Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 190306
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
706 PM PST Thu Jan 18 2018

.UPDATE...

Precipitation has overspread northeast California and far
northwest Nevada so far and is now working its way into the Tahoe
Basin. We have noted just recently that snow has started to fall
near Donner Summit and points west with light rain near lake
level. As the evening progresses and more moisture works its way
east of the Sierra...gusty winds will continue to subside. We are
currently seeing gusts mainly below 55 mph in the lee of the
Sierra...but will allow the High Wind Warnings to continue until
10 pm. We have allowed the Lake Wind Advisory for Pyramid Lake to
expire as the lake is closed at sunset and winds are decreasing.

A few minor changes were made to the evening forecast to lower
pops for areas in the lee of the Sierra and south of I-80 while
raising pops and QPF across far northeast California and northwest
Nevada. Before the precipitation ends...some parts of the Surprise
Valley and far northern Washoe County could see up to 2 inches of
snow...but for now it is rain below 5000 feet. XX

&&

.PREVIOUS DISCUSSION... /Issued 343 PM PST Thu Jan 18 2018/

SYNOPSIS...

A winter storm will impact the Sierra and western Nevada through
Friday. Strong, gusty winds will decrease overnight into Friday.
A period of heavy snowfall is expected in the mountains tonight
through early Friday morning, producing hazardous driving
conditions with poor visibility. Colder air with snow showers is
expected Friday. More storm systems are possible next week.

SHORT TERM...

The storm remains on track with a few modest adjustments. Primary
forecast concerns continue to be strong winds preceding the
surface front, heavy snow in the Sierra, and the potential for
morning snow in western Nevada. More details are below.

The main forecast changes were made to expected QPF for the Sierra
Front. While model precipitation totals are meager, wind profiles
and the conceptual model associated with Atmospheric Rivers favor
more spill over. Considering the winds increasing through 500 mb
and relative position of the jet, precipitation totals along the
foothills into the Sierra Front warranted an increase. Still,
totals will be fairly light, in the 0.10-0.3" range. Most of this
will fall overnight in the form of rain.

Also, the coldest air is lagging a little behind the frontal
boundary now. Coupled with more liquid precipitation, this has
implications for Friday morning`s commute for the metro areas. It
is now a little less likely that the 1/2 inch or so of snow
forecast will be as problematic since roads will be fairly wet.
However, it would still be prudent to plan additional time in case
the colder air "catches up" to the front overnight.

Another change was an increase in shower coverage Friday
afternoon, especially over the Tahoe Basin. Flow will likely
remain upslope behind the surface front. However, the upper front
will not pass through until tomorrow evening. The result is
upslope in a convectively unstable environment. Additional
accumulations for the Tahoe Basin northward to Truckee are
generally up to 3 inches. It should be noted that some light
accumulations will be possible for the Sierra Front, but the
nature of these showers makes it very difficult to predict. Keep
your eyes out for shadowed areas where showers have moved through;
these areas may be slick.

Otherwise, the forecast remains on track even though snow levels
are starting marginally higher than forecast. These are still
expected to fall once precipitation moves in. Winds are on the
rise and will continue to increase into the evening hours. Once
moisture begin spilling over, winds will likely fall off somewhat,
but remain gusty. Expect gusty winds and snow in the Sierra to
create difficult travel conditions. It would be best to hold off
on any travel plans over the passes until tomorrow at this point.
Boyd

LONG TERM...Sunday night through Thursday...

Summary of the long term forecast: a weak system with light/very
light snow possible Sunday night will be followed by a much stronger
system with more significant Sierra (and possibly lower valleys...
but low confidence) snowfall for the middle of next week.
Temperatures should remain near average for the first half of next
week, possibly falling to considerably below average for late week.

Sunday night, the GFS and EC are indicating a period of warm air
advection (WAA) lift to the south of an upper disturbance moving
through the Pacific Northwest. Simulated soundings show isothermal
profiles for northeast California and much of western Nevada, which
would tend to keep snow levels at or near valley floors. The WAA
is not overly strong so light precipitation (mainly snow) is
expected at best; however, light snow could easily cause slick
roads for Monday morning`s commute so be aware of the latest
forecast and forecaster thinking for your area.

After some residual light showers Monday, a break is expected before
the next system arrives for late Tuesday and Wednesday. Some
simulations show a period of light warm air advection precipitation
(mainly snow) ahead of the system across northeast California and
far northwestern Nevada. On Wednesday, forecast uncertainty grows
considerably as some simulations have a drier, weaker system moving
in while others have a much more consolidated (and stronger) system
with a long Pacific moisture fetch. The stronger solutions would
even possibly feature a thin moisture tap/weak AR. Both types of
systems would bring some snow to the Sierra with light rain and snow
showers for the lower valleys; however, the stronger simulations
would bring much heftier amounts to the Sierra and possibly even
some western Nevada valleys. -Snyder

AVIATION...

*WIND: Winds have been a bit lower than expected near the ground for
far western Nevada and Mono County terminals so far today, with
generally expected values for KTRK/KTVL. However, crosswinds just
off the deck continue to be possible until FROPA, along with
considerable mountain wave turbulence downwind of the Sierra. Winds
will begin to decrease behind the cold frontal passage with some
dampening of the mountain wave activity. Approximate FROPA times:
KRNO/KCXP, 03-04Z (7-8 PM PST), and for KHTH/KMMH, 10-13Z (2-5 AM
PST).

*RAIN & SNOW: Snow begins at KTRK and KTVL around 00-02Z (~06z
KMMH), with periods of moderate to heavy snow possible in the 03-12z
window (09-18Z KMMH). Gusty winds and high ratio (dry/light)
snowfall may produce periods of near 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 this evening with a change
over to snow expected behind the front around 12Z/4 AM PST. Snow
accumulations are expected to be limited (0.5" or less) with the
fast cold frontal passage and a likely sharp cutoff to precipitation
after FROPA. There is a chance for another round of -shsn late in
the afternoon Friday for western NV terminals, although warmer
ground temps by then should limit accumulations substantially.
-Snyder/Hoon

&&

.REV Watches/Warnings/Advisories...
NV...Winter Storm Warning until 4 PM PST Friday NVZ002.

     High Wind Warning until 10 PM PST this evening NVZ003.

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

     High Wind Warning until 4 AM PST Friday CAZ073.

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

     Winter Storm Warning until 4 PM PST Friday CAZ072.

&&

$$

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



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