Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 170301

Area Forecast Discussion
National Weather Service Reno NV
701 PM PST Thu Feb 16 2017


Winds have diminished across Lake Tahoe and Pyramid Lake this
evening, and the Lake Wind Advisories have been allowed to expire.
Localized choppy waters will remain through the evening and
caution is advised. Lake waters will calm down overnight with
light winds anticipated.

Otherwise the forecast remains on track with multiple systems
anticipated over the weekend and into next week. Please see the
previous discussion for additional details. -Dawn


.PREVIOUS DISCUSSION... /Issued 338 PM PST Thu Feb 16 2017/


Another storm system moves into the region Friday into Saturday
with less wind but additional valley rain and Sierra snow. A much
stronger storm is still on track to bring wind and more
significant precipitation early next week. Keep flood mitigation
in place or readily available through at least next week.


Gusty winds continue this afternoon behind a storm system that
passed through the region last night. Winds will continue to
weaken into tonight, so we will go ahead and drop the wind
advisories for today. Gusty winds up to 40 mph will continue
through this evening, so lake wind advisories for Tahoe and
Pyramid will remain in place. Very little precipitation is
expected tonight, although we will start to see increasing chances
for precipitation again by early Friday morning as another storm
approaches the region.

By Friday morning, the next Pacific storm system arrives into
Central California and pushes up into the Sierra and western Nevada
through the day. The majority of the dynamics with this system
remain in Southern CA, although good moisture and broad upper
level divergence/lift will bring precip into our area Friday and
into Friday night. This storm will be moderate by Sierra standards,
with snow levels down near 5000 feet Friday morning and remaining
between 5000-6000 feet through Friday night.

We have issued Winter Weather Advisories for the Sierra as we expect
to see moderate snow accumulations Friday into early Saturday
morning. Snow amounts of 8 to 16 inches are expected in the Sierra
above 7000 feet, with as much as 2 feet possible along the Sierra
Crest in Mono County. Around 4 to 8 inches of snow are expected
around the Tahoe Basin and along the US-395 corridor through Mono
County below 7000 feet. For the Sierra north of Tahoe, lesser
amounts are expected, although 4 to 8 inches of snow can be
expected for the elevations above 5000 feet in Lassen, eastern
Plumas and eastern Sierra counties.

Around western Nevada, a mix of rain and snow is expected down to
the valley floors, although little to no accumulation is expected
below 5000 feet. Above 5000 feet in western Nevada, we could see
light accumulations up to a couple inches, including the north
valleys of Reno, Virginia City, and foothill areas. Snow amounts
will be less during the day on Friday, although once the sun goes
down Friday evening, we should see some additional accumulations
for elevations above 5500 feet.

Some light showers are expected on Saturday as the trough axis
passes over the region. Snow levels by Saturday should be near
6000 feet with light accumulations mainly for areas above 7000
feet. Hoon

LONG TERM...Sunday through Thursday...

The main change to the extended forecast was to significantly
increase the precipitation (QPF) forecast for late Sunday night
through Monday night and to raise snow levels Monday afternoon and
evening. The Monday-Monday night system has come in very wet, with
atmospheric river moisture transport (IVT) probabilities from the
GFS ensemble increasing to an almost certainty for the coast of
California, with a medium to high probability for the AR to make it
past the Sierra into western Nevada. As far as strength, it looks
moderate to strong per IVT forecasts from the GFS/GFS ensemble.

A breakdown of the various features of the Monday-Tuesday system:


With strong moisture transport with the system, liquid or liquid
equivalent precipitation late Sunday night into Tuesday morning is
projected to be between 3 and 5 inches (locally up to 6 inches) for
the northern Sierra down to near Carson Pass, with that much west of
Highway 395 in Mono County. In the Sierra Front and foothills from
eastern Lassen County down through Mono County, we are currently
expecting between 1 and 3 inches of precipitation with the highest
amounts in the foothills west of Highway 395/I-580. Finally, out in
the Basin and Range a solid 0.25" to 0.50" is expected.


As far as widespread flooding early next week, it all comes down to
snow levels. Late Sunday night and Monday morning, a period of warm
air advection is shown in the GFS simulations to bring isothermal
profiles, especially for areas north of Highway 50. This will serve
to hold down snow levels substantially, with snow possible down to
near all valley floors north of I-80. However, during the day Monday
snow levels are expected to rise significantly as the core of the
atmospheric river hits northeast CA.

Our current forecast indicates snow levels of 7000 to 8000 feet in
the Sierra down through Alpine County, and 7500-8500 feet in Mono
County Monday afternoon and evening. However, the greatest
uncertainty lies in this period as the GFS begins knocking down snow
levels significantly (to ~5000-6500 feet) Monday evening...while the
EC keeps them above 7000 feet until Tuesday morning. This
discrepancy will go a long way to determining the possibility for
flooding of small creeks and streams, areas with large snow berms,
and lower elevations river basins such as the Feather and Pit Rivers.
Stay tuned as simulations hopefully consolidate.


Current projections are for 2-4 feet of snowfall (starting as a
layer of "Sierra Cement") above 7500 feet in the Sierra. Areas
between 5000 and 6500 feet in the northern Sierra, and between 6000
and 7500 feet in Mono County, will have the most uncertainty as the
snow level will be in play. If it falls faster, substantially more
snow will be possible.


The most significant period of winds is likely to be Monday just
ahead of the atmospheric river. Once the moisture plume moves in,
rapid moistening of the atmosphere is likely to dampen winds
substantially despite 700 mb (ridgetop) sustained winds of 50-75
kts. It will still be breezy with rain in valleys, but nothing
remarkable for the region.


The pattern turns much colder with snow possible at all elevations
on Wednesday as the main trough axis moves over eastern CA and
western NV. Some simulations even bring one more, inside slider-type
system through on Thursday but there is low confidence in that
feature at this point. Snyder


A series of storms will impact the Sierra through the middle of
next week. The first storm is still creating gusty ridge wind
gusts to around 75 mph. Surface winds will continue to diminish
with gusts to 35 knots through this evening.

A few lingering showers will continue into this evening and then
the next storm arrives early Friday morning and into Saturday
morning. Snow accumulations are likely Friday through Friday night
at KTRK/KTVL/KMMH with up to 3-7 inches possible. Slushier
accumulations are likely during the afternoon on Friday. Periods
of MVFR/IFR CIG/VIS and moderate turbulence are likely as well,
although winds will be much less than this morning. Conditions
will begin to improve on Saturday with a break in the weather
before the next storm moves in Sunday night. Hoon


.REV Watches/Warnings/Advisories...
NV...Flood Watch from Monday afternoon through late Monday night

     Winter Weather Advisory from 7 AM Friday to 10 AM PST Saturday

     Flood Watch from Monday afternoon through Tuesday morning NVZ003.

CA...Flood Watch from Monday afternoon through Tuesday afternoon

     Winter Weather Advisory from 7 AM Friday to 10 AM PST Saturday
     above 5000 feet in CAZ071.

     Flood Watch from Monday afternoon through Tuesday morning CAZ073.

     Winter Weather Advisory from 4 AM Friday to 10 AM PST Saturday

     Flood Watch from Monday afternoon through late Monday night

     Winter Weather Advisory from 7 AM Friday to 10 AM PST Saturday



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