Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 182316
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
316 PM PST Sat Feb 18 2017

.SYNOPSIS...

Snow and rain showers linger through this weekend as weak storm
systems move across the region. A powerful atmospheric river
storm will bring gusty winds and very heavy precipitation with
higher snow levels Sunday night into Tuesday. Keep flood
mitigation in place or readily available. Colder storms with lower
elevation snow are then possible by the middle of next week.

&&

.SHORT TERM...

A strong atmospheric river storm will move into the region Sunday
through Tuesday, bringing heavy rainfall, heavy snow to the High
Sierra, and strong gusty winds to the region. Otherwise, a few
light showers remain in the forecast tonight and during the day on
Sunday. We could see a couple inches of light snow in northern
Lassen County and Surprise Valley into early Sunday morning as
snow levels remain around 4500 feet. Winds will start to increase
on Sunday afternoon as the powerful storm system moves closer to
the California coast. Wind gusts up to 45 mph may be possible
Sunday, with Sierra ridges increasing to over 70 mph.

By Sunday night, the powerful Pacific storm moves into the Sierra
and begins to spill over into western Nevada as a deep atmospheric
river (AR) noses in. Multiple impacts are expected with this
storm, including heavy rainfall, flooding, heavy snow, and strong
winds.

HEAVY PRECIPITATION...Over the last 24 hours models have continued
to show and increase in the precipitation for the Sierra and into
western Nevada, and we`ve adjusted the ongoing forecast to show
this as well. The strong AR will push into the region late Sunday
night and remain over the region through the day on Monday and
into Tuesday morning. The heaviest precipitation amounts look to
be Monday and Monday night with the best moisture transport into
the region. Impressive 700mb flow will peak out around 60-80kts
during this time and will bring very efficient spillover into the
eastern Sierra, Lake Tahoe Basin, and western Nevada. Overall, we
expect to see around 3-6 inches of precipitation in the Sierra,
with 1-2 inches of precipitation into the valleys of western
Nevada and the lower valleys of northeast CA. Higher rainfall
amounts of up to 3 inches are expected in the foothills west of
US-395, including the foothills of Reno-Carson City and around
Washoe Valley. This AR storm will bring bring moderate to heavy
amounts of rainfall into west-central Nevada of 0.50-1.0". Flood
watches remain in effect for the Sierra zones and the Western
Nevada Sierra Front, as heavy rainfall will cause flooding,
mudslides, rock slides, and increases to the mainstem rivers and
smaller creeks and steams. See our hydrology section below for
more details on flooding. Heavy precipitation will taper off
Tuesday morning, although we should see light to moderate snow in
the Sierra continuing through the day on Tuesday.

SNOW LEVELS...We will see snow levels rise to around 7000-7500
feet on Monday, which will also lead to increases flooding
concerns in the Sierra below 7000 feet. Snow levels could get
pushed down by as much as 500-1000 feet lower during the heaviest
precipitation. We have issued Winter Storm Warnings for the Tahoe
Basin and Mono County as this heavy snowfall will cause many
problems. Elevations above 7000 feet will see significant
snowfall during this period, with 3-5 feet of snow especially
along the Sierra crest. This heavy snow in the Sierra will likely
have an impact on avalanche potential as well. For Mono County,
heavy snow of up to 2-4 feet can be expected for elevations above
7000 feet. Finding places to put all of this excessive snow will
continue to be a challenge especially around Mammoth Lakes and
other locations in the Sierra above 6500-7000 feet. Snow levels
will lower Tuesday morning to near 4500-5000 feet around Lassen
Plumas, with snow levels around 5500-6000 feet around the Reno-
Carson-Tahoe areas.

STRONG WINDS...Gusty winds up to 45 mph can be expected Sunday as
the storm approaches the area. As the stronger winds aloft push
in by Monday, we can expect to see strong winds gusting around
55-60 mph to the valleys, especially for areas south of Highway
50 in Mono, Mineral and Southern Lyon Counties. As for the Sierra
ridges, we could see extreme gusts over 140 mph during the main
storm on Monday. Further north around the Reno-Carson-Tahoe areas,
we could also see strong winds on Monday, although the heavy
rainfall will likely dampen the winds down slightly with gusts up
to 45 mph. Hoon

.LONG TERM...Tuesday Night through Saturday...

More moisture is poised to pushed through the region Tuesday night.
However, this wave is expected to be much faster and colder than
Monday`s drenching. Lower snow levels will aide in shutting the bulk
of runoff into area rivers, streams, and creeks by Wednesday
morning. Still, expect ongoing flooding concerns to continue into
mid/late week as water moves down basin. During periods of weaker
precipitation, expect winds to increase; staying gusty Tuesday
evening and again Wednesday ahead and along a frontal boundary.

Additional snow amounts through Wednesday evening in the Sierra will
likely total around 8-12 inches around Lake Tahoe level with another
12-20 inches for the higher Sierra. Since instability is
substantial, there will be some spill over into the Sierra Front and
Basin and Range. While the Reno Airport area will most likely see
token accumulations of around an inch, 1-4 inches will be more
towards the average for other Sierra Front locations: Reno west of
395, Washoe Valley, Carson City, Douglas County. Up to 6 inches will
be possible in the foothills and around Virginia City. It also
wouldn`t be too surprising to accumulate an inch or two out in the
Basin and Range as well.

