Area Forecast Discussion
Issued by NWS Reno, NV

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655
FXUS65 KREV 061051
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
251 AM PST Tue Dec 6 2016

.SYNOPSIS...
An active weather pattern will push multiple storms across the
region over the next several days. The storm Wednesday night into
Thursday morning may produce freezing rain in valley locations and
light snow accumulations over the Sierra passes. A moderate
atmospheric river is forecast to move into the Sierra Thursday
afternoon with a period of heavy rain and snow into Friday Night.

&&

.SHORT TERM...

The main concern continues to be the potential freezing rain event
for the valleys late Wednesday night to Thursday morning. While the
window for freezing rain is small, even minimal ice accumulations
can lead to high travel impacts.

Main takeaway...Thursday morning commute will likely be impacted
by snow and/or freezing rain.

The initial storm approaching the Pacific Northwest this morning
will bring light snow accumulations north of Susanville to Lovelock
through the afternoon. While locally higher amounts up to 2 inches
will be possible it is likely that roads will be slick, including
Highway 447 from Gerlach to Cedarville and I-80 east of Lovelock.
The bigger impact from this storm will be the cold air behind it.
Lows for Wednesday morning will drop to 20 or less across most
western Nevada valleys with single digits in the Sierra. Afternoon
highs by Wednesday afternoon will struggle to reach freezing in some
valleys. Sub freezing air in these valleys will directly
contribute to an increased threat for an unusual mixed
precipitation Wednesday night into early Thursday morning.

By Wednesday night, warmer air ahead of the approaching atmospheric
river will lead to light overrunning precipitation across the Sierra
and western Nevada. There will be little to no mixing ahead of this
storm, so the sub-freezing air will stay trapped in the valleys.
Model soundings still indicate a rather shallow warm layer, but it
still may be just enough to allow snow to melt to rain before the
hydrometeor falls back into the sub-freezing surface layer.
Liquid amounts from the overrunning are still showing below a 0.25
inch in the Sierra with less than 0.10 inch across the NV
valleys. Although those are relatively light amounts, that would
be sufficient to produce a light coating of ice on roads, trees,
and powerlines.

If the warm layer is deep enough the potential precipitation
types are...
* Above 7000-7500 feet...Snow (1-4 inches possible)
* 5500 - 7000 feet...A mix of snow and rain
* Below 5500 feet...A mix of snow, rain and freezing rain

If the warm layer ends up being too shallow, then the main
precipitation type will be snow at all elevations Wednesday night
and Thursday morning. Snow should begin to transition to rain as
the much warmer air moves into the Sierra and western Nevada in
the afternoon. Snow levels will rise through Thursday afternoon,
and could climb as high as 8000 feet in the Sierra as the
precipitation from the atmospheric river begins. Periods of heavy
rain will begin as the first significant moisture plume impacts
the Sierra between 11AM and 5PM on Thursday afternoon. Once the
rain starts Thursday afternoon, it will likely remain rainy for
the entire Sierra through the end of the week. Although snow
levels will remain around 8000 feet for Thursday and Friday
(largely above the Sierra passes) there could still be travel
impacts from ponding of water and slick road conditions. Periods
of heavier rainfall could briefly limit visibility so take your
time and increase braking distance on the roads, even if it is
just rain! - Edan

.LONG TERM...Friday through Monday...
A moderate AR event still looks on track to continue through at
least Friday if not Friday night depending on the model. The last
impulse of the system looks to move through either Friday (GFS) or
Friday night (EC). Either way, the last part of it will have snow
levels 7-8000 feet and centered around the Tahoe Basin. Some
spillover into western NV is possible, but it looks spotty as the
forcing and thermodynamic/kinematic profiles are not very favorable.

Some lingering showers are expected Saturday as colder air filters
in for the weekend. These showers could continue into Sunday, but it
is looking more and more like there will be a dry period between
systems on Sunday into Sunday night. Right now, kept a slight chance
to low chance of precip as previous runs showed some moist zonal
flow continuing. Snow levels will be lower and around 6000 feet or
so, but they still remain variable at this point in time.

Another moderate AR system is possible late Monday into Tuesday.
Increased the threat of precip during this period and also started
raising snow levels. Currently, the models are targeting the area
from I-80 northward. However, this can change and the GFS/EC
ensembles overall are a bit further south than their respective
deterministic runs. Have gone with a very large mixed layer for
rain/snow due to the changes in snow levels in the latest models and
the previous runs. One other thing to watch would be the possibility
of FZRA and snow again in the valleys north of Susanville/Gerlach
depending on how much cold air filters in over the weekend.

&&

.AVIATION...
A few snow showers with LCL MVFR CIGS/VIS in -SHSN north of KSVE and
KLOL 12Z to 03Z Wed. Otherwise VFR with some W-NW winds with peak
gusts to 25-30 kts. Some FZFG formation is possible after 09Z
tonight for KTRK.

The next system is set to arrive Wednesday night with snow and MVFR
CIGS/VIS. It gets more interesting with the possibility of a period
of -FZRA for valleys and terminals east of the Sierra including
KRNO/KCXP. The highest chance for this FZRA would be 09-18Z before
changing to all rain. As for KTRK/KTVL, the rain should go from -SN
to -RA as those locations are high enough in elevation to be within
the warm nose. Rain will continue into Friday at times, mostly in
the Sierra. Winds will also increase with mtn wave turbulence.

&&


.REV Watches/Warnings/Advisories...
NV...None.
CA...None.
&&

$$

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



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