FXUS66 KMTR 090501
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1001 PM PDT Wed Apr 8 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1118 AM PDT Wed Apr 8 2026
- The forecast has trended wetter and more active
- Unsettled weather returns as early as Thursday and continues
through the weekend
- Thunderstorms are possible Thu-Sun, with the best chances
Friday and Saturday
&&
.UPDATE...
Issued at 434 PM PDT Wed Apr 8 2026
High based showers have developed across the higher terrain this
afternoon. Although most of the resulting precipitation has not
reached the ground, and no lightning has been reported with these
cells, a couple of the strongest showers could still result in
precipitation, with the possibility for brief periods of rain and
gusty winds to 30 mph through sunset. Otherwise, no changes to the
forecast.
DialH
&&
.SHORT TERM...
Issued at 1118 AM PDT Wed Apr 8 2026
(This afternoon through Thursday)
We didn't expect any rain today, but were surprised by a
few small showers in the North Bay this morning. These are
associated with a remote outer rain band from the cut-off low
pressure system over the Eastern Pacific. These early showers
demonstrate how moist the atmosphere is. The 12Z sounding found a PW
of 0.92", well above the 90th percentile for this time of year.
This moisture is being fed by large scale southerly winds pumping
a band of tropical moisture to all areas east of the cut-off low.
While the widespread clouds and rain are still well off-shore,
the column is already gaining moisture effectively. When the
weather does roll in to this moist environment, rain showers and
thunderstorms will likely overperform earlier forecasts.
I'm getting ahead of myself, focusing back on the short term
forecast, the marine layer is starting to mix out with cooler
temperatures aloft decreasing the stability. As a result
temperatures today will remain above normal, and broadly similar
to yesterday. We'll trade the stratus for passing mid level
clouds associated with the outer rain bands of the cut-off low.
As the low drifts closer to the coast on Thursday, the
troposphere will start to become saturated and a blanket of high
clouds will move overhead. Despite the continued southerly winds,
the increased cloud cover will cause max temperature to drop
around 5 degrees. Light prefrontal rain is possible Thursday,
particularly in the North Bay. We could even see a couple
thunderstorms up there, but the main event begins Friday.
&&
.LONG TERM...
Issued at 1118 AM PDT Wed Apr 8 2026
(Thursday night through next Tuesday)
Bottom line, the forecast has taken a turn towards the
wetter and more active direction for Fri-Sat. The Friday rain is
caused by the cut-off low finally approaching the coast. This will
bring stronger southerly winds, enhancing the advection of tropical
moisture in the warm sector. The exact speed and eventual
location of this low is hard to pin down due to a lack of steering
flow. The greatest impacts will be near the low pressure center,
where the lift and vorticity is strongest. Rain will likely start
well before the sun comes up Friday morning and continue, off-and-
on, through the day. The best chance for stronger showers and
thunderstorms is during the afternoon hours.
The Saturday rain is more dynamic. It's caused by a robust,
reinforcing short wave trough quickly moving from the Gulf of Alaska
directly towards the Bay Area. While this feature is bringing a
drier air mass, the lingering moisture from the stalled low will
remain in place as the two features combine. We're basically adding
the jet stream back into this cut off low. That means colder air
aloft and much stronger winds. The shear between the surface
southerlies and upper level westerlies creates favorable conditions
for longer lived thunderstorms that can separate their up-drafts
and down-drafts. The low-level shear also looks favorable (20-25
kt 0-1 km) and we can't rule out rotating storms capable of
producing tornadoes. Again the most active time looks to be in the
afternoon during peak day-time heating. Meanwhile this system
will also bring a colder air mass. The 850 mb temperature should
get close to freezing and surface temperatures will struggle to
get out of the upper 50s or low 60s.
By Sunday the showers should become more scattered or isolated as
the drier air takes over. The upper levels will continue to cool,
and we could even get a couple novelty snow flakes across the
highest mountain peaks Sunday morning. There is still a chance for
thunderstorms due to steep lapse rates, but the decreasing
moisture will be a major barrier to convection. The sun will
likely make a return in the afternoon or early evening. All told
we are expected to pick up around 0.5" to 1.0" across the valleys
and 1.0" to 2.0" in the coastal mountains. Overall that's
a beneficial amount of rain, but some localized flooding is
possible due to the showery nature of this rain. This impact is
most likely in urban areas with poor drainage.
The pattern stabilizes Monday-Tuesday as the trough moves inland
and ridging starts to take its place. That will kick off a warming
trend that lasts at least through the middle of the week before
the uncertainty grows late week. There's roughly an equal chance
of the ridge hanging on or a new trough moving in late next week.
&&
.AVIATION...
(06Z TAFS)
Issued at 1001 PM PDT Wed Apr 8 2026
IFR stratus has developed at the coast and will expand inland
through the night, as moderate onshore winds become light through
Thursday morning. Timing of stratus impacts is low to moderate
confidence as the pace of expansion has been slower than the model
output has depicted. Stratus will lift and retreat to the coastal
regions through the course of Thursday morning, with mid- to high
level clouds across the region as a system approaches, causing
southwest winds across the region on Thursday afternoon. Some
isolated showers are possible in the coastal region and North Bay
valleys, but most terminals should not see rain until Friday past
the end of the TAF period. Also beyond the TAF period, expect
scattered thunderstorms to develop late Friday morning and continue
through the day.
Vicinity of SFO... Moderate northwest winds will diminish overnight
with IFR stratus developing in the next few hours and lasting
through late Thursday morning. Exact stratus formation time is
uncertain owing to poor model depictions of the present situation.
Ceilings will lift late Thursday morning into the early afternoon,
with moderate winds resuming from a southwesterly direction in
advance of a system that will bring MVFR-IFR ceilings and some
isolated showers to the region on Thursday evening into Friday
morning, but with most of the rain expected to arrive after the end
of the TAF period. Also beyond the TAF period, expect scattered
thunderstorms to develop late Friday morning and continue through
the day.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... It appears that large-scale southwest flow
is already impacting the stratus pattern at the terminals with the
stratus deck deflected to the north of MRY and SNS as of the latest
satellite imagery. Expecting this deck to inch its way southward as
light winds continue at the terminals, but timing of impacts is
uncertain as models are not capturing this situation well. Stratus
will scatter out through the course of Thursday morning, leaving a
layer of mid- to high level clouds with moderate onshore winds
through the afternoon. Some isolated showers could arrive Thursday
evening but most impacts, including scattered thunderstorms, should
arrive beyond the TAF period on Friday.
&&
.MARINE...
(Tonight through next Tuesday)
Issued at 1001 PM PDT Wed Apr 8 2026
Gentle to moderate northwest winds across the coastal waters are
shifting to the south and southwest tonight as a low pressure
system approaches the coast. This system will result in showers
and a chance of thunderstorms Thursday through Sunday, with SW
winds increasing to a strong breeze on Saturday. As high pressure
builds behind the system, fresh northwest winds will develop
Sunday through Monday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...DialH
MARINE...DialH
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Prepared by Boulder-Creek.com Weather at: Wed Apr 8 22:30:02 PDT 2026
From the National Weather Service
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