Snow tapers off by Thursday with weak, transient ridging over the
region. Expect another wave later in the week. While uncertainty on
timing remains, Friday for GFS and Saturday for the EC, this low
appears to be a fast-moving, colder wave with a north-to-south
trajectory. Models show more of a westward dig keeping most of the
forcing west of the Sierra. This trajectory favors shadowed
conditions for western Nevada and lower snow accumulations for the
Sierra. Precipitation chances were increased to around 30% for
Sierra and northeastern California locations. Boyd

&&

.AVIATION...

Relative light winds and VFR conditions will persist this evening.

Conditions begin to deteriorate again Sunday ahead of another storm
system. Gusty winds begin to develop by late in the day. Sunday
night into Monday precipitation develops and becomes widespread with
lowering cigs/vsbys. Snow levels increase through Monday afternoon
before falling again. This will bring snow accumulation to the
Sierra valleys and KTRK/KTVL/KMMH by late Monday.

Aside from cigs/vis concerns, expect gusty winds as well Sunday
afternoon through Wednesday. There will be periods of lower surface
winds, but expect winds aloft to remain strong; turbulence and
periods of LLWS through Wednesday can also be expected. Generally,
gusts will be 30-45 kts with the strongest winds occurring Monday.
Boyd

.HYDROLOGY...

* Moderate flooding expected for the Middle Fork of the Feather
  River near Portola.
* Truckee River at Reno and Vista along with the Carson River near
  Carson City forecast to be near monitor stage.
* Flooding possible for creeks, streams, urban areas and drainage
  basins in the Tahoe Basin, eastern Sierra, northeast California
  and western Nevada early Monday through early Tuesday.

A strong atmospheric river will push into the region late Sunday
night through Tuesday morning bringing heavy rain and rising snow
levels on Monday. Currently, snow levels are expected to be around
6500 feet in northeast California and around 7500 feet for much
of the Sierra and western Nevada. Precipitation totals in the
Sierra could reach 3 to 6 inches, with 2 to 3 inches in the
Carson Range and much of the Susan, Pit, and Middle Fork of the
Feather river basins. 1 to 2 inches of rain is likely through much
of the Greater Reno/Sparks/Carson City metro areas and along the
Highway 395 corridor into Mono County.

Unprecedented amounts of rain and snow have already fallen this
winter and the ground is at record saturation levels. It will not
take much to bring renewed flood concerns throughout the Sierra,
northeast California, and in western Nevada. Around the Tahoe Basin,
the eastern Sierra, and western Nevada, main threat for flooding
will be along creeks, streams, and drainage basins/farmland where
water collects from creeks running out of the mountains. Flooding is
also possible through both the Carson Valley and Washoe Valley along
with Hardscrabble Creek in the Virginia Range. Urban flooding is
probable in the Tahoe Basin as snow berms won`t allow for water to
properly drain from the region. The other concern will be the
increased potential for rock and mudslides in areas of steep
terrain, with road closures possible.

The river basins to watch at this time will be the Middle Fork of
the Feather, the Susan, and the Pit in northeast California. Snow
levels may be just low enough for keep the Susan and Pit rivers from
flooding, but with copious amounts of additional rainfall expected,
these will need to be closely monitored. The Middle Fork of the
Feather is currently forecast to reach moderate flood stage, with
only an additional 500 cfs/0.3 feet needed to reach major flood
stage.

Snow levels below 8000 feet usually prevent flooding along the
Carson, Truckee and Walker Rivers. However with significant runoff
anticipated, the drainages which flow into the mainstem rivers may
overcome the slightly lower snow levels. This includes, but is not
limited to, Steamboat Creek and the North Truckee Drainage. The
current forecast takes the Carson River near Carson City and the
Truckee River at Reno and Vista to near monitor stage. Small changes
to precipitation totals and/or snow levels may result in minor
flooding.

Snow levels are anticipated to fall by Tuesday morning or afternoon,
which also coincides with weakening moisture transport. This will
help lessen new flood risks, but it may take a day or two for
ongoing flood waters to subside. -Dawn

&&

.REV Watches/Warnings/Advisories...
NV...Flood Watch from late Sunday night through Tuesday morning
     NVZ002.

     Winter Storm Warning from 10 PM Sunday to 4 AM PST Tuesday above
     6500 feet in NVZ002.

     Flood Watch from late Sunday night through Tuesday morning
     NVZ003.

CA...Flood Watch from late Sunday night through Tuesday afternoon
     CAZ071.

     Winter Weather Advisory from 10 PM Sunday to 4 AM PST Tuesday
     above 5500 feet in CAZ071.

     Flood Watch from late Sunday night through Tuesday morning
     CAZ073.

     Winter Storm Warning from 10 PM Sunday to 4 AM PST Tuesday above
     7000 feet in CAZ073.

     Flood Watch from late Sunday night through Tuesday morning
     CAZ072.

     Winter Storm Warning from 10 PM Sunday to 4 AM PST Tuesday above
     6500 feet in CAZ072.

&&

$$

For more information from the National Weather Service visit...
http://weather.gov/reno



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