Area Forecast Discussion
Issued by NWS Reno, NV

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402
FXUS65 KREV 220519
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
919 PM PST Sat Jan 21 2017

.UPDATE...

Major winter storm is set to push into the region over the next
24-36 hours. The latest forecast models look on track, with heavy
snow starting in the Sierra and western NV tonight through Monday
morning. Forecaster confidence remains high for the storm, with
multiple feet of snow in the Sierra and 8-16 inches possible for
the western NV Sierra front.

Periods of blizzard conditions are possible in the Tahoe Basin
and Mono County during this storm, especially early Sunday
morning between 4am to noon as the cold front moves into the
region. Travel is extremely discouraged in the Sierra Sunday.
Heavy snow, combined with gusty winds, low visibility, and high
walls of piled up snow along the Sierra roads and highways will
make travel very difficult and even dangerous for the Sierra. The
storm does still look to come through in two parts, one burst
tonight into Sunday morning, then the second burst behind the
front Sunday evening into early Monday morning.

Avalanche danger will be increasing over the next 24 hours as well,
in fact the Placer County Sheriff has issued a voluntary evacuation
order for the Alpine Meadows areas that are susceptible to
avalanches. We have also shared their informational video on our
Facebook page.

Check out the latest satellite images from GOES-West, you can see
the intensity of the low pressure system off the Pacific Coast.
We`ve posted a loop of the satellite on our Facebook and Twitter
accounts. This low is exhibiting extreme cyclogenesis (also
called bombogenesis) with a deepening of nearly 26mb by 15z Sunday
morning and bottoming around 964mb. This is a strong indicator of
the powerful storm that is expected to affect the region over the
next 24-36 hours. Hoon

&&

.PREVIOUS DISCUSSION... /Issued 335 PM PST Sat Jan 21 2017/

SYNOPSIS...

A very strong winter storm will impact the region late tonight
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...

Alright folks...it`s almost showtime! If you haven`t heard by now
or are living under a rock, a very potent winter storm promises
widespread heavy snow for the Sierra and western Nevada from late
tonight into Monday morning. Forecast changes were generally
minimal through Monday, although snow chances were reduced Monday
in the Basin north of about Highway 50.

Checking out the current conditions, the infrared satellite shows
the rapidly intensifying winter storm with a developing surface
low about 500 miles west of San Francisco (at about 130 W longitude).
Satellite winds from this morning verified model simulations
indicating a very strong jet core in excess of 170 kts approaching
California. GFS/NAM simulations show the surface low bottoming out
at a very stout 963-967 mb off the Oregon coast by Sunday morning.

The incoming storm will feature a couple waves of heavier snow
(and possibly a mix of rain and snow out in west-central NV) with
possible lessening of precipitation rates in between, especially
well east of the Sierra in the Basin and Range. The two waves will
be: 1) warm air advection period late tonight and Sunday morning
followed by (2) precipitation with and behind a strong cold front
Sunday afternoon and evening.

1) The warm air advection period: (Approx midnight tonight through
tomorrow morning): Substantial lift with warm air advection will
combine with a short period of moisture transport from an
atmospheric river to brings the onset of snow. As the main push of
moisture comes over the Sierra it will easily push across the
Sierra with little impediment providing a similar start time for
snowfall in western Nevada as compared to the Sierra.

Higher snowfall amounts are expected roughly from Washoe Valley
through Carson Valley as wind orientation and distance from the
terrain will favor heavier spillover. See the Winter Storm Warning
for details on snow amounts.

There is medium confidence that snow levels will rise a bit
during the afternoon Sunday well east of Reno as the warm air
advection period wanes. Therefore, some valleys below 4,000 to
4,500 feet out in the Basin and Range (Fernley to Yerington eastward)
could see rain begin to mix with snow or even briefly change to
rain by late morning and then turn back to snow late Sunday
afternoon and early evening as the cold front moves through that
area. Jet forcing intensifies out in west-central NV north of
Highway 50 in the afternoon so it is possible that cooling with
steadier precipitation there will hold any changeover to near and
south of highway 50.

