000
FXUS66 KSTO 202112
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
212 PM PDT Tue Apr 20 2021

.SYNOPSIS...
Cooler today with the chance of mainly mountain showers and
thunderstorms. A few showers possible Wednesday in the Sierra
south of Tahoe. Otherwise dry and warmer weather returns through
the end of the week. Chance of precipitation returns this weekend.

&&

.DISCUSSION...
A robust short wave and vort max is begining to push into the NW
part of the state this afternoon. This vort max has brought
showers to far northern CA this morning and we are starting to see
these showers spread south. Mesoscale Analysis has generally
100-250 j/kg of uncapped CAPE from about Chico north and models
build in as much as 500+ j/kg of CAPE by the evening mainly from
Sacramento north. Showers and a few thunderstorms will be
possible even in the Valley given the CAPE as the vort max pushes
over the region but the best chances will remain over the higher
elevations. Current PWATS are between 0.20 and 0.60" and we will
see moisture continue to build into the region this evening with
PWATS reaching 0.75" in some areas. This will bring a heavy rain
threat with any convection but soundings due have an inverted V
look to them in the Valley and that should limit the heavy rain
potential for lower elevations and bring more of a gusty wind
threat. With more moist low levels over the higher elevations that
is where the heavy rain threat will be greatest. Rain rates are
expected to remain below flash flood guidance over the burn scars
but up to 0.60" of rain in an hour will be possible over portions
of the North Complex, Camp Fire Burn Scar and August Complex.
With limited low level shear storms will likely pulse up and
quickly pulse down.

Shower chances will continue to push south into the evening but
dry low levels and limited CAPE will really limit activity as it
reaches the Sacramento area. Mainly dry conditions can be
expected overnight. The upper level trough will push into NV
during the morning tomorrow and we will see some CAPE build in
over the Sierra from I-80 south. This will bring a threat of a few
showers and thunderstorms once again for tomorrow but coverage
will be very limited to I-80 south over the Sierra. Wind will be
the other impact with this system and we are seeing some breezy
northerly winds over the northern Sacramento Valley this
afternoon. These winds will calm some overnight before picking
back up tomorrow mid to late morning into the afternoon. Most of
the region is seeing onshore flow today and that is keeping
humidity levels up but that will not be the case tomorrow and
some single digit humidity reading look possible over the northern
Sacramento Valley during the afternoon. This will bring some
increased fire weather concerns given the increased wind with it.

Upper level ridging builds in on Wednesday and that will bring a
period of quiet weather into Friday with less wind. A short wave
trough will approach the northern part of the state Friday night
and we could see a few light showers develop over the Shasta
County Mountains overnight. Highs will continue to run 6-12
degrees above average this week with today being the coolest day
in the short term.

-CJM

&&

.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)...
Cluster analysis has come into better agreement about an
unsettled weather pattern impacting Northern California this
Sunday into next week. We will likely see cooler temperatures as a
trough makes it way into the region. We are also forecasting
widespread precip chances across Northern California Sunday into
early next week. Best chances will be for higher elevation
locations, however Valley rain will be possible. Snow showers will
likely bring travel impacts to Sierra Passes particularly on
Sunday. Some uncertainty still exists with timing and amounts as
depicted by cluster analysis so check back for updates.

&&

.AVIATION...
Widespread VFR with MVFR possible due to passing shower activity.
Isolated thunderstorms could impact Sacramento Valley and
Sacramento Metro area TAF sites. IFR possible under any isolated
heavy showers that may develop. Locally gusty northerly winds 20
to 30 kts over Northern Sacramento Valley and Southwesterly gusts
up to 25 kts through Delta region decreasing after around 4 UTC.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$