Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 181124
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
324 AM PST Thu Jan 18 2018

.SYNOPSIS...

A winter storm will impact the Sierra and western Nevada today and
Friday. Strong winds will develop today with a period of heavy
snowfall in the mountains from late this afternoon through early
Friday morning, producing hazardous driving conditions with poor
visibility and blowing snow. Much colder air with snow showers is
expected Friday, with additional storm systems possible next week.

&&

.SHORT TERM...

We will keep all existing winter and wind headlines in place, and
add Mono County to a winter weather advisory for late tonight
through Friday morning. This storm is more of a typical January
storm with a relatively short period of significant travel impacts
for eastern CA (primarily from late this afternoon into early
Friday morning from Alpine County northward, and from late evening
into Friday morning for Mono County), but the fast movement will
produce rapidly worsening travel conditions with poor visibility
in wind-swept snow, especially for the Sierra passes. Considering
the mild winter we`ve received so far, this sharp change in the
weather pattern could produce greater than usual issues for
motorists not yet accustomed to dealing with snow on the roads.

Main concerns with this storm:

* Strong gusty winds today: A strong thermal gradient and
  approaching upper level jet stream are still on track, with 700
  mb winds increasing to near 55 kt. Model soundings indicate a
  shallow stable layer between 675-725 mb which could lead to
  localized downslope enhancement this afternoon during lee side
  precipitation shadowing. We are still anticipating peak wind
  gusts around 60 mph for many locations along the Sierra Front
  from the Reno-Carson vicinity southward to Mono County, with
  potential gusts up to near 75 mph in wind prone areas. Sierra
  ridge gusts are expected to surpass 100 mph this afternoon and
  evening.

* Heavy mountain snow with potential for white-out conditions:
  Periods of heavy snow are still expected to begin in northeast
  California by mid-afternoon, then push south into Tahoe and
  Alpine County during the late afternoon/early evening and
  continue overnight. Farther south into Mono County, the moisture
  band is projected to lose some of its punch, but it appears
  that snowfall will be sufficient to produce travel impacts from
  US-395 westward during the late night and Friday AM commute.
  When combined with the gusty winds and cooler air mass, periods
  of white-out conditions are likely. Travel in the Sierra should
  be avoided tonight if possible.

* Western Nevada snow through Friday AM: Low confidence, but
  potential for travel impacts. Current estimates for snow
  accumulation around the Reno and Carson metros are still less
  than 1 inch, and the model guidance is trying to cut off the
  precip from the urban areas. However, since some forcing is
  still likely along the cold front and moisture isn`t overly
  shallow (moist layer up to near 600 mb on some of the wetter
  model soundings), we are not going to rule out the possibility
  of a brief period of snow with reduced visibility before the
  precip winds down. This may be more of an issue for untreated
  roads near and above 5000 feet, leading to potential impacts for
  the Friday morning commute.

* Friday afternoon/evening commute issues: Also a lower confidence
  item, but with some instability projected for north central CA
  Friday afternoon associated with the upper trough passage, snow
  shower bands developing from Lassen County southward to the
  Tahoe basin could spread into the urban areas when temperatures
  are more likely to be near or below freezing. Widespread heavy
  snowfall amounts are unlikely, but a few locations could receive
  a quick inch or more of snow, if a band persists over the same
  area for a sufficient amount of time. These types of snow shower
  bands are notorious for setting up along portions of I-80, so
  keep aware of this possibility if planning travel over the
  Sierra Friday.

By Friday night, drier air with north flow aloft behind the upper
trough passage should lead to the snow showers diminishing. Shortwave
ridging on Saturday will keep most areas dry with below average
temperatures, except some weak warm advection aloft could produce
isolated very light snow showers north of Susanville-Gerlach. MJD

.LONG TERM...Sunday through next week...

Latest projections continue to take the next shortwave into the
Pacific Northwest on Sunday-Monday, which unfortunately will take
most of the moisture and dynamics north of our area. There is enough
moisture that will push down into the northern Sierra late Sunday
into Monday morning. The best snow chances look to occur from far
southwest Lassen County southward to the Sierra crest west of
Tahoe, where we could see up to 8 inches of snow. Elsewhere across
eastern CA including lake level around Tahoe and southward to
Mono County, snowfall amounts are likely to be less than 4 inches.

Snow levels will be 4000-4500 feet Sunday night, with the
boundary layer isothermal due to weak warm air advection aloft.
Along with this we will see broad isentropic lift that will
spillover light precip into the valleys of western Nevada. Doesn`t
look heavy at all, but we could see a light dusting of snow
(generally less than one inch) on the valley floors in northeast
CA and western NV. Anyone with travel plans in the Sierra should
definitely be prepared for winter driving conditions and
potentially a slick Monday morning commute with light snow on the
valley floors. Keep posted to our latest weather forecasts for
the Monday morning commute, as these lighter snow events
typically can catch folks off guard after the weekend.

The unsettled weather pattern continues through next week with
another storm system moving into the West Coast Wednesday-Thursday.
This doesn`t look like an overly strong storm, but will likely still
bring travel difficulties over the Sierra and possibly for western
NV valleys as well. -Hoon

&&

.AVIATION...

A Winter Storm will impact the Sierra and western Nevada terminals
today. Impacts include: strong and gusty winds, turbulence and wind
shear, accumulating snowfall for Sierra terminals.

*WIND: Winds are currently increasing over the Sierra Crest with
gusts already up to 60 kts reported on mountaintop sensors. This
will create areas of moderate turbulence this morning. Wind gusts
will continue to strengthen to near 100 kts this afternoon with
moderate to severe mountain wave turbulence expected along and
east of the Sierra.

Terminals should begin to see strong winds develop after 18-20z with
gusts at area terminals reaching 35-50 kts. Low-level wind shear,
moderate to severe mountain wave turbulence and rotors downwind of
the Sierra are likely. Winds will begin to decrease behind the
cold frontal passage sometime this evening after 06z.

*RAIN & SNOW: Snow begins at KTRK and KTVL this afternoon after
00z (06z KMMH) with periods of moderate to heavy snow possible in
the 03-15z window (09-18Z KMMH). Gusty winds and high ratio
(dry/light) snowfall may produce periods of zero visibility (white
out conditions) in blowing snow. Accumulations of 4-6 inches are
possible for KTRK/KTVL, 2-4 inches at KMMH through Friday morning.

KRNO and KCXP will see light rainfall by this evening with a change
over to snow expected behind the front around 10-12z. A light
snowfall accumulation of 0.5-1.0" is possible Friday morning.
-Fuentes/Hoon

&&


.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory until 4 PM PST this afternoon for Lake Tahoe
     in NVZ002.

     Winter Storm Warning from 4 PM this afternoon to 4 PM PST Friday
     NVZ002.

     High Wind Warning from 10 AM this morning to 10 PM PST this
     evening NVZ003.

     Lake Wind Advisory until 7 PM PST this evening for Pyramid Lake
     in NVZ004.

CA...Winter Weather Advisory from noon today to 10 AM PST Friday
     CAZ071.

     High Wind Warning from 4 PM this afternoon to 4 AM PST Friday
     CAZ073.

     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Friday CAZ073.

     Lake Wind Advisory until 4 PM PST this afternoon for Lake Tahoe
     in CAZ072.

     Winter Storm Warning from 4 PM this afternoon to 4 PM PST Friday
     CAZ072.

&&

$$

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



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