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

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, X, and YouTube at:
www.facebook.com/nwsbayarea
x.com/nwsbayarea
www.youtube.com/nwsbayarea


  
Prepared by Boulder-Creek.com Weather at: Thu Apr 9 12:30:03 PDT 2026