Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 201630 AAA
AFDREV

Area Forecast Discussion...UPDATED
National Weather Service Reno NV
830 AM PST Mon Feb 20 2017

.UPDATE...
Updating for a couple things this morning. First, snow levels are
around 6500-7000 feet from Tahoe to Mono this morning so lowered
them through mid afternoon. The heavy precip already occurring
will keep them there for the next few hours before the main warm
air arrives this afternoon. Still, with heavy precip already
occurring, they may not want to come up much at all. Given this,
snow amounts may be much higher in the 7000-7500 foot range.

Second, heavy precip is already spilling over into the Sierra
Front of Western NV. Rainfall rates of around 0.33"/hr are
already occurring around the Tahoe Basin and near 0.2"/hr for the
Sierra Front with some places seeing over 1/4" per hour. The HRRR
also shows only minimal let up until the heaviest precip arrives
this afternoon. Mainstem rivers are already responding as well as
the smaller creeks. If this continues, and we have another 12-15
hours of heavy precip to go, the mainstem Truckee and Susan Rivers
may flood, although it would probably be minor. However, the
smaller creeks and streams such as Steamboat, N Truckee Drain, Ash
Canyon, etc. would have significant issues. We will continue to
monitor and additional flood and/or flash flood warnings may be
needed in the next few hours. Wallmann

&&

.SYNOPSIS...

A powerful atmospheric river storm will bring strong winds and  very
heavy precipitation with higher snow levels today through  Tuesday.
Keep flood mitigation in place through or readily  available. Weaker
but colder storm systems may bring snow to lower elevations at times
from Tuesday night through next weekend.


&&

.PREVIOUS DISCUSSION... /Issued 327 AM PST Mon Feb 20 2017/

SYNOPSIS...

A powerful atmospheric river storm will bring strong winds and
very heavy precipitation with higher snow levels today through
Tuesday. Keep flood mitigation in place through or readily
available. Weaker but colder storm systems may bring snow to lower
elevations at times from Tuesday night through next weekend.

SHORT TERM...

And so it begins...satellite and radar loops are showing rapid
increase in precipitation across northern and central CA, with a
mix of rain and snow from Bogard southward to the Tahoe basin and
light snow at Mammoth airport. Surface temps in these areas are
near or slightly above freezing. Radar estimated snow levels vary
from 6000 to 8000 feet along the western Sierra foothills from
Chico to Colfax, further complicating the snow level picture. As
precip becomes more consolidated, we should see snow levels become
more uniform, averaging near 6500 feet for northeast CA, 7000-7500
feet around Tahoe, and 7500-8000 feet for Alpine-Mono counties.
However, the varying intensity of precip will battle with a milder
subtropical air mass today through this evening, resulting in snow
levels fluctuating by a few hundred feet on either side of these
average projections. Even with the bulk of this storm occurring in
the next 24 hours, model guidance sources still disagree on which
portions of eastern CA-western NV will receive the heaviest
precip, and how high or low the snow levels will set up.

The general idea of our forecast spreads precip efficiently across
the Sierra and into most of western NV this morning, with the
maximum moisture feed extending from northern Mono County to near
I-80. extending east across the the US-50 corridor. By this
afternoon, the heaviest precip rates lift northward, targeting
Tahoe and Reno more directly, then an even more intense band
targets Plumas and Lassen counties (but rain and snow should
still be ongoing at a substantial rate around Reno-Tahoe and most
of the Sierra) by this evening. The most intense precip rates then
settle down later tonight into Tuesday, but areas of moderate to
heavy rain and snow will continue in the Sierra and northeast CA,
with bands of steady light-moderate precip spreading into lower
elevations at times. Colder air will bring snow levels steadily
downward during the day Tuesday, with snow level falling further
to many valley locations Tuesday night, although precip will also
be diminishing to scattered showers by this time. The projected
totals and various impacts of this storm are highlighted below:

HEAVY RAIN...Rainfall amounts of up to 3-6 inches are likely in
the Sierra through Tuesday morning, with 1-2 inches in the valleys
of northeast CA and western NV and 2-3 inches in the foothills
west of Highway 395. For northwest and west central NV, between
0.50 and 1 inch is probable north of US-50 with less than 0.50
inch south of US-50. Heavy rain is expected to bring widespread
flooding concerns to the region--including small stream flooding,
urban flooding, mud and rock slides, as well as increases to
mainstem rivers. See our hydrology section below for more details
on flooding.

SNOW...Elevations above 7500 feet will get very heavy snowfall,
around 2-5 feet through Tuesday afternoon, with the highest
amounts along the Sierra crest west of Tahoe. An avalanche watch
is in effect from the Sierra Avalanche Center. Finding a place to
put all the excessive snow will be a major challenge for Sierra
communities above 7000 feet, including around Tahoe and Mammoth
Lakes. Snow amounts between 6500-7500 feet will be more variable
with roughly 1-3 feet likely at that elevation. Below 6500 feet,
more limited snow amounts are likely, but not until snow levels
begin to drop late tonight north of I-80 and Tuesday morning
elsewhere. Final totals at lake level around Tahoe and northward
across Truckee and the Sierra Valley above 5000 feet could still
end up in the 3-8 inch range by early Wednesday morning. Snow
amounts in western NV look more limited with the latest guidance
data, with less than 1 inch in most valleys and maybe up to 3
inches in foothill locations around Reno-Carson and Virginia City.

