High Wind in Nevada, February 16, 2017

Area Forecast Discussion
National Weather Service Reno NV
345 PM PST Wed Feb 15 2017

.SYNOPSIS...

Thursday will feature a windy day with mountain snow and lower
valley rain. Friday into Saturday brings another system with less
wind but additional rain and higher elevation snow. There will be
a brief break before a much stronger storm brings wind and more
significant precipitation early next week. Keep flood mitigation
in place or readily available through at least the next week.

&&

.SHORT TERM...

Main changes to the short term forecast were to put wind
advisories and winter weather products in place for Thursday for
eastern CA and portions of western NV. Thursday's system is
generally on track to bring strong, gusty winds to the Sierra
Front and a fairly short, but possibly heavy burst of higher
elevation snow to the Sierra and northeast California, especially
from Alpine County northward.

Winds are increasing over the Sierra this afternoon as low pressure
approaches from the eastern Pacific, with winds gusting over 50 mph
for exposed areas along the crest. By early Thursday morning, winds
over Sierra ridges are expected to be between 100-120 mph as the
incoming system and its atmospheric river tighten the thermal gradient
aloft. By between 4 AM and 7 AM, model cross sections show stronger
winds beginning to duct down towards the surface in the lee of the
Sierra.

By late morning along the Highway 395/I-580 corridor from Susanville
to Minden, moisture is simulated to increase substantially as the
atmospheric river reaches the area. This should decrease the
chance for strong downsloping, although it will remain breezy
through the day. Farther south along Highway 395 in extreme
southern Douglas and through Mono Counties, the moisture increase
will be slower and more tenuous as the PWAT feed weakens and winds
are likely to stay strong well into the afternoon. As far as
speeds in the immediate lee of the Sierra Thursday, still thinking
most areas will see winds gusting to 45-55 mph during the peak of
the event as moisture increases during the day Thursday (to limit
the strongest downsloping). However, the foothills and wind prone
areas along and west of Highway 395/I-580 should gust higher, with
winds as high as 60-70 mph. Wind advisories are out for the
Sierra Front from Reno-Sparks south through Mono County. While
Tahoe, eastern Sierra and Plumas, and Lassen County will
experience strong winds ahead of the precipitation, the expected
winds there are covered with winter statements.

Turning to snow Thursday, no big changes to thinking with the highest
accumulations for snow above 6000 to 7000 feet from Lassen to Mono
County, respectively. The event will be in the form of a fairly
quick burst of heavier precipitation as the upper wave works on
the atmospheric river. The heavier burst is shown in simulations
to bring down snow levels via wet-bulbing, so the forecast has
trended towards the lower end of snow level guidance, especially
from Tahoe north. In northeast CA, expect snow impacts to last
into early afternoon for Fredonyer Pass and Highway 44 west of
Susanville. Farther south, northern Sierra passes are expected to
experience travel issues due to snowfall. It is still a bit
uncertain below 6500 feet in the Tahoe area where a mix of rain
and snow should turn over to snow in the afternoon. However,
impacts could be more limited as roads will begin to heat up as
precipitation turns to all snow, especially as precipitation
should be winding down in intensity by the afternoon.

Thursday night will bring a brief break before the next wave
approaches Friday. That wave will bring the strongest impacts
south of the region as it dives into southern CA. However, expect
additional snowfall for the Sierra (highest south of Tahoe) with
bands of scattered to numerous showers spreading over northeast
CA and western NV as divergent flow aloft and no downslope (south-
southeast flow aloft) allow for better precipitation coverage in
the lee of the Sierra. A winter weather advisory may be needed
for Mono County on Friday into Saturday. -Snyder

.LONG TERM...Saturday through Wednesday...

The strongest system in this series of storms is expected to impact
the region late Sunday night through Monday. The main forecast
changes today were to continue to increase QPF, raise snow levels,
and increase winds.

There will be a decent break in the weather this weekend before the
next storm arrives. Saturday could see some light to moderate snow
showers, mainly along the Sierra crest, with lighter showers
possible on Sunday.

