FXUS66 KMTR 091840
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1140 AM PDT Thu Apr 9 2026
...New SHORT TERM, LONG TERM, 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...
(18Z TAFS)
Issued at 1134 AM PDT Thu Apr 9 2026
Quiet weather as we await our next chance for rain. Satellite shows
a mix of low, mid, and high level clouds with the lower clouds
gradually eroding. Conditions vary from IFR to MVFR right now, and
should become MVFR to VFR for most sites over the next couple of
hours. Cumulus clouds may build up and may lead to some VCSH around
areas of the North Bay. By later this evening and into the overnight
hours, the first round of rain showers will move through starting in
the north and gradually spreading south. MVFR to IFR conditions are
expected with this, and should improve to MVFR to VFR after the
front moves through. There's also 15-20% of thunderstorms with this
system. Should we get enough moisture, instability, and lift tomorrow
afternoon, we could see some vicinity thunderstorms around the north
bay terminals.
Vicinity of SFO...Clearing skies around SFO, with VFR conditions
expected for several hours. Lingering low level clouds around the
site are expected but shouldn't return to MVFR status in until 01-
03Z. Onshore flow is expected to bump up to around 10-15kt this
afternoon, before decreasing shortly after sunset. Rain showers
arrive after 6Z and will be intermittent through the morning hours.
CAMs show precip chances dwindling after that, with perhaps a few
showers skirting by during the afternoon. As noted above, there is a
chance for isolated thunderstorms, but confidence was too low to add
any mention of VCTS or PROB30 Thunder.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Stratus hugs the Monterey Bay this morning,
which has left the TAF sites with MVFR and IFR cigs. Satellite shows
it slowly clearing and models didn't catch on to this very well.
Timing of VFR cigs may need to be adjusted if the clouds take longer
to dissipate. Low cigs and vis return tonight, with the chance for
rain arriving late tonight and into tomorrow morning.
&&
.MARINE...
(Today through Tuesday)
Issued at 1134 AM PDT Thu Apr 9 2026
Unsettled weather is expected through the weekend, including
increasing winds, building seas, and thunderstorms. Thunderstorms
will be capable of producing gusty and erratic winds, 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. Fresh north winds last through midweek.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...KR
MARINE...KR
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Prepared by Boulder-Creek.com Weather at: Thu Apr 9 14:30:02 PDT 2026
From the National Weather Service
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