Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 211548 AAA
AFDREV

Area Forecast Discussion...UPDATED
National Weather Service Reno NV
748 AM PST Tue Feb 21 2017

.UPDATE...
A band of heavy snow is affecting the Reno/Sparks area with rates
up to 2 inches per hour. These rates will result in slushy roads
even on valley floors with more snow packed roads above 5000 feet.
Issued a winter weather advisory through noon for the rest of the
messy commute. The band is esxpected to weaken close to noon.
Further south, lighter bands are expected with up to 2 inches.
In addition, winds are much weaker due to the heavy spillover so
canceled the wind advisory for the Sierra Front. Wallmann


&&

.SYNOPSIS...

A powerful storm will continue to bring snow and windy conditions
through tonight, with snow levels lowering to most valleys by this
afternoon or evening, and light accumulations possible tonight.
Lighter snow showers may linger through Wednesday night, mainly
north of I-80. Cold conditions will continue into the weekend, when
another storm could bring widespread snow even to some valleys.


&&

.PREVIOUS DISCUSSION... /Issued 342 AM PST Tue Feb 21 2017/

SHORT TERM...

The powerful storm has brought some of the most extreme Sierra
ridge winds seen in many years, with peak gusts of 199 and 193 mph
at Alpine and Squaw Summits respectively around 11 pm Monday
night. The highest sustained wind was 148 mph at Alpine. Currently
the winds are "only" gusting between 130-140 mph.

Winds in the advisory and warning areas have been erratic--an
isolated gust of 71 mph occurred at Reno airport shortly after 1
am near the edge of a convective shower band. Considering the
strong ridge winds still occurring and the banded nature of the
precip pushing over the Sierra, we will keep the wind headlines up
from I-80 southward. Farther north in the cooler air mass, winds
have not been as strong and with the cold front continuing to push
south, we will drop the advisory for northern Washoe and Surprise
valley although these areas could still receive occasional gusts
above 40 mph.

Precipitation wise, most areas in the Sierra are now snowing,
while in lower elevations the rainfall rates have decreased
overnight. For additional information on flooding concerns, see
the Hydrology segment below.

Snow will continue to spread across the winter-related warning
and advisory areas not only through today, but into tonight as
well. For northeast CA, a secondary shortwave moving inland across
the northwest portion of the state will provide forcing to keep
bands of light to moderate snow going even into Wednesday morning.
The snowfall rates will not be as intense compared to yesterday
(except locally in heavier convective bands), but should still be
sufficient to produce around a foot of additional snowfall
especially above 7000 feet near Tahoe, and several more inches in
northeast CA, at lake level around Tahoe, and in western Mono
County. Therefore we will extend the winter headlines through
tonight from Tahoe to Mono Counties, and through Wednesday morning
for Lassen-Plumas counties. For lower elevations of eastern CA-
western NV, latest model guidance keeps steady precip going for
most of the day along US-395, with areas of lighter rain and snow
extending into west central NV mainly north of US-50. For most
lower valleys around Reno-Carson-Susanville, a rain-snow mix is
anticipated which will limit snow accumulations, while for
foothill locations above 5000 feet and around Virginia City a few
inches of snow may accumulate mainly on unpaved surfaces through
this evening. A more significant hazard may be wet pavement
freezing overnight as temperatures drop further, with a light
coating of snow covering the ice. If conditions favor formation of
black ice across the urban areas tonight through Wednesday
morning based on the precip activity today, an advisory may be
needed.

Otherwise from Wednesday-Thursday, the coldest air we`ve seen this
month will spread across the region, with highs mainly in the 30s
to lower 40s in lower elevations and below 30 near the Sierra.
Weak disturbances within the cold west-northwest flow aloft could
keep the risk of light snow showers going for both days especially
north of I-80, around the Tahoe basin, and in eastern portions of
Pershing and Churchill counties. MJD

LONG TERM...Friday through Monday...

Significant changes were made to the extended forecast this period
as models are now coming into better alignment with a cold system
over the weekend into the early part of next week. QPF and Pops were
increased Sunday through Monday...but there is likely more room to
increase these as we start getting closer to the event.

Friday looks mainly dry with a weak flat ridge aloft...but this
gives way to an elongated trough dropping south across the region by
early Saturday. ECMWF has now trended toward the previous and
current GFS solutions of developing a closed low offshore
Saturday, then bringing this low into southern California Sunday.
The models are also showing a secondary wave/low that drops in
behind the first one on Monday.

