Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 072327

Area Forecast Discussion
National Weather Service Reno NV
327 PM PST Wed Dec 7 2016


An unsettled weather pattern will continue over the next several
days. The next system will impact the region tonight into Thursday
morning and will bring snow accumulations to the Sierra and
western Nevada, along with light freezing rain in the valleys.
More rain and high elevation snow is likely Thursday through
Saturday with additional systems possible into early next week.



Unusual weather pattern will unfold over the next 24 hours,
featuring trapped polar air in the valleys and warm frontal
subtropical moisture overrunning the cold air as it moves in from
the Pacific. Temperatures in the valleys have struggled to break
the freezing mark today, with high temperatures generally in the
upper 20s and low 30s. This cold air in the valleys will have
trouble getting mixed out as this next weather system moves in
tonight. Precipitation has already started to move through
Northern CA, associated with warm air advection and isentropic
broadscale lift. This will bring light snow to the Sierra and into
the valleys of western Nevada as well, with potential impacts to
this evening`s commute. Several inches of snow may be possible in the
valleys of western NV and northeastern CA tonight before we see a
change over to rain and/or freezing rain.

This warm frontal system will increase temperatures aloft,
bringing a variety of winter weather impacts early Thursday
morning down to valley floors. As the warm air advection continues
aloft, this will raise temperatures above the 6000 foot level to
above freezing, causing snow to change over to rain in the early
morning hours Thursday. Freezing air trapped in the colder
valleys of the Sierra, northeast CA and western NV will then re-
freeze the rain on surfaces such as roads, sidewalks, trees, and
powerlines. This is a very sensitive weather pattern, as some
valleys may see ice accumulations from freezing rain, some valleys
may stay all snow and switch over to rain, and then other valleys
may not see much of anything at all. Also, just a small amount of
ice on the roads and sidewalks can make for very dangerous
conditions. Best chances for freezing rain/drizzle Thursday
morning will be between 1am to 6am in the Sierra and 4am to 8am in
the valleys of western Nevada.

Motorists should prepare for slow travel, possible chain restrictions
and should allow for extra time to reach their destinations. This
warming aloft will eventually raise the snow levels to near 8000
feet Thursday afternoon. Also, the freezing temperatures in the
valleys should mix out by the afternoon which will end the
freezing rain potential. Storm total snow amounts through Thursday
will be 6 to 18 inches of snow for elevations above 7000 feet and
up to 6 inches around Lake Tahoe level. Snow amounts in the
valleys of northeastern CA and western Nevada will be less,
although amounts up to 1-4 inches will be likely.

Winds aloft will also be increasing Thursday. By Thursday
afternoon as the colder air gets mixed out, we will see wind gusts
up to around 35-45 mph and even stronger winds on Friday with
gusts up to 50 mph or more. Latest GFS and NAM model runs are
showing some strong mountain wave signatures for Friday, which we
will need to keep an eye on. Dangerous boating conditions, cross
winds for high profile vehicles and turbulence/shear may be
possible for local flights.

A weak/moderate atmospheric river will also push into the Sierra
on Friday, bringing moderate to heavy rainfall and high elevation
snow. With snow levels expected to stay above 8000 feet, most
mountain passes will be spared from winter travel conditions on
Friday. The only exceptions will be the highest Sierra passes,
including Mount Rose Summit, Carson Pass, and Conway Summit
(US-395). Hoon

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

Active weather pattern will continue into the middle next week, with
several contrasting scenarios, especially with precip chances
and snow levels Saturday through Monday. Overall, the best chances
for substantial rain and snow is expected Saturday, and again next
week from late Monday through Tuesday.

For Saturday, medium range guidance continues to indicate wide
variations for precip potential, and we are continuing to favor
the wetter scenario since the atmospheric river is still projected
to be affecting the Sierra and northeast CA. However, this feature
will likely be weakening, so we are only expecting moderate
additional rain and snow amounts for the Sierra (up to 0.75"
liquid and up to 6 inches snow above 7000 feet, lesser snow amounts
between 6000-7000 feet), with lighter rain for western NV. Snow
levels for the Sierra are projected to drop from 7000-7500 feet
in the morning to near lake level by late afternoon. For northeast
CA snow levels could drop to near 5000-5500 feet during the day.

For Saturday night through Sunday night, the guidance continues to
indicate diverging scenarios. While the original consensus
favored a relatively quiet period with only light showers mainly
near the Sierra, the GFS brings through another band of
moisture. This appears to be more of an overrunning/warm
advection aloft event with relatively cool air at the surface, so
precip could fall as snow in lower elevations. The location of
this possible moisture band has varied by a couple hundred miles on
the two latest GFS runs, so we are not going very aggressive with
precip chances or amounts at this time.

For Monday and Tuesday, there is a little better consensus for
another atmospheric river bringing a more substantial amount of
precip. Milder air is likely to spread northward across the
region, although on Monday this may be a slow or unsteady process,
so we kept a wider range of elevations with rain-snow mix. By
Monday night or Tuesday, snow levels in the Sierra should rise
above 8000 feet. While today`s guidance was not quite as
impressive with two-day precip totals, there remains a possibility
for a decent period of heavier precip rates which could produce
hydrologic concerns for the Sierra and northeast CA.

Wednesday could be a little less active with the moisture feed
directed farther north, possibly even missing the Sierra. Since
the location of the best moisture has varied during the past few
days of model runs, we will keep chances for for rain and high
elevation snow (snow levels generally above 8500 feet for Sierra)
Temperatures will continue to be several degrees above average,
with highs possibly near 60 degrees for warmer valleys of western
NV and breezy conditions. MJD



Multiple weather-related impacts to aviation will occur from tonight
through the next few days.

For tonight-Thursday morning: Light snow will spread quickly across
the main terminals this evening, starting between 00-02z and
continuing through most of the night. MVFR CIGS/VIS are most likely,
with occasional reduction to IFR at times especially for the Sierra
terminals. A period of -FZRA could affect valleys and terminals east
of the Sierra including KRNO/KCXP after 08z, with greatest risk
between 12-16Z before precip changes to all rain or ends. As for
KTRK/KTVL, the transition should be directly from snow to rain
Thursday morning. For part of the morning, rain could be falling
around Tahoe but still snowing in western NV valleys. Farther south
along the Sierra KMMH will likely stay all snow with only brief rain
possible shortly before precip ends.

Winds will begin increasing Thursday afternoon with periods of gusty
winds continuing through Friday. Mountain wave turbulence is
expected, but LLWS could also occur at times during periods of
lighter near-surface winds while stronger winds continue above
the surface.

More rain and snow is expected for the Sierra Thursday night
through Saturday, with much higher snow levels and mainly rain even
at the Sierra terminals. Lighter and shorter duration periods of
rain are expected for western NV during this time frame. MJD


.REV Watches/Warnings/Advisories...
NV...Winter Weather Advisory until 10 AM PST Thursday NVZ005.

     Winter Weather Advisory until 10 AM PST Thursday NVZ002.

     Winter Weather Advisory until 10 AM PST Thursday NVZ001-003-004.

CA...Winter Weather Advisory until 10 AM PST Thursday CAZ070-071.

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

     Winter Weather Advisory until 10 AM PST Thursday CAZ072.



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