FXUS66 KMTR 081827
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1127 AM PDT Wed Apr 8 2026

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

.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

&&

.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...
(18Z TAFS)
Issued at 1037 AM PDT Wed Apr 8 2026

VFR conditions look to persist through much of the day with 
onshore flow increasing. LIFR/IFR/MVFR ceilings return late this 
evening near the coast and more so into Thursday morning across 
the remainder of the region. Fairly high confidence for MVFR/VFR 
ceilings return by mid morning on Thursday. 

Vicinity of SFO...VFR. Onshore winds increase this afternoon out 
of the west-northwest to between 10-15kt before easing a bit late 
in the night. Moderate to high confidence for sub-MVFR conditions 
to late tonight and potentially lowering to LIFR at times (lower 
confidence for this to occur). Fairly high confidence for MVFR 
ceilings to return by 18Z Thursday and VFR by 21Z.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. Onshore winds increase this 
afternoon out of the northwest before easing this evening. High 
confidence for IFR ceilings to return late this evening. There is 
moderate to high confidence for LIFR conditions overnight 
gradually improving to MVFR by mid morning. 

&&

.MARINE...
(Today through Monday)
Issued at 1037 AM 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 Friday 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...RGass
MARINE...Flynn

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Prepared by Boulder-Creek.com Weather at: Wed Apr 8 12:30:03 PDT 2026