FXUS66 KMTR 100503
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1003 PM PDT Thu Apr 9 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1134 AM PDT Thu Apr 9 2026
- Widespread rain showers and thunderstorms this weekend
&&
.SHORT TERM...
Issued at 1134 AM PDT Thu Apr 9 2026
(This afternoon through Friday)
The sky is chaotic over the Bay Area, filled with marine
layer stratus, mid level cumulus, and high level cirrus all stacked
on top of each other with sunbreaks in between. The cut-off low is
now 450 miles due west of the Monterey Peninsula and has picked up
some speed, now moving at about 15 mph to the West. This system will
continue at a similar speed and direction for the next 24 hours,
before it starts to feel the effects of the jet stream, opens up
into a wave and moves inland to Northern California.
While the sun will come out at times this afternoon, conditions
will be cooler than the last couple days, and noticeably more
humid. Dew points are in the mid 50s, with 100% relative humidity
observed across the majority of cwa in this morning. This humid
air is thanks to SW winds ahead of the cut-off low pumping
tropical moisture into California. The PW has remained over 0.9"
for the last 3 balloon launches going back to Wednesday morning.
Shower and thunderstorm chances will start later today, but mainly
confined to the North Bay.
More widespread rain is expected by Friday morning. The low pressure
system will make landfall somewhere along the coast in far northern
California, bringing a pseudo-cold front to the Bay Area. High
resolution models are starting simulate this first band of rain and
the consensus is for an organized, but somewhat weak initial push
of showers moving through from around 2 AM to 9 AM Friday before
showers become more isolated through the day. Despite the isolated
nature of the showers, there is still a 20% chance for
thunderstorms in the afternoon, though most of the activity is
expected in the Central Valley and across the Sierra. All told we
expect less than 1/4 inch on Friday.
&&
.LONG TERM...
Issued at 1134 AM PDT Thu Apr 9 2026
(Friday night through next Wednesday)
If the rain on Friday leaves you unimpressed, wait until
the weekend is over before putting your rain boots back in the
attic. A reinforcing short wave trough will move quickly down from
the Gulf of Alaska, colliding and merging with the pre-existing cut-
off low and associated moisture. This convergence is expected in the
afternoon or early evening, which is when the peak of the storm
activity will arrive to the Bay Area and Central Coast. This frontal
passage will also serve as a lifting mechanism for convection, which
is arriving at a good time of day to capitalize on the maximum
surface based instability generated from afternoon insolation. The
combination of low-level moisture left over from the tropical air
mass advection, instability from relatively warm surface
temperatures quickly cooling with height (850 mb temperatures near
freezing), and a lifting mechanism from the cold front, are all
favorable for thunderstorms to develop Saturday afternoon/evening.
The addition of wind shear delivered from the jet stream allows
these storms to separate updrafts and downdrafts, prolonging their
lifespan. If we get enough low-level shear we could even see some
rotating thunderstorms, and we can't rule out a weak waterspout or
even brief tornado. Anglers should take special note of the
expected weather and have a plan to reach safe harbor quickly when
the weather turns. Even without thunderstorms, the marine
conditions will deteriorate through the day Saturday. If
thunderstorms develop, the associated wind gusts are capable of
capsizing small vessels. Fishing leads to more lightning deaths in
the U.S. than any other activity. When thunder roars, head
indoors!
One change to the forecast is that Sunday now looks more active. It
should still be a transition day as high pressure starts to rebuild,
but the rain may not scatter out until the afternoon, rather than
the morning. Adding it all up, we are expecting anywhere from 1-2"
across the majority of the Bay Area and Central Coast. That number
has continued to trend up. The floor of the probabilistic forecast is
pretty high. There is a 90% chance that SFO gets at least 1" of rain
according to the ECMWF ensemble. Typically the "dry" scenario is
much lower for our rainy forecasts, but it seems unlikely that this
particular storm will be a dud.
Monday still looks dry for now. As the storm breaks, we'll be
left with cold temperatures. Lows will be in the 40s for most, and
upper 30s in the coldest pockets. A gradual warming trend is
expected through mid-week, stabilizing near normal with much drier
conditions expected.
&&
.AVIATION...
(06Z TAFS)
Issued at 1003 PM PDT Thu Apr 9 2026
MVFR-IFR stratus is developing at the coast and moving inland
through the next few hours, and will be reinforced overnight by the
first band of showers expected to arrive sometime Friday morning. As
the showers pass, ceilings should lift to become generally VFR-MVFR,
along with isolated to scattered thunderstorms across the region,
with the best chances focused on the North Bay and East Bay on
Friday afternoon and evening. Have decided to go ahead and put VCTS
in for the STS, APC, and LVK TAFs for the 18-00Z time period, but
note that this is a lower confidence forecast. Chances for
thunderstorms decrease on Friday evening, but scattered showers
remain possible through the end of the TAF period, with another band
of widespread rainfall expected after the end of the TAF period on
Saturday afternoon through Sunday. Onshore wind flow will continue
to diminish tonight, with light winds overnight before moderate
southwest winds prevail through much of Friday. Gustier winds are
expected where thunderstorms do develop.
Vicinity of SFO... VFR conditions continue through the next couple
of hours. MVFR conditions will set up overnight as a band of showers
passes through the terminal area through Friday morning, with VFR
conditions with mid-level clouds returning once the showers pass.
Friday afternoon should see a moderate southwesterly flow develop
through the evening. There is around a 25-35% chance of
thunderstorms at the terminal Friday afternoon and evening, but
confidence is too low to put VCTS or PROB30 in the TAF at this time.
Chances for showers continue through the end of the TAF period.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... MVFR stratus is developing around the
terminals with a "sucker hole" at present around MRY, with a band of
showers expected to pass through the region through the course of
Friday morning. VFR conditions return as the showers pass with
breezy southwest winds developing Friday afternoon. There is a 20-
35% probability of thunderstorm development at the terminals Friday
afternoon and evening, but confidence is too low to put VCTS or
PROB30 in the TAF at this time. Chances for showers continue through
the end of the TAF period, with a low confidence forecast that winds
at the terminals will abate overnight with potential development of
MVFR stratus.
&&
.MARINE...
(Tonight through next Wednesday)
Issued at 1003 PM PDT Thu Apr 9 2026
Unsettled weather is expected Friday through the weekend,
including increasing winds, building seas, and a few rounds of
thunderstorms. Thunderstorms will be capable of producing frequent
lightning, gusty and erratic winds, localized heavy rain, small
hail, and waterspouts. Gentle to moderate south to southwest winds
will become fresh to strong by Saturday, with occasional near
gale force gusts. Winds become westerly late Saturday and
northerly by late Sunday as the low pressure system passes near
the northern waters. Fresh north winds last through midweek.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Gale Watch from Saturday morning through late Saturday night for
Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt
Piedras Blancas 0-10 nm.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...DialH
MARINE...DialH
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Prepared by Boulder-Creek.com Weather at: Thu Apr 9 22:30:03 PDT 2026
From the National Weather Service
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