FXUS66 KMTR 082336
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
436 PM PDT Wed Apr 8 2026
...New UPDATE, 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...
(00Z TAFS)
Issued at 434 PM PDT Wed Apr 8 2026
Generally VFR conditions through the region with some convective
clouds and virga developing over the higher elevations. Moderate
onshore flow continues into the evening hours, diminishing to become
light overnight. MVFR-IFR stratus will return to much of the inland
regions through the evening and overnight hours, with LIFR and
patchy fog at the coastal terminals. Stratus will lift and
retreat to the immediate coast through Thursday morning, with mid-
to high level clouds across the region as a system approaches,
causing southwest winds and isolated showers through Thursday
afternoon and evening.
Vicinity of SFO... VFR with some high clouds and moderate to fresh
west-northwest winds through the evening hours. IFR stratus returns
to the terminal late this evening as winds become light overnight.
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, but with most of
the rain expected after the end of the TAF period.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... VFR with some high clouds through the
evening as moderate northwest winds continue. IFR-LIFR stratus will
develop across the region this evening through the overnight period,
with the stratus gradually lifting through Thursday morning and
afternoon. Mid-to high level clouds will then persist through the
end of the TAF period. Winds will become light overnight and
Thursday morning before an onshore breeze resumes in the afternoon.
&&
.MARINE...
(Tonight through next Monday)
Issued at 434 PM PDT Wed Apr 8 2026
Gentle to moderate NW winds across the coastal waters will
gradually shift to southwest over the next 24-48 hours 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-Monday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...DialH
MARINE...Flynn
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Prepared by Boulder-Creek.com Weather at: Wed Apr 8 18:30:03 PDT 2026
From the National Weather Service
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