NATIONAL WEATHER SERVICE FORECAST DISCUSSION
Area Forecast Discussion
Issued by The National Weather Service
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000
FXUS66 KSTO 160647
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1045 PM PST Fri Feb 15 2019

.SYNOPSIS...

A strong storm continues to bring precipitation to the region
through the weekend. Heavy snow will continue in the mountains with
minor accumulations possible in the foothills through early Sunday.
Additional rainfall expected in the Valley today with chances
decreasing through the weekend. Next week begins dry before
returning precipitation chances by Wednesday.

&&


.DISCUSSION...
Evening Update: Multiple short waves will be working through the
upper trof over Norcal tonight and Sat bringing scattered showers to
the Valley and widespread/numerous showers to the mountains at
times. The WNWly flow aloft will result in rain shadowing on the lee-
side of the coastal range (in the Valley) so amounts will likely be
lower compared to the W slope Sierra where upslope flow will
continue through the weekend.

Forecast soundings indicate a mid level cap around 12 kft so more
low-topped showers are expected, but shear parameters appear weaker
than previous days. Thus, thunder may be a little more problematic
than last couple of days.

Lowering snow levels through the weekend will likely bring snow down
into the foothills and even the Nrn Sac Vly Sat nite/Sun morning
although QPF will be much, much lower than previous storms as the
subtropical air mass has been shunted well Swd. This shows up in the
TPWs forecast over the Bufkit sites of only a few tenths of an inch.
   JHM

.Previous Discussion...
Afternoon satellite imagery shows the axis of the upper level low
pressure system finally making landfall over the West Coast. The
upper level low will be slow to move east this weekend with winter
weather continuing through early Sunday.

The main hazard this weekend will be the snowfall impacting the
Sierra-Southern Cascades and Shasta County mountains. Heavy snow
has fallen all day, and Interstate 80 has remained closed since
last evening. Snow levels are around 2000 to 3000 feet today and
Saturday with an additional 2 to 4 feet expected through early
Sunday morning, with local amounts up to 5 feet possible.
Additionally, areas around Interstate 5 in Shasta County may see
up to a foot of snow, mainly through Saturday morning, creating
hazardous travel conditions, although additional light amounts are
still possible through the day Saturday. A shortwave will move
across NorCal Saturday night into Sunday bringing with it colder
air and lower snow levels. Models continue trending colder with
this impulse with snow levels dropping for a brief period of time
early Sunday morning to 650 to 1500 feet, before rising to
1500 to 2000 feet by late morning. However, precipitation during
this time will be diminishing and mostly scattered, so low
elevation snow below 1500 feet will be hit or miss with only light
accumulation generally under an inch expected. At high
elevations, only 2 to 4 inches of additional snow is expected
Sunday, mainly tapering off by the evening.

For the Valley and foothills, scattered showers are expected
tonight through Saturday night with periods of dry weather
possible. Additional amounts up to half an inch are possible in
the Valley, with half an inch to 2 inches in the foothills.

Dry weather and sunny skies are in store for early next week.

.EXTENDED DISCUSSION (Tuesday THROUGH Friday)

An upper ridge generally centered over 150-140W longitude will be
a fixture in the forecast across the eastern Pacific. Systems
traversing the Gulf of Alaska will then dive southward with
potential influences to the West Coast. The 18Z GFS trended deeper
with an upper low paralleling the coast on Tuesday morning.
However, at this time, any appreciable impacts should remain well
west of the region. 1000-500 mb thicknesses are quite low (530-535
dm) at this point which supports slightly below normal
temperatures. It will prove to be quite chilly Tuesday morning
with optimal radiational cooling effects in place. Valley lows
will likely sit near the freezing mark with low 20s in the
foothills and readings near 10 degrees in the mountains.

By Wednesday, cloud cover will increase in advance of the next
upper low sweeping down from far western British Columbia. The 18Z
GFS/12Z ECMWF agree on a sub-530 dm mid-level low crossing
northern California late Wednesday. This particular track would
not have access to appreciable Pacific moisture. However, the cool
air aloft will support an uptick in shower activity Wednesday
afternoon into the overnight hours. It is a rather quick moving
upper low so conditions are forecast to clear out from north to
south on Thursday morning. Snow levels will again be low with this
system, potentially below 2,000 feet which would support
accumulations into the lower foothills.

Beyond this period, conditions dry out for the remainder of the
work week although the guidance show another system potentially
impacting the region next weekend. Models are not the most
consistent with this feature so confidence is on the lower end at
this time. BRO

&&

.AVIATION...

Ceilings have remained at VFR across all TAF sites although
reduced visibilities within passing showers and thunderstorms may
lower flight rules to MVFR/IFR. IFR to LIFR conditions are more
likely across mountain locations given continued moderate/heavy
snow. Scattered rain showers to continue over the Valley into
Saturday with south winds gradually decreasing in strength by this
evening, except in the vicinity of any thunderstorms.  BRO
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 4 AM PST Sunday for West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Winter Storm Warning until 4 AM PST Saturday for Burney Basin /
Eastern Shasta County-Shasta Lake Area / Northern Shasta County.

&&

$$
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