the
terminals, MVFR conditions will likely prevail through the morning.
Lower probability for drizzle, but if it occurs, lower ceilings and
visibilities are expected. VFR is expected by late-morning. Diurnal
winds will prevail.

&&

.MARINE...
(Tonight through next Tuesday)
Issued at 933 PM PDT Wed Sep 16 2026

Conditions will improve tonight as northwesterly breezes diminish
to  become gentle to moderate. Hazardous conditions for small
craft will  return Friday into the weekend due to fresh to strong
northwesterly  breezes. Slight to moderate seas will prevail
through Sunday.

&&

.FIRE WEATHER...
Issued at 412 AM PDT Wed Sep 16 2026

A passing trough is allowing the marine layer to expand with the
depth estimated around 2000-2500 feet in the area around the Timber
and Plaskett Fires. Above the marine layer inversion, humidities
remain low with relative humidity values as low as 15% observed. The
trend for deeper marine layers is expected to continue into Friday,
with humidities in higher elevations expected to rise as a result.
Some nocturnal drying is possible in the highest peaks, but minimal
coverage is expected.

DialH

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Sarment
MARINE...Sarment

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