680
FXUS66 KSTO 202010
AFDSTO
Area Forecast Discussion
National Weather Service Sacramento CA
110 PM PDT Tue Oct 20 2026
For additional details on weather and expected impacts over the
next 7 days, please visit weather.gov/sto/briefing.
.KEY MESSAGES...
- Widespread precipitation late today through Thursday, with
below normal temperatures, thunderstorm chances, and high
elevation snow showers.
- Ridging builds into the region end of the week and into the
weekend, with a slight chance for precipitation Sunday into
early next week.
&&
.DISCUSSION...
...Today through Thursday...
Satellite has a broad area of broken clouds across CA this
afternoon, with a few isolated showers in parts of the Sierra.
Current HIRES guidance has showers and isolated thunderstorm
potential along the Sierra as the main upper level trough remains
offshore pushing moisture and instability into CA. Best chances
for any shower or isolated storm for today will be this evening
and into the overnight hours for the Sierra before the main upper
level system starts to shift onshore. HRRR and NAM guidance has
the most robust chances, while others favor more isolated shower
potential. None the less, chances are 10-20 percent for areas in
the Sierra this evening and overnight for thunderstorm potential.
A few storms may be on the stronger side, with small hail, brief
heavy rain, gusty winds, and frequently lightning being the main
hazards.
Further instability and moisture push into the Valley, Foothills,
and Mountains Wednesday. Best chances for widespread precipitation
will be during the afternoon through the evening hours. This
timeframe is when the highest PWATS, Shear, Instability, and Lift
will be within the region. Shear is roughly around 20kts through
the first 6km and instability is sitting around 500-750J/kg. This
combined with the strength of the forcing moving through the low
to mid levels will bring the thunderstorm chances through the
mountains and into the foothills/valley. A few of the storms could
be on the stronger side once again, with the potential for a low
top supercell. The chance of a low top supercell is low for the
region, but the area of interest for this development is within
the southern Sacramento Valley, northern San Joaquin, and into the
adjacent terrain. Main hazards in the stronger storms would be
small hail, gusty winds, lightning, and brief heavy rain. We will
continue to monitor the latest CAMS and observations as the system
moves into the region bringing the thunderstorm chance.
Current NBM guidance increased snow levels to 8500 feet and above
Wednesday through Thursday. Snow totals at the highest elevation
(areas south of Highway 50) are roughly 3-6 inches at this time.
Higher chances and totals are toward the Yosemite region.
Conditions slowly dry out on Thursday with lingering thunderstorm
potential in the mountains Thursday afternoon and evening. By the
overnight conditions dry and skies start to clear. Besides the
thunderstorms, rain, and snow chances, temperatures will continue
to be below normal to near normal across the region through the
end of the week.
...Friday and beyond...
As the system tracks east of the region late Thursday into Friday,
conditions dry and slight ridging builds into the area. Winds will
turn toward the north to northeast on Friday, with a few gusts up
to 30mph. Minimum RH values will be in the 20s to 30s for
portions of the northern Sacramento Valley and adjacent Terrain
Friday, but at this time chances for fire weather concerns are
low. Dry and below normal temperatures continue through Saturday
and into Sunday, with slight chances for precipitation in the
higher terrain late Sunday and into early next week. Guidance
continues to struggle with the ridge evolution and if any trough
can break through and bring further unsettled weather.
&&
.AVIATION...
VFR conditions prevail across the region through the TAF period.
Winds will remain light and less than 12kts. A few isolated
showers will be possible in the Valley after 05Z Wednesday,
before more widespread showers move into the area after 18Z
Wednesday.
&&
.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
