Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 212248

Area Forecast Discussion
National Weather Service Sacramento CA
348 PM PDT Wed Mar 21 2018

Wet weather continues into the weekend. Snow will remain above
pass levels through the evening, then lower with significant
accumulations at the higher elevations into the weekend.


Brief break in widespread precip early this afternoon with a few
cloud breaks adding to increased instability. Breezy south winds
have developed and will last into early Thursday with a few gusts
to 40 mph. Short term mesoscale models including HRRR are
indicating scattered thunderstorm development around 4 pm lasting
to as late as midnight. A strong shear environment is in place
with helicity values 300-400. Instability will be conditional and
dependent on cloud breaks, but Bufkit shows values 200-600 j/kg.
Dewpoints have soared into the lower 60`s over the srn Sacramento
valley with plenty of fuel for destabilization. High shear
environment could lead to rotation and a brief funnel cloud or
isolated tornado in stronger cells, as outlined in latest SPC
thunderstorm discussion. High freezing levels will likely lower
the threat of hail except in the tallest/strongest storms.

Decided to upgrade winter storm watch to warning, although snow
level forecast remains a bit uncertain. NBM/Nam both bring snow
levels down to 6800 feet around 11 pm tonight and down to 6000
feet by late Thursday afternoon. In this scenario just a 500 foot
change in snow level could drastically change snow amounts along
I80 from Soda Springs to Donner pass where current forecast is
for 1 to 2 feet of snow through Thursday afternoon, as snow is
forecast to just barely make it down to pass level.

Thunderstorms with small hail will also be possible Thursday, as
a very unstable airmass moves over NorCal. Additional valley
rainfall amounts up to 1.50 inches are possible with mountain
values 2.00-4.00 inches are possible during the next 24 hours.
This will increase the threat for minor small stream and poor
drainage area flooding.

A brief break in the wet weather looks likely Late Thursday night
and Friday morning, before a colder system brings potentially
heavier and lower snow Friday night. Exact details of this system
remain a bit uncertain, but decided to issue Winter storm watch
for mountain areas above 3000 feet of the srn Cascades and Sierra
including I80 and Highway 50. At this time the heaviest snow
should move over the Sierra after 8 pm with 1 to 2 feet possible
through Sunday morning. Timing and amounts with this system could
still change, as confidence is just moderate.


Main upper trough shifts east of the state during the day on
Sunday bringing the last of the precipitation for a while as upper
ridging builds over the eastern Pacific. Precipitation chances
will mainly be restricted to east of the central valley.
Precipitation amounts will be quite light but snow levels will be
abnormally low at around 1500 to 2500 feet. A few lingering
showers may still be possible Sunday night but building high
pressure will bring an end to any precipitation threat by Monday.
Temperatures warm by several degrees on Monday but are still
forecast to run slightly below normal. Upper level and surface
gradients indicate breezy north winds which will help to start
drying things out. Temperatures continue to warm on Tuesday as
upper ridge pushes inland with highs expected to warm to several
degrees above normal. Even more warming is expected on Wednesday
with highs throughout the valley expected to reach the mid 70s in
the northern Sacramento valley. Breezy north winds will continue
through the period between an upper low over the southwest U.S and
building high pressure over the eastern Pacific.



Mainly MVFR/IFR conditions next 24 hours TAF sites as a Pacific
storm system pushes through the region, with isolated
thunderstorms through 06z in the Valley. Mainly IFR over
mountains. Snow levels dropping from around 8000 feet this
afternoon to around 6500 feet after 06z. Wind gusts 20-30 kt
through this evening in the Valley, locally stronger in
thunderstorms. Southerly winds into Thursday with gusts 20 to 30
knots northern in the Central Valley, 40-50 knots over the higher


Winter Storm Warning from 11 PM this evening to 11 PM PDT
Thursday for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

Winter Storm Watch from Friday morning through late Saturday
night for Burney Basin / Eastern Shasta County-Shasta Lake Area
/ Northern Shasta County-West Slope Northern Sierra Nevada-
Western Plumas County/Lassen Park.


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