Area Forecast Discussion
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FXUS66 KSTO 120828

Area Forecast Discussion
National Weather Service Sacramento CA
128 AM PDT Fri Apr 12 2024

Unsettled weather arrives today, with periods of showers, isolated
thunderstorms, mountain snow, and breezy to gusty southerly winds
into the weekend. Drier weather is then expected from Monday into
next week, with increasing northerly winds possible through mid week.


As of early this morning, mostly quiet weather continues throughout
interior NorCal. An area of increasing cloud cover is evident across
Shasta and Tehama counties, that may result in some light showery
activity over the next few hours. Otherwise, light winds and low
temperatures about 5 degrees cooler than previous nights are
expected this morning. Upper level troughing can also be seen
digging toward the northern CA coast this morning via latest GOES-
West satellite imagery. This system is expected to continue building
southward today toward the central CA coast, before beginning its
inland trek by Saturday morning. While precipitation will provide
the primary impacts from this incoming system, some breezy to gusty
south-southwesterly winds are expected today and Saturday. Gusts of
20 to 30 mph will be possible throughout the Valley, with strongest
gusts of 40 to 50 mph possible over the Sierra.

Overall precipitation impacts are generally expected to remain tied
to the approaching trough as it progresses eastward. There is a
nonzero signal within some deterministic guidance, however,
indicating that some showers and isolated thunderstorms may develop
this afternoon and evening ahead of the system from the northern San
Joaquin Valley into the Motherlode and Sierra. Current guidance is
only assigning a 20% to 40% chance of this activity occurring
though, as it will be highly dependent on the timing of moisture
advection and attendant lifting mechanisms into interior NorCal. If
it does indeed come to fruition, precipitation should remain
relatively light, with accumulations up to 0.25" possible, and
locally higher in any thunderstorms that develop.

Rather, the more likely scenario is that outside of a few isolated
showers today, increasing precipitation chances hold off until late
this evening/early Saturday morning as the main band of
precipitation associated with the upper trough moves through on
Saturday. This swath of precipitation may include some periods of
moderate intensity precipitation and possibly an embedded
thunderstorm or two throughout the day as well. Current
probabilities of exceeding 0.50" of liquid precipitation remain
around 20% to 40% for much of the Valley, and around 60% to 80%
across the northern Sacramento Valley, foothills, and mountains. The
general expectation is for 0.25" to 1.0" of liquid precipitation in
the Valley, and 0.75" to 1.5" for the foothills and mountains.

Snow levels are expected to be somewhat variable with this system as
well. With still near normal temperatures expected this afternoon,
snow levels of 6000` to 8000` are anticipated through today. As
the trough moves inland, snow levels look to drop to near 4000` to
5500` by Saturday morning, then rapidly down to 3500` to 4000`
from Saturday afternoon through Sunday morning. Despite this, the
majority of accumulating snowfall is anticipated above 5000`, with
snowfall accumulations of 3" to 8" possible above 6000`. Highest
snowfall totals are expected to occur generally south of Highway
50 and north of I-80, with locally highest snowfall at highest
peaks. Latest probabilities of snowfall accumulations exceeding 4"
are generally 15% to 40% from Highway 50 northward, with 50% to
80% probabilities for Lassen Peak and areas south of Highway 50.

As mentioned prior, the primary band of precipitation is expected to
move through interior NorCal throughout the day on Saturday,
reaching the Sierra in the afternoon hours. The upper trough looks
to linger over northern/central CA on Sunday before ejecting
eastward by Monday though. This will likely result in some areas of
lingering showers on Sunday, with areas from I-80 northward favored
for this additional activity. As soon as the trough exits, upper
level ridging will attempt to slide into place moving into early
next week. This will lead to temperatures warming from well below
normal, back to near normal by Monday afternoon. There is some
uncertainty then looking into the extended, but overall dry and
quiet weather is expected for Monday.



While dry weather is expected to prevail through mid to late next
week, it may not necessarily be a quiet period. At this time, there
are significant differences between ensemble suites with regards to
the progression of an upper trough digging down from British
Columbia. There is a nearly 50/50 split in cluster analysis at this
time on the track that this trough will take, which will have a
bearing on the magnitude of appreciable weather through mid next
week. Regardless of outcome, winds are expected to become northerly,
but in essence, a further eastward trajectory would result in breezy
winds at most, while a further westward trajectory would result in
gusty to potentially strong winds associated with more of an inside
slider track. With ensemble membership hedging toward the eastward
trajectory though, resultant probabilities of gusts exceeding 40 mph
have trended downward to around 15% to 30% in the Valley at this
time. Otherwise, temperature trends generally favor near to slightly
above normal temperatures throughout the extended.



VFR conditions next 24 hours except areas MVFR with showers over
the Coast Range and northern mountains after 06Z Saturday. Areas
southerly surface wind gusts 20-30 kts develop after 18Z.



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