Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 221739
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
939 AM PST Sun Jan 22 2017

.UPDATE...
Strong winds and heavy snow will continue to create hazardous
travel in the Sierra today and tonight along with high avalanche
danger in the backcountry. For western NV, a break between the
two waves of the storm is allowing improving road conditions below
around 5,000 feet for many locations. However, as the cold sector
of the storm crosses the Sierra this afternoon, moderate to heavy
snow rates should develop. The next wave should be reaching the
Highway 395/Interstate 580 corridor around 4 pm and then spreading
across western NV this evening. This will bring worsening
visibilities across west NV after 4 pm. The winter storm warning
has been updated to reflect these conditions. JCM

&&

.PREVIOUS DISCUSSION... /Issued 802 AM PST Sun Jan 22 2017/

UPDATE...
The warm sector of the storm has all but finished crossing the
central Sierra with cooling aloft about to ensue. Snow levels east
of the Sierra currently range from 4,500 to 5,000 feet MSL with a
snow/rain mix or rain below 5,000 feet for parts of west NV
including along the I-80 corridor and in the Washoe Valley. In the
mountains, I-80 is currently closed between the Nevada state line
and Colfax, CA due to heavy snow. This storm is just beginning
with snow rates expected to increase this afternoon and evening,
bringing worsening visibilities and road conditions. The colder
sector of the storm is on track to arrive after noon. Thus, the
winter storm warning through 4 am Monday morning is on track for
all areas. Please see www.weather.gov/reno for the latest winter
storm information.

The Forest Service Sierra Avalanche Center has issued an Avalanche
Warning for the backcountry in effect through at least Monday
morning. Heavy snowfall rates and strong ridge winds have created
high danger with potential for avalanches to be large, deep and
run long distances. Travel near avalanche terrain is strongly
discouraged. JCM

PREVIOUS DISCUSSION... /Issued 333 AM PST Sun Jan 22 2017/

SYNOPSIS...

A very strong winter storm will impact the region today through
early Monday, bringing heavy snowfall to the Sierra and western
Nevada, with the Sierra seeing periods of blizzard conditions. Dry
but cold weather is expected for the middle to end of next week,
with strong valley inversions developing by late week.

SHORT TERM...

The first wave of our strong winter storm is pushing precipitation
across nearly the entire state of California with spillover snowfall
making its way across western Nevada. Moderate to isolated areas of
heavy snow have been reported so far this morning from Reno to
Carson City with steady snowfall occurring across the Sierra. Snow
covered roads and chain controls are up all across the Sierra with
roads starting to become snow covered on untreated surfaces across
far western Nevada.

No significant changes with the forecast with the expected snow
amounts and timing still holding as of early this morning. As a
recap from the discussions over the last few days we expect this
storm to play out in two main waves. The first band moving through
now is a warm air advection driven band and will intensify through
the early morning across both the Sierra and western Nevada with
moderate to heavy snow showers persisting til about noon. During mid-
afternoon today is when we may see some rain mix with snow mainly
for valleys below 4,000 feet across the western Nevada Basin and
Range areas including areas such as Lovelock and Fallon. By late
afternoon and evening precipitation will turn back to all snow as a
cold front approaches and heralds the arrival of the second wave of
the storm.

The second wave will be a post cold frontal regime driven by higher
instability and colder air moving in aloft, so snow ratios will be
higher with lighter and fluffier snow expected. This greater
instability will allow the development of smaller banded and
convective features which can produce heavier snowfall rates in the
2-4"/hr range. This secondary wave will roughly occur from late
afternoon until about midnight, where snow showers will then begin
to diminish in intensity through Monday morning.

Overall, this storm matches pretty decently with composite synoptic
patterns for heavy snow in western Nevada with a deep sub 520dm
500mb low off the Pacific Northwest coupled with an upstream 150W
ridge near Alaska and 500mb temps near -30C across northern Nevada.
The main difference with this storm is that it posses a very
powerful 180kt jet which is aiding in spillover and also in the deep
bombing out of the surface low off the Pacific Northwest.

This jet translates to strong 700mb winds as well which will
result in powerful winds across the Sierra ridge tops with gusts
over 140 mph expected. The main forecast concern which we will be
monitoring is the potential upgrade to a Blizzard Warning across
the Sierra. This storm differs some than our previous Blizzard
earlier in the month in that, that previous storm possessed a
prolonged period of strong winds leading to whiteout conditions.
While, no doubt this storm with have periods of Blizzard
conditions, the winds will be weakening by late morning across the
Tahoe Basin with more prolonged winds in Mono County. The winds
aloft will become very strong through the morning but winds tend
to have a difficult timing mixing down in a warm air advection
regime. Will closely monitor still functioning mountain wind sites
and will analyze the 12z upper air sounding to see if this
upgrade is needed for the Tahoe Basin and Mono County. Fuentes

LONG TERM...Tuesday through Sunday...

A few lingering snow showers are possible Tuesday morning, but any
additional accumulations will be light. By Tuesday night into
Wednesday, the system exits the region with mainly dry conditions
likely the remainder of the week. The one exception will be Thursday
when a weak wave drops through the region. The trend has been taking
this wave eastward and absorbing it into a larger low positioned
across much of the CONUS. This could bring flurries or very light
snow showers to the area and have included this in the forecast.
Without much forcing or additional moisture to work with, doubt this
feature would result in much of any accumulation.

With light winds and plenty of cold air trapped in valleys,
inversions are expected to develop from midweek onward. Freezing fog
and stratus are definite possibilities in valley locations, but we
will need to see how the ongoing snow event plays out first to
determine the extent. Ridge builds into the west for the weekend
with a 1044 mb surface high building across the Great Basin,
resulting in easterly flow and strengthening the inversions. Expect
poor mixing and particulate build-up in the lower valleys with
possible reductions to air quality.

The good news for those trying to dig out of the incredible amount
of snow that had fallen this month is that the long range forecast
keeps the area dry through the start of February. There are some
indications that moisture will return to the west by the end of the
first week of February, but as always there is low confidence that
far out. -Dawn

AVIATION...

The start of a significant winter storm is affecting the region with
snow only expected to intensify over the next several hours through
Sunday morning. There will be a brief lull in the intensity Sunday
afternoon, but the second portion of the storm will follow Sunday
evening into Monday morning. Very heavy snow is likely with
widespread LIFR-IFR conditions and mountain obscuration. 20 to 35
inches of accumulation is expected for Sierra terminals by Monday
morning with 8 to 12 inches of accumulation possible for KRNO with
12 to 16 inches for KCXP.

With a strong jet directly over the region, turbulence is likely.
Winds are fairly strong not too far off the deck, but for the most
part are not fully mixing down to lower valleys. This will make for
areas of LLWS through the day today. However, there will be times
that gusts will make it down to the surface, so erratic strong gusts
are possible.

Conditions will begin to improve Monday, but snow showers will still
linger, especially in the Sierra. -Dawn

&&

.REV Watches/Warnings/Advisories...
NV...Winter Storm Warning until 4 AM PST Monday NVZ005.

     Winter Storm Warning until 4 AM PST Monday NVZ002.

     Winter Storm Warning until 4 AM PST Monday NVZ001-004.

     Winter Storm Warning until 4 AM PST Monday NVZ003.

CA...Winter Storm Warning until 4 AM PST Monday CAZ071.

     Winter Storm Warning until 4 AM PST Monday CAZ070.

     Winter Storm Warning until 4 AM PST Monday CAZ073.

     Winter Storm Warning until 4 AM PST Monday CAZ072.

&&

$$

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


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