2) Precipitation near/behind cold front: (Sunday evening into
early Monday morning): The front will be strong and moist-
adiabatic and unstable profiles will provide good spillover for
western Nevada. Although some areas of gusty winds are possible
Sunday morning in valleys and the foothills, the main window for
blowing snow issues may be toward Sunday evening as snow becomes
drier/fluffier and blows around easier. Snow will start to
diminish by early Monday morning and really taper off by early
afternoon across the Tahoe Basin and western Nevada with areas in
Mono County hanging on through the day.

Overall, conditions will really go downhill tonight and Sunday so
its best to avoid any travel if possible in the Sierra and across
far western Nevada. Seeing the potential for widespread snow like
this in western Nevada has not occurred for several years. As a
bit of climatology, the chance of seeing snow amounts >10" at the
Reno airport when it does snow are generally less than 1% for any
given winter, but this storm has a good mix of cold air and moisture
to provide it. Some analogs can be made to the copious snow
producer in January of 2005 which produced 1-2` of snow in the
valleys but that storm had some differences and perhaps an even
better setup for spillover and cold air. Overall it will be an
interesting storm to see unfold. Snyder/Fuentes

LONG TERM...Monday night through Saturday...

The last vestiges of the ongoing winter storm will drop south of the
forecast area Monday night into early Tuesday morning. Snow showers
continue through the night but gradually decrease in intensity and
areal coverage resulting in little in the way of additional
accumulation...maybe an inch or two in the highest elevations of the
Sierra.  A few showers are also possible through the day
Tuesday...but will be isolated in nature.

Latest model simulations show northerly flow developing aloft by late
Tuesday into Wednesday as a ridge builds to the west. Light east to
northeast flows develops at the surface and dry air begins to move
into the region. This light flow over existing snow cover could
produce low clouds in many of the valleys...but if the air is dry
enough clouds are not as likely. The presence of clouds versus no
clouds is critical to temperatures in the valleys. Lows would be
affected more than highs, but both will be below normal either
way.

To account for the possibility of less cloud cover but widespread
snow cover we have lowered the lows a little through the extended
period...single digits to teens with some negative readings in the
Sierra valleys. Highs were also lowered a little below guidance to
account for snow cover that should be present after this weekend`s
storm. If we are clear with the expected significant snow cover
low`s could be far colder than we currently depict...with some areas
in the colder valleys approaching 15-20 below zero.

The GFS persists in dropping a short wave south over California
Thursday. The ECMWF is less aggressive with this feature...but does
show a weak area of vorticity. This feature could trigger a few snow
showers Thursday and we will add slight chances to the Sierra and
northeast california. If low clouds are present east of the Sierra a
few flurries are possible.

The ECMWF is more aggressive in building a high over the region by
late Friday into Saturday while the GFS keeps the ridge to the east.
Either way we are still looking at below normal temperatures and
light winds into the weekend.

Beyond the seven day period there is little in the way of
precipitation hinted at through the bulk of the next week. Some of
the very long range forecasts hint at the development of another
plume of moisture late next weekend...but it is far too early to
place any confidence in a forecast that far out. 20

AVIATION...

Snow showers continue through this evening in the Sierra with
several inches of additional accumulation likely for Sierra
terminals. Western Nevada remains largely shadowed, though a few
stray showers may pass through the area. Periods of IFR conditions
in the Sierra with widespread mountain obscuration throughout the
Sierra and western Nevada.

Main event is late this evening into Monday when a significant
winter storm moves into the Sierra. Very heavy snow is expected with
15 to 30 inches of accumulation for Sierra terminals and widespread
LIFR/IFR conditions. Across western Nevada, periods of heavy snow
should move into the region after midnight and persist into Sunday
Night or early Monday morning. 8 to 14 inches of accumulation is
possible for KRNO with 12 to 18 inches for KCXP.

With a strong jet directly over the region, turbulence is likely.
With winds ramping up aloft and not fully mixing down to lower
valleys, LLWS is expected tonight into Sunday across all areas.
-Jon/Dawn

&&

.REV Watches/Warnings/Advisories...
NV...Winter Storm Warning from midnight tonight to 4 AM PST Monday
     NVZ005.

     Winter Storm Warning until 4 AM PST Monday NVZ002.

     Winter Storm Warning from 4 AM Sunday to 4 AM PST Monday
     NVZ001-004.

     Winter Storm Warning from midnight tonight to 4 AM PST Monday
     NVZ003.

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

     Winter Storm Warning from midnight tonight to 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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