Area Forecast Discussion
Issued by NWS Reno, NV

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000
FXUS65 KREV 191112
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
312 AM PST Sun Feb 19 2017

.SYNOPSIS...

Light snow and rain showers will linger today, then a powerful
atmospheric river storm will bring gusty winds and very heavy
precipitation with higher snow levels tonight through Monday
night. Keep flood mitigation in place or readily available. Weaker
but colder storm systems may bring snow to lower elevations at
times from Tuesday through next weekend.

&&

.SHORT TERM...

Weak shortwave passage across the Pacific Northwest has kicked up
winds early this morning with Sierra ridge gusts around 70 mph,
and some gusts 40-50 mph from north Reno to Cold Springs. These
breezy conditions will continue today, mixing down more
efficiently to most valley locations by this afternoon.

As for precipitation today, bands of light snow and rain west of
the Sierra will spread to near the crest during the day, with
limited moisture pushing very far beyond the crest. Also, areas of
light snow remain possible from far northern Lassen County to the
Oregon border including the Surprise Valley, with accumulations
generally 1 inch or less.

Then from tonight through Monday night, a strong atmospheric
river storm remains on track to make a direct hit to the Sierra,
northeast CA and most of western NV, bringing extreme amounts of
rain and higher elevation snow, similar to some of the strongest
AR systems already experienced so far this season. The progression
of this system is a little faster, with the leading edge of the
more intense precip rates arriving by early Monday morning. Anyone
planning travel in the Sierra is strongly urged to arrive at
their destinations today, as main highways are likely to become
impassible due to very heavy snow in higher elevations and heavy
rain capable of producing large mud and rock slides in lower
elevations. Expect a myriad of major weather impacts with this
storm.

HEAVY PRECIPITATION...Model guidance remains consistent with very
large amounts of precip for all of eastern CA and most of western
NV. The heaviest precip rates are projected to target the Tahoe
basin for most of this event, with only slightly less intense
precip rates southward across Alpine and Mono counties. By Monday
evening, most guidance sources lift the heaviest precip rates
northward into northeast CA, then returning south across Tahoe
again late Monday night although precip rates will finally begin
to decrease by that time. Impressive 700 mb flow will peak out
around 60-80 kts Monday-Monday night, bringing very efficient
spillover into western NV mainly from US-50 northward. Overall, we
expect to see around 3-6 inches with locally higher amounts of
precip in the Sierra and northeast CA west of Susanville, with
1.0-2.5 inches into the valleys of western Nevada, lower valleys
of northeast CA, and Mono County near US-395. Higher rainfall
amounts greater then 3 inches are expected in the foothills west
of US-395, including areas west of Reno-Carson City and around
Washoe Valley. This storm will also bring moderate to heavy
amounts of rainfall between 0.50 to 1 inch into northwest and west
central Nevada, except lesser amounts south of US-50 into Mineral
County. 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.

SNOW LEVELS/AMOUNTS...We still expect snow levels rising to
around 7000-7500 feet from Tahoe northward and 7500-8000 feet
south of Tahoe to Mono County Monday morning through Monday
evening. This will lead to increased flooding concerns in the
Sierra below 7000 feet. Snow levels could get pushed down by as
much as 500-1000 feet lower during periods of heaviest precip. All
winter related headlines remain intact as this heavy snowfall
will cause many problems. Elevations above 7500 feet can expect
2-5 feet of snowfall with highest amounts likely along the Sierra
crest west of Tahoe, which will likely have an impact on
avalanche potential as well. 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 7000 feet.
Snow amounts between 6500-7500 feet will vary widely, ranging
anywhere from about 6 inches to at least 2 feet depending on how
much rain mixes in during the peak precip rates, while most areas
below 6500 feet receive limited snowfall amounts. Snow levels
will lower by early Tuesday morning north of US-50 to near
5000-5500 feet around Lassen Plumas, and 6000-6500 feet around the
Reno-Carson-Tahoe areas.

STRONG WINDS...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.
Farther 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.
MJD

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

Primary changes to the long term forecast this morning involve
increasing snow levels slightly for early Tuesday and bumping up
QPF a little for Tuesday...then lowering QPF slightly for
Wednesday.

Overall...the operational models are a little slower with lowering
snow levels Tuesday. While this may not make a great deal of
difference to snow amounts in the Sierra and northeast
California...it could increase the flooding possibilities slightly
in the lower elevations east of the Sierra and the lower portions of
the river basins in northeast California. QPF numbers are a little
higher for early Tuesday...but then the amount of precipitation
should decrease through the day as the moisture tap becomes cut
off and slides south. The biggest increase in QPF was for Mono
County where additional snow is expected at the higher elevations.

Beyond Tuesday the models have trended a little drier Tuesday night
into Wednesday night. The main upper trough begins to slide over the
region Wednesday. This will lower snow levels to all valley floors
and could result in a few inches of accumulation in the western
Nevada and northeast California valleys. There may even be an inch
or two in the basin and range. With the slight decrease in expected
QPF for Wednesday the amount of snow for the Tahoe Basin and Mono
County for Wednesday and Wednesday night was lowered
slightly...generally less than a foot at lake level and around
Mammoth Lakes with in excess of a foot at the higher elevations.

Snow showers linger into early Thursday afternoon before weak
ridging temporarily develops and brings an end to the precipitation
for a short time. Operational models and ensemble members are still
struggling with forecast agreement beyond Thursday. Some want to
drop a long wave trough south over the region with precipitation
redeveloping Friday afternoon/evening and extending into Saturday.
Others show a slower development of a closed low dropping south
along the coast. This would be a slightly drier solution although
some snow would be possible in the Sierra and northeast California.
Whatever solution prevails we should be cold enough for mainly snow.

Given the disagreements among the model solutions beyond Thursday we
have left small pops in the forecast from Friday night through
Saturday night.

&&

.AVIATION...

Increasing winds today ahead of an approaching storm system will
lead to turbulence aloft and near the surface along with Low Level
Wind Shear possibilities.

Mainly VFR conditions east of the Sierra this morning with fog in
some Sierra valleys producing IFR/MVFR conditions give way to all
VFR into the late afternoon hours. By this evening conditions begin
to deteriorate again as moisture and lower cigs/vsbys push into the
region from the west. Snow levels rise through the night into
Monday. The heaviest precipitation is likely during the day Monday.
All rain at the terminals by then. Gusty winds remain a possibility
east of the Sierra late Monday afternoon into Monday evening when
precipitation starts to decrease.

Lower cigs and vsbys return Monday night and snow levels start to
lower. This will produce IFR conditions and accumulating snows for
the Sierra terminals. By late Tuesday snow levels are likely to the
lower valley floors with minor accumulations east of the Sierra. But
the amount of precipitation will be much lower by then.

&&

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...Lake Wind Advisory from 10 AM this morning to 10 PM PST this
     evening for Lake Tahoe in NVZ002.

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

     Flood Watch from late tonight through Tuesday morning NVZ002.

     Flood Watch from late tonight through Tuesday morning NVZ003.

     Lake Wind Advisory from 10 AM this morning to 10 PM PST this
     evening for Pyramid Lake in NVZ004.

CA...Flood Watch from late tonight through Tuesday afternoon CAZ071.

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

     Flood Watch from late tonight through Tuesday morning CAZ073.

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

     Lake Wind Advisory from 10 AM this morning to 10 PM PST this
     evening for Lake Tahoe in CAZ072.

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

     Flood Watch from late tonight through Tuesday morning CAZ072.

&&

$$

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


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