Weather Service Report


553 
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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Western Regional Climate Center, wrcc@dri.edu