064
FXUS66 KSTO 212024
AFDSTO
Area Forecast Discussion
National Weather Service Sacramento CA
124 PM PDT Wed Oct 21 2026
For additional details on weather and expected impacts over the
next 7 days, please visit weather.gov/sto/briefing.
.KEY MESSAGES...
- Scattered showers, thunderstorms, and high elevation snow
showers expected through late Thursday.
- Drier weather moves in Friday and into the weekend, with breezy
north to east winds Friday.
- Ridge builds in over the weekend, with slight chances for
showers in Shasta county.
.DISCUSSION...
...Today through Thursday...
Current radar has a swath of rain and isolated thunderstorms
through the Coastal Range down toward the Sierra Foothills. The
track is to the north-northwest. We have seen a few lightning
strikes over the past couple of hours, but they remain few and far
between storms. There are gaps of coverage in the rain as it
moves through the area, but HIRES guidance continues to show the
swath of rain filling in completely over the next several hours
and begin to push toward the north and east as the main upper
level system off the coast moves onshore. Instability has been
lacking in the latest observations, with only up to 250 J/kg
across the Valley, foothills, and mountains. This will likely
continue for the rest of the afternoon. Cloud coverage is the key
into keeping the instability lower than expected. None the less,
moisture content is high and lift is present to development
thunderstorms and at least maintain them through this evening. The
question remains on if these storms will be able to sustain hail
shafts long enough to produce nickel or even quarter size hail. As
of right now, we are leaning toward small hail as a possibility
and brief gusty winds in the strongest of storms. Brief heavy rain
and frequent lightning will also be a hazard to watch out for.
We will see the precipitation spread across the region this
evening and into the overnight, with a brief break in activity for
the Valley during the late overnight hours into tomorrow morning.
The mountains and foothills will see continued chances but on an
isolated basis late in the overnight. Further chances for
thunderstorms tomorrow afternoon and evening as the trough moves
onshore, instability increase, and moisture remains on the higher
side. We will see another chance for a few stronger storms
tomorrow afternoon/evening with the main hazards being lightning,
small hail, gusty winds, and frequent lightning. We will continue
to monitor observations, the environment at hand, and future runs
of the HIRES guidance on any potential of stronger storms today
and again tomorrow. Within the Valley rainfall amounts will
generally be up to 0.50 inches and up to 1.5 inches within
mountains.
Snow levels continue to be around 8000-9000 feet in the Sierra
today and into tomorrow, with expected snowfall totals at the
highest peaks of 3-6 inches. Best chance for snow will be toward
Mono county and into the Yosemite area. Minor impacts are possible
at the highest elevations in Tuolumne county and toward Alpine
due to the snow.
...Friday and beyond...
Lingering mountain showers are possible early Friday before
conditions clear out across the area. Dry conditions are expected
by Friday afternoon/evening, with RH values dropping to the 20s
for portions of the northern Sacramento Valley. As the trough
digs into the southwestern United States winds will shift toward
the north in the Valley and the northeast within the mountains. We
will see gusty winds during the afternoon/evening on Friday with
gust up to 35mph within the northern Sacramento Valley for areas
along and west of I-5. Areas within the mountains may see gusts up
to 40mph especially within the gaps. Otherwise, we will see below
normal temperatures continuing with ridging developing across the
southwestern United States. Couple ridge riders are possible over
the weekend and into early next week that may bring a brief
isolated shower across Shasta County.
&&
.AVIATION...
Showers and isolated thunderstorms will be possible through the
afternoon and into the evening hours, with lowered ceilings.
Ceilings should remain VFR, but chances are there for MVFR for all
sites post 00z. Best timeframe for thunderstorms will be 2000z
through 0400z for all sites. Winds will remain light throughout
the day. Areas in and around thunderstorms may see erratic winds,
brief moderate rain reducing visibilities and IFR ceilings.
Otherwise, we are looking at a lull in activity later tonight
around 09-10z through the mid morning hours. During this lull
isolated to scattered showers at times for the sites. Near the end
of the TAF period thunderstorm chances start to increase once
again.
&&
.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
