Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 052226
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
226 PM PST Mon Dec 5 2016

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

&&

.SHORT TERM...

Main concern is a potential freezing rain event in 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.

Bottom line up front...Thursday morning commute could be impacted
by snow and/or freezing rain.

First system is running a bit slower in the model data, with
light snow north of Susanville to Lovelock from late Tuesday night
into Wednesday afternoon. High end accumulations could reach 2
inches with some slick roads, including Highway 447 from Gerlach
to Cedarville and I-80 east of Lovelock. The bigger impact from
this storm is the cold air behind it. Lows for Wednesday morning
should drop to 20 or less across most western NV valleys with
single digits in the Sierra. Afternoon highs wednesday afternoon
may struggle to reach freezing in some valleys, which brings us to
an increased threat for an unusual mixed precipitation event.

By Wednesday night, warmer air ahead of the approaching
atmospheric river will lead to light overrunning precipitation
while the sub-freezing temperatures are trapped in the valleys.
Overrunning events don`t produce much wind, it is likely valley
locations will remain below freezing due to a lack of mixing for
much of Thursday morning. The big question is if the above
freezing warm layer is deep enough to allow snow to melt to rain
before the hydrometeor falls back into the sub-freezing surface
layer. Liquid amounts from the overrunning should remain below a
0.25 inch in the Sierra with less than 0.10 inch across the NV
valleys. That doesn`t seem like much, but would be enough for a
light coating of ice on roads, trees, and powerlines.

If the above freezing 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 above freezing layer is too shallow the main precipitation
type will be snow at all elevations Wednesday night and Thursday
morning, though there could also be some rain from 5500-7000 feet
and some sleet below 5500 feet.

By Thursday afternoon there should be enough warm air pushed into
the region to raise valley temperatures above freezing and melt
any icy roads. 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. Brong

.LONG TERM...Thursday Night through Monday...

Active weather pattern is likely to continue into early next week.
There is quite a wide contrast in potential scenarios, especially
with precip amounts and snow level/temperatures late Friday into
Saturday. In general, the higher potential for more substantial rain
and snow is expected from Thursday night through Saturday, with
lesser chances and lighter precip amounts Saturday night through
Monday.

For Thursday night and Friday, ongoing atmospheric river feature
directed toward the Sierra gets a boost of energy as shortwave
passes through the Pacific Northwest. This should provide a decent
round of precip for the Sierra and northeast CA, with QPF values
between 1-2 inches near the crest/west of CA-89, and between 0.50
and 1 inch for northeast CA and the majority of the Tahoe
basin/western Mono County. However, snow levels will be rather high,
with the best potential for accumulating snow (early projection 8-16
inches) near and above 8000 feet. There is likely to be a short
period of spillover rain into parts of western NV Thursday night
with occasional showers on Friday, although precip amounts will be
much lighter.

Gusty winds will also accompany this storm system. Gusts of 40-50
mph are possible for eastern CA and into far western NV, with
maximum Sierra ridge gusts up to 100 mph.

Friday night and Saturday becomes the most complicated part of the
extended forecast. Scenarios in the medium range guidance range from
spotty light snow/rain showers with cooler air (snow levels 5000-
6000 feet) to a moderate-heavy precip/spillover event with a much
milder air mass (snow levels 8000+ feet). This does not look like a
good boost for the Sierra snowpack, except for very high elevations
if the wetter outcome occurs. The forecast was initially in the
cooler/drier end of the above range of scenarios, but we didn`t want
to dismiss the wetter/warmer end of the range either. Therefore, we
trended a little milder with increased precip chances. Snow levels
in the Sierra will be highly variable, with a general trend toward
lower snow levels by Saturday afternoon-evening, and we kept a wide
elevation range for rain-snow mixture.

For Sunday and Monday, some unsettled showery conditions could
continue mainly near the Sierra and northeast CA as weaker shortwave
energy moves across the region. At this time, we are only expecting
light amounts of rain or snow. Temperatures should be near seasonal
averages, although Sunday appears a few degrees cooler than Monday.
Moderate winds are expected at lower elevations with short periods
of stronger gusts (30-40 mph) possible during shortwave passages.
Sierra ridge winds will remain rather gusty for both days, with
gusts generally between 50-70 mph but extreme winds are not
expected. MJD

&&

.AVIATION...

VFR conditions with limited aviation impacts are expected through
Tuesday morning. Later on Tuesday, winds will increase with gusts 25-
30 kt for western NV terminals and around 35 kt for Sierra
terminals, leading to more turbulence. Light snow showers will
spread across northwest-west central NV and far NE CA during the
day, but most of this activity will remain north of KSVE-KLOL. Brief
-shsn may reach as far south as KRNO-KTRK but only a 20% possibility
of brief MVFR cigs/vsby.

VFR conditions with lighter winds are expected Tuesday night thru
much of Wednesday. The next series of storms are set for Wed night
through Saturday with periods of MVFR and IFR cigs/vsby, especially
in the Sierra. Winds will also be an issue with mtn wave turbulence
and possible LLWS at times. MJD

&&

.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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