This one-two punch has enough over water trajectory to tap into a
moisture plume by late Sunday and bring higher amounts of integrated
water vapor into the region through Monday. The highest
precipitation in the current set up would be from Interstate 80
southward, and along and west of Highway 395.

The atmosphere is generally cold enough for all snow throughout this
event...but there are some signs that snow levels could briefly rise
Sunday afternoon to around 5000 feet. Overall...initial indications
are between 1 and 2 feet of snow in the Sierra above 7500 feet with
up to a foot below that level. In northeast California and far
western Nevada the valleys could see 2 to 6 inches of snow by late
Monday.

This forecast is likely to change in the details. We could see more
precipitation if the system lingers over the region...and less if
the first low tracks farther west.

AVIATION...

Bands of precipitation continue today. A cold front should slide
through the region later this afternoon and drop snow levels to all
valley floors by the evening. Additional snow accumulations of 3 to
8 inches are possible at the Sierra terminals while an inch or
two is possible for places like KSVE/KRNO/KCXP/KMEV by early
Wednesday. MVFR/IFR conditions persist for much of the day in the
higher terrain along with obscured mountain tops. In the Nevada
and northeast California valleys...VFR conditions early today
should give way to periodic IFR/MVFR conditions due to showers.
More widespread precipitation in the evening is likely to produce
a better chance of MVFR conditions with intermittent IFR. Showers
persist into early Wednesday but become less widespread with
higher cigs/vsbys.

The other story continues to be strong winds aloft and occasional
gusty winds at the surface. Very strong winds over the mountain
ridges are likely to produce continued turbulence through the day.
The decrease in precipitation at the surface is allowing a
resurgence of gusty winds in the valleys east of the Sierra. Low
Level Wind Shear is less of a possibility this morning as gusty winds
should line up direction-wise with the winds off the
surface...except for KTVL...but there will still be turbulence even
into the low levels. The winds will begin to decrease later today
and this evening as colder air begins to settle over the region and
upper level winds begin to decrease.

HYDROLOGY...

* Flooding likely for creeks, streams, urban areas and drainage
  basins in eastern Sierra, northeast California and western Nevada
  through late this morning.
* Minor flooding of the Pit River basin in northwest Lassen County
  possible later today and tonight.
* Moderate flooding possible for the Middle Fork Feather River near
  Portola.

Widespread heavy precipitation has mostly ended and we are
transitioning to more showery type precipitation...but this will
persist through much of the day into tonight.

Snow levels remain in the 5500-6000 foot range for northeast
California...6000-6500 foot range for the Tahoe area...and around
6500-7000 feet for Mono County. Snow levels may be a bit higher east
into western Nevada. These lower snow levels have caused less rises
on some of the smaller streams and river systems than were initially
feared...but some areas are still seeing near bank full or slightly
over bank flow as run off continues from yesterday`s rain.

Additional rain below the snow level today will aggravate flooding
concerns in the lower valleys of northeast California and western
Nevada with some areas seeing minor flooding of poor drainage areas
and low water crossings from Susanville south to Carson City and the
typical flashy small streams in the southern part of the Truckee
Meadows through Washoe Valley into the Carson City area and the
Carson Valley. Areal Flood Warning continue through these areas.

The Middle Fork Feather River is still forecast to rise above flood
stage later today...but the lower snow levels are likely to keep it
from reaching major flood stage. The Pit River through northwest
Lassen County may exceed its banks later this evening...but only
minor flooding near the river is expected.

We will cancel flood watches for the area this morning as the areas
of greater concern for flooding have flood warnings in effect now.

One last note. The Humboldt river in northeast Pershing County
should continue to slowly rise through the week. The river could
reach flood stage near Imlay by late Friday or early Saturday. This
will continue to bear watching as we move through the week.

&&

.REV Watches/Warnings/Advisories...
NV...Winter Storm Warning until 4 AM PST Wednesday NVZ002.

     High Wind Warning until 4 PM PST this afternoon NVZ001.

     Winter Weather Advisory until noon PST today NVZ003.

     Wind Advisory until 4 PM PST this afternoon NVZ004.

CA...Winter Weather Advisory until 10 AM PST Wednesday CAZ071.

     Winter Storm Warning until 4 AM PST Wednesday CAZ073.

     Winter Storm Warning until 4 AM PST Wednesday CAZ072.

&&

$$

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


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