Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 181737
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
937 AM PST Wed Jan 18 2017

.UPDATE...

Updated to move up start time of the Tahoe area winter weather
advisory to noon, with no other forecast changes for today.

The latest HRRR indicates a start time of heavier snow between 11
AM and noon for the crest and in the higher elevations of the
Carson Range, especially north of Highway 50. Given warmer
temperatures and some areas of rain below 6500 feet, snow should
mainly affect roads near the crest and above 7000 feet in the
Carson Range for this afternoon (I-80, Hwy 50, and Mt Rose Summit).
The Tahoe Basin is still expected to have travel issues by this
evening as any rain changes to snow. -Snyder

&&

.SYNOPSIS...

A more active storm pattern is expected to bring a series of storms
to the Sierra and western Nevada today through the weekend. The
first moderate storm today into Thursday will bring snow, wind, and
very low visibility to the Sierra with wind shear impacts to
western Nevada. Another, colder storm is expected by Thursday
night with the potential for a strong storm for the end of the
weekend.

&&

.PREVIOUS DISCUSSION... /Issued 343 AM PST Wed Jan 18 2017/

SHORT TERM...

Primary changes to the short term forecast were centered around
increasing precipitation amounts in the Sierra. This resulted in a
bump in forecast snow especially west of Highway 89 for the Tahoe
Basin northward through Lassen County. Ultimately, this will be a
higher end Winter Weather Advisory with some locations west of
Highway 89 accumulating around a foot or more of snow.

The main concerns remain unchanged. Moderate to heavy snow from a
winter storm will create very low visibility with periods of
whiteout conditions in the Sierra starting this morning for
western Lassen/Plumas Counties. This system will spread down the
Sierra today bringing its impacts to the Tahoe Basin after 4 PM
and Mono County late this evening after 10 PM. As a split system,
liquid totals will be muted for western Nevada due to an
appreciable amount of shadowing - generally less than 0.2 inches
and in the form of mostly rain. Foothill locations could receive a
couple of inches of snow accumulations mainly above 5500 feet.
Fortunately, flooding concerns are much more limited. Still, there
should be some modest rises mainly on area creeks.

Winds will increase through the Sierra with gusts generally up to
40 mph for valleys and 110+ mph for the ridges. More locally,
gusts on and around Lake Tahoe, mainly northern and eastern
shores, could reach 50 mph today as the relative warmth of the
water aids mixing faster winds aloft to the surface. This will be
especially problematic when coupled with snow late this afternoon
and evening; whiteout conditions will be possible. Western Nevada
winds will be initially more subdued as stronger valley inversion
keep winds decoupled until the front moves through the region.
Expect wind shear and turbulence to be a factor for aviation; more
in the aviation section below. Once valleys mix out, likely by, with chain controls likely to pop
up in those areas. late afternoon/evening, wind gusts will be
consistent around 40-45 mph. Wind prone locations will experience
higher gusts, sooner, generally up to 55 mph. These locations
include Washoe Valley and Highway 395 from Doyle to Susanville.

There will be a brief break between wave Thursday before another,
colder system moves through the region Thursday night through
Friday night. QPF totals are lower, but snow ratios will be
higher. Additionally, instability appears to be higher allowing
for some precipitation to reach western Nevada. Most, if not all,
precipitation should fall as snow. Tahoe/Truckee could see another
6-10 inches of snow with 8-14 inches possible west of Highway 89.
Also, 2-5 inches could occur across western Nevada; Fallon may
miss most of this snow, however, since precipitation will likely
be mixed east of Highway 95 and South of Interstate 80.

LONG TERM...Saturday through Tuesday...

Minimal changes were made to the inherited forecast this cycle. Main
changes were to show a more defined break in pcpn over wrn Nevada
late Saturday and to increase pops for Sunday into Sunday evening
for most of the area.

Upper trough associated with the first system moves east on
Saturday. While the Sierra are likely to continue to receive light
snow...much of the area east of the Sierra should break out by the
afternoon and into the early evening. Thus...pops were lowered just
a bit over wrn Nevada for that time frame.

Models are still in general agreement with the next system moving
into the region starting late Saturday night from northeast
California down to the Tahoe basin then farther south and east
through the day Sunday. This remains a more dynamic storm than the
previous ones and we should be able to spill over into wrn Nevada
fairly easily through the day Sunday. Snow levels will be low enough
that all valleys receive snow...some may have fairly significant
snowfall for valley locations and warnings will likely be needed in
many locations.

QPF in this system is not much different than the previous shift
saw...but pops were raised Sunday and Sunday evening. The biggest
change is the GFS trending toward a slower exit of this system on
Monday. While still not as slow as the ECMWF...which the trough over
the area for much of the day...the GFS is holding better chances for
pcpn in the region through late Monday afternoon. GEFS ensemble
members are largely trending this way as well so pops were increased
through Monday.

The trough begins moving east late Monday night..but there could
still be some lingering snow showers into Tuesday...the morning
especially...so this was left as inherited. 20

AVIATION...

First band of warm advection clouds/pcpn is pushing through the
region this morning. Very light sleet was reported at KRNO and our
office so icing is a good possibility in this band. The main band of
pcpn starts to move in later today affecting the Sierra and
northeast California first. Ahead of this pcpn we should see gusty
surface winds of 30-35 kts and stronger winds aloft creating low
level wind shear and turbulence.

With this approaching pcpn comes lower cigs/vsbys with MVFR/IFR
conditions overspreading the region from west to east through the
evening and overnight hours. VFR is likely to hang on east of the
Sierra until early Thursday morning when pcpn finally spills over.

Conditions improve marginally late Thursday outside of the Sierra.
There MVFR conditions will remain. Another system moves in for
Friday with snow most areas. Then a stronger system starts to affect
the area late Saturday into Sunday with gusty winds and snow. 20

&&

.REV Watches/Warnings/Advisories...
NV...Winter Weather Advisory until 4 AM PST Thursday NVZ005.

     Lake Wind Advisory until 4 PM PST this afternoon for Lake Tahoe
     in NVZ002.

     Winter Weather Advisory until 4 AM PST Thursday NVZ002.

CA...Winter Weather Advisory until 4 AM PST Thursday CAZ070-071.

     Winter Weather Advisory from 7 PM this evening to 7 AM PST
     Thursday above 6000 feet in CAZ073.

     Lake Wind Advisory until 4 PM PST this afternoon for Lake Tahoe
     in CAZ072.

     Winter Weather Advisory until 4 AM PST Thursday CAZ072.

&&

$$

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


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