000
FXUS66 KSTO 022220
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
220 PM PST Thu Feb 2 2023

.SYNOPSIS...
Cool and dry weather continues today, then precipitation chances
return for the end of the week and the weekend. Dry weather
expected through much of next week.

&&

.Discussion...
Cloud cover has increased across NorCal this afternoon ahead of
the approaching trough that is currently spinning near 40N,135W.
As this trough continues to move northeast, marginal moisture
associated with a weak and decaying AR will move inland across
the PacNW, brushing NorCal in the process. This first system will
bring light precipitation to portions of NorCal, leading to minor
impacts. Latest guidance indicates a narrow band of light
precipitation moving into interior northern CA between 3am-5am
Friday. As this band progresses eastward, it dissipates with lack
of dynamic support, resulting in the `heaviest` precipitation
expected to be along and west of I-5, northwest of Sacramento.
Precipitation totals expected with this band for the area
northwest of Sacramento range from 0.25" to 0.50", with locally
higher amounts north of Redding. Areas east and southeast of the
I-5 corridor can expect precipitation totals of trace to 0.25"
Friday. Snow amounts of 3 to 8 inches are expected to be limited
to the higher elevations of the Coastal Range and southern
Cascades, producing limited impacts. Winds with this system will
also be minor, with southerly winds gusting to 20-30 mph.

A brief period of mostly dry conditions is expected late
afternoon Friday through Saturday morning, before another weather
system moves in.

The next weather system is expected Saturday afternoon - Sunday,
bringing more widespread precipitation, including heavy mountain
snow to the Sierra, stronger winds, and even some scattered
thunderstorms. Still some uncertainty in the timing of the second
system as the the latest ECMWF is a tad faster than the GEFS with
bringing in the second trough. Regardless it does look like the
heaviest precipitation is expected to arrive Saturday late
afternoon/evening through Sunday morning. Precipitation totals
range from 0.25" to 0.75" across the Valley (with highest amounts
across the eastern Valley), 0.75" - 1.75" in the foothills, and
1.00"-2.50" in the mountains. Snow levels will initially be around
5000`-6000` feet Saturday, dropping to 4000` Sunday. With the
heaviest precipitation expected Saturday-Sunday morning, the
heaviest snow is expected above 6000`, with totals of 15 to 30
inches. As snow levels decrease through the morning Sunday, snow
amounts of 6 to 15 inches will be possible above 4000`. With heavy
snow expected across the northern Sierra/Lassen Park, decided to
issue a Winter Storm Watch, highlighting the likely winter travel
impacts over the weekend.

Added dynamics with this cold trough will allow for decent
instability to develop Sunday, bringing the potential for some
scattered thunderstorms. Timing and cloud cover will be key
factors in determining where thunderstorms may develop Sunday.
Even so, latest guidance is depicting CAPE values of 200-350 J/kg
between 10am - 7pm Sunday.

Slightly stronger southerly winds are expected with the second
weather system Saturday-Sunday, with gusts to 25-35 mph in Valley
and foothills, and gusts to 45-55 mph in the mountains.

//Peters

&&
.EXTENDED DISCUSSION (Monday THROUGH Thursday)...

Ensembles and cluster analysis depict a ridge of high pressure
building across interior NorCal for the extended period. This
will result in dry conditions and near normal daytime highs.
Temperatures will range from the upper 50s to low 60s in the
Valley, with 40s and mid 50s over the mountains. Overnight lows
are expected to range from the mid 30s to upper 30s in the Valley.

&&

.AVIATION...


VFR conditions over interior NorCal next 24 hours. Surface winds
generally below 10-15 kts through this evening. Increasing
southerly wind gusts up to 15-25 kts in the Valley and up to 25-40
kts over higher terrain after 06Z Friday. Pacific system will
gradually spread precipitation after 10Z-12Z Friday with local
MVFR/IFR conditions in rain.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Saturday afternoon through Sunday
evening for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&

$$