**Sunday/Monday Precipitation**

Confidence continues to increase that the Sunday/Monday storm is
going to be the biggest precipitation producer of the week. However,
there is increasing uncertainty on how fast snow levels will fall.
Earlier model simulations had a very robust cold front driving snow
levels down quickly as the system moved into the Sierra. More recent
model runs have a secondary wave rounding the back of the trough
forcing it to dig in slightly farther off shore. This delays the
arrival of the coldest airmass until Wednesday.

Our current snow level forecast is in the 6500-7000 foot range
during the heaviest precipitation late Sunday night into Monday and
dropping below 6000 feet late Monday night near the end of the main
precipitation event. I will repeat this: the snow level forecast
remains highly uncertain at this point and this part of the forecast
could change by 1000 feet or more.

The current hydrologic situation is already primed for flooding and
moderate amounts of precipitation have shown quick responses on area
rivers, and especially smaller creeks and streams. Consider that
creeks, streams, and rivers are already running with high base flows
and there are 2 light to moderate storms anticipated before the
Sunday/Monday event. In addition, the very deep snowpack in the
Truckee/Tahoe and Mammoth Lakes areas have shown how quickly water
drainage areas can dam up causing significant urban flooding. Not to
mention all the rock and mudslides that have continued to impact
area roads, even the last few days when it wasn't raining. All the
areas that have seen flooding during the past few weeks: including
the Susan, Pit, and West Fork of the Feather Rivers could very well
see flooding again if snow levels don't fall quickly with this
storm.

**Monday Winds**

Winds have continued to come in stronger with this system as well.
700 mb winds are projected to reach 60-70 kts by Monday morning with
ridge winds again gusting in the 120-140 mph range Sunday evening
into Monday. Currently there is moderate confidence that these winds
could mix down into western Nevada on Monday which would create
another hazardous travel day for high profile vehicles and air
travel.

**The rest of next week**

The stronger cold front could still make its way into the Sierra
next week. Current projections show the colder airmass arriving in
the Sierra on Wednesday. This could bring another round of snow to
the region with snow levels dropping near all valley floors during
the middle of next week. While this system doesn't have as strong of
a moisture tap it could bring decent amount of snow to lower
elevations. The pattern stays conducive to allow a colder slider
type system into the region before a drier period of weather is
expected near the end of the month. -Zach

.AVIATION...

A series of storms is expected to impact the Sierra and western
Nevada tonight through the middle of next week. The first storm is
currently making landfall along the northern CA coast with ridgetop
winds already gusting to 50-60 mph and turbulent lenticular clouds
in western Nevada. Ridge winds will continue to increase overnight
into tomorrow gusting to 120+ mph. Surface winds are expected to
pick up overnight peaking at KRNO/KCXP/KTVL/KTRK between 12-18Z
Thursday and gusting to 30-45 knots. Winds at KMMH will pick up a
few hours later, could gust most of the day and reach 40-55 knots.
Low level wind shear and rotors are also a concern across the
region Thursday morning before the wind direction and speed
better lines up between the surface and midlevels.

Precipitation is expected to be a relatively short duration with
this first storm mainly between 14Z/15-00Z/16. Snow levels are
expected to start above Sierra terminals and with a few inches of
snow accumulating near the end of the event. All other terminals
including KMMH should see only -SHRA. -Zach

&&

.REV Watches/Warnings/Advisories...
NV...Winter Weather Advisory from 4 AM to 4 PM PST Thursday NVZ002.

     Wind Advisory from 4 AM to 4 PM PST Thursday NVZ003.

     Lake Wind Advisory from 10 AM to 6 PM PST Thursday for Pyramid
     Lake in NVZ004.

CA...Winter Weather Advisory from 7 AM to 4 PM PST Thursday above
     5500 feet in CAZ071.

     Wind Advisory from 7 AM to 4 PM PST Thursday CAZ073.

     Winter Weather Advisory from 4 AM to 4 PM PST Thursday CAZ072.

&&

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