STRONG WINDS...Impressive 700 mb flow will move into the Sierra
today and peak near 80 kt by tonight, bringing strong winds
across the region. Widespread gusts 45-55 mph are expected for
valley locations, which is actually limited due to steady periods
of heavier precipitation. All wind advisories and warnings remain
intact, with the strongest wind potential for Mineral-Southern
Lyon Counties where more shadowing is expected. If the moisture
feed lifts farther to the north this evening, some of these
stronger winds could surface across the Reno-Carson vicinity and
parts of west central NV. In the Sierra, the passes and ridges
will see extreme winds today through Tuesday with gusts to 150 mph
possible at times tonight. High winds and heavy snowfall in the
high elevations will create whiteout conditions. MJD

LONG TERM...Wednesday through Sunday...

The primary changes made to the extended forecast this cycle were to
continue the trend of lowering pops and QPF for Wednesday into early
Thursday.

Operational models and most GEFS ensemble members are in good
agreement early in the forecast period...sliding the main upper
level trough over the region late Wednesday then east of the area on
Thursday. This will help usher much colder temperatures into the
region with snow levels down to even the lowest valley floors.
Precipitation becomes mainly showery by late Wednesday with less
coverage than previous days and far less QPF as the main moisture
tap has shifted far to the south and east. Still a few inches of
snow are possible in the Sierra Wednesday/Wednesday night with an
inch or so possible in the lower valleys of northeast California and
western Nevada.

By late Thursday/early Friday a weak flat ridge tries to develop
with drying over the region. H700 temps by early Friday dip into the
-12 to -16 C range from south to north across the CWA. This would
mean lows in the teens and 20s with some single digits in the
colder valleys. Highs Thursday through Saturday will struggle to
get out of the 30s in the western Nevada and northeast California
valleys and reach only the lower 40s in the basin and range
country. In the Sierra highs may not reach 30.

Model agreement breaks down by Saturday...continuing a trend from
the previous couple of days. The GFS is developing a closed low in
an east/west oriented trough that drops into central/southern
California by early Sunday. It is farther east than previous runs
showed. This trajectory would favor higher precipitation amounts in
the Sierra Saturday/Saturday night. The ECMWF has more of an open
trough with an overland trajectory. This would be a faster system
with less precipitation. The GEFS ensembles show support for both
solutions. Given the disparity between the model solutions we have
opted to maintain slight chance to chance pops Saturday into
Saturday night. Models show a few lingering showers early Sunday
then drying.

AVIATION...

Extreme winds aloft including at near surface levels will create
turbulence and strong low level wind shear today through tonight.
Southerly peak gusts 30 to 45 kts are expected at TAF sites with a
chance for erratic gusts over 60 kts.

A strong atmospheric river will bring heavy precipitation to the
area today through Tuesday. Expect moderate to heavy rain to create
MVFR cigs and vsbys at all TAF sites today through 12z Tuesday. Rain
is forecast to then transition to snow at Sierra terminals after 12z
Tuesday leading to IFR conditions and accumulating snows at those
elevations. By late Tuesday snow levels are likely to fall to the
lower valley floors with minor accumulations possible east of the
Sierra. JCM

HYDROLOGY...

* Major flooding now 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 and Susan River at Susanville forecast to reach
  monitor stage.
* Flooding likely 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 today through
through Tuesday morning bringing heavy rain and rising snow levels.
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 are likely through much of the Greater
Reno/Sparks/Carson City metro areas and along the Highway 395
corridor into Mono County.

Due to very wet antecedent conditions we will not need much
precipitation to bring renewed flood concerns throughout the Sierra,
northeast California, and western Nevada. Around the Tahoe Basin,
the eastern Sierra, and western Nevada, the main flood threat will
be along creeks, streams, poor drainage areas, and ranch/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 possible in the Tahoe Basin as snow berms may not
allow runoff to properly drain. 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 now forecast 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 smaller streams and creeks that drain into the
mainstem rivers may overcome the slightly lower snow levels. This
includes, but is not limited to, Steamboat Creek and the North
Truckee Drain, White`s and Dry Creeks, Eagle Valley Creek, and small
drainages in the foothills west of Carson City.

The current forecast takes the Carson River near Carson City and the
Truckee River at Reno and Vista to monitor stage. Small changes to
precipitation totals and/or snow levels may result in minor flooding.

Snow levels are critical to flood potential. The latest model
simulations show a wider spread in solutions with regard to snow
levels and positioning of the heaviest precipitation. If snow levels
stay a bit higher than forecast in the Susan, Carson and Truckee
basins this afternoon and evening, the Susan River could reach flood
stage along with the Truckee River at Vista. The Carson River would
be just below flood stage. Again, these snow levels are critical and
we may not know how this event will evolve until much later today.
The river forecasts will be updated more frequently than usual
through Tuesday to try to incorporate the most recent changes to
snow levels and precipitation amounts.

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.

&&

.REV Watches/Warnings/Advisories...
NV...Wind Advisory until 4 PM PST Tuesday NVZ005.

     Flood Watch through Tuesday morning NVZ002.

     Winter Storm Warning until 4 PM PST Tuesday NVZ002.

     High Wind Warning until 4 PM PST Tuesday NVZ001.

     Flood Watch through Tuesday morning NVZ003.

     Wind Advisory until 4 PM PST Tuesday NVZ003-004.

CA...Flood Watch through Tuesday afternoon CAZ071.

     Winter Weather Advisory until 4 PM PST Tuesday CAZ071.

     Wind Advisory until 4 PM PST Tuesday CAZ070.

     Flood Watch through Tuesday morning CAZ073.

     Winter Storm Warning until 4 PM PST Tuesday CAZ073.

     Flood Watch through Tuesday morning CAZ072.

     Winter Storm Warning until 4 PM PST Tuesday CAZ072.

&&

$$

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


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