Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 240243
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
643 PM PST Tue Jan 23 2018

.UPDATE...
Quiet so far this evening with the cloud cover beginning to
increase and lower ahead of the next storm. Winds for tomorrow
look good, but made some adjustments to the statements for
tomorrow night into Thursday.

Added the Sierra Front to the Winter Weather Advisory for tomorrow
night with the frontal passage. The thinking from earlier has not
changed much, but with higher confidence in 1-3 inches of snow for
the foothills and Carson Valley as described below. Much of the
precip in the lee side looks to be right along the front with a
little more post-frontal than we have seen. Snow levels still look
to be 4500-5000 feet on average limiting much of Reno-Sparks to
accumulations of a skiff to 1 inch at best.

However, with higher elevations of the Carson Valley and a more
favorable spillover profile there in SSW flow, a solid 1-3 inches
is looking more likely. These amounts are also possible in the
western foothills of Reno around Galena and Caughlin Ranch.
Another concern is that even with only a rain/snow mix in the
lower valleys, very cold air is coming in behind. This could
result in wet roadways freezing creating very slick conditions
overnight and for the early portion of the Thursday morning
commute.

For the Sierra, everything is on track for the front tomorrow
night. However, after a 6 hour break Thursday morning, another
wave moves in on the backside. There will be favorable jet
dynamics as well as an unstable profile up through 500 mb.
Moderate to heavy snow showers are likely to develop again from
CA Highway 88 north to CA Highway 36 in the Sierra Thursday
afternoon and evening for another burst of snow. Amounts with the
snow showers could approach those seen along the front tomorrow
night.

Updates to the winter weather statement are out. Grid updates out
within the next hour. X

&&

.PREVIOUS DISCUSSION... /Issued 310 PM PST Tue Jan 23 2018/

SYNOPSIS...

One more colder storm system will impact the region Wednesday
into Thursday morning. This storm will bring strong winds,
moderate snow amounts to the Sierra, and lighter snow for lower
elevations. Expect winter travel impacts Wednesday evening through
Thursday. Drier conditions return for next weekend with mild
temperatures.

SHORT TERM...

A winter storm is expected to bring periods of moderate to heavy
snowfall in the Sierra Wednesday into Thursday. Strong and gusty
winds will be the main impact for western Nevada with chances for
snow showers by Wednesday night. Anyone with travel plans through
the Sierra late Wednesday into Thursday should expect winter
driving conditions and possibly extended delays. A few snow
showers will linger mainly across the Sierra Thursday afternoon
and evening before ending Friday morning.

WINDS:

Winds will increase across the Sierra ridgetops tonight with gusty
conditions beginning to surface across the Tahoe Basin tomorrow
morning and western Nevada valleys in the afternoon. The strongest
winds will be concentrated through the Highway 395/I580 corridor
area from Carson Valley up to near Susanville. Gusts in these
locations could reach 50-60 mph. Wind prone locations such as
Washoe Valley and the stretch of Highway 395 from Cold Springs to
Honey Lake could see gusts reaching in the 70-80 mph range.

Peak winds are expected Wednesday afternoon and evening before
winds begin to weaken Wednesday night. Travel restrictions can be
expected with crosswinds creating favorable conditions for
overturning high profile vehicles and semis. Boating is not
advised with dangerous conditions expected. Localized tree and
fence damage is also possible.

SNOWFALL:

*Tahoe into Lassen County: Snow levels will be low from the onset
 and generally below 5,000` across the Sierra and NE California
 Wednesday afternoon. Could see accumulating snow on Fredonyer and
 Yuba Passes by 1pm with snow reaching Donner Summit after about
 4pm. The heaviest snow will be roughly 5pm-Midnight. High winds
 combined with heavy snow could produce periods of zero visibility
 in white out conditions.

 Snowfall amounts through Thursday morning will be around 4 to 8
 inches between 5500-7000 feet (including around the Tahoe
 Basin). For elevations above 7000 feet from Tahoe north, expect
 heavier snowfall totals around 8 to 14 inches.

*Alpine & Mono Counties: Initial snowfall looks to after about 5pm
 in Alpine with snow moving through Mono County through after
 about 9pm. Heaviest snow should arrive after 8pm in Alpine county
 and about 10pm to 2am through Mono County. Overall snowfall
 amounts look less with 3 to 6 inches along and west of US-395,
 with 5 to 10 inches along the Sierra crest.

*Western Nevada: A mix of spillover rain and snow could begin
 after 7pm Wed evening with snowfall possible for areas above
 5,000-5,500`. Snow levels are expected to fall to the valleys
 floors after a few hours later with snow showers providing some
 light accumulations.

 Looking up to around 0.5-1.0" for most locations with higher
 chances for about 1-3" in foothill areas and south through
 Carson Valley. Roads could become icy Wednesday night into
 Thursday morning so give yourself extra time on your Thursday
 morning commute. Fuentes

LONG TERM...

Daytime high temperatures are expected to warm around 10 degrees
for the weekend as high pressure begins to build over the
southwest U.S. Moisture moving into northern California will keep
skies partly to mostly cloudy for the weekend with moderate
breezes Saturday, especially in the mountains.

The storm track for the weekend has shifted slightly south with a
good amount of moisture moving through northern California. In the
Sierra, from Tahoe north, into NE California and NW Nevada could
see some snow showers continuing early Saturday, but total
accumulations are expected to be very light. By Sunday the storm
track shifts farther north into Washington. A trough digging into
the Great Basin midweek will bring the chance for a slider type
system to clip the region, though little to no precipitation is
currently expected with this system.

For more on the building ridge along the west coast and outlook
for precipitation check out the Week 2 Outlook below. -Zach

Week 2 Outlook...Jan 31st-Feb 6th...Issued 3pm 1/23

Another period of dry weather is looking more and more likely
along the west coast for the first part of February. Something the
meager snowpack here in the Sierra doesn`t need.

Long range ensemble models and short term climate models are in
good agreement of a ridge building off the U.S. west coast as a
trough moves through the Midwest and begins to amplify in the
central and northeastern states. This trough along with a trough
deepening in the central Pacific will help to lock the ridge in
the west in place with a dry weather pattern for the west.
Atmospheric River tools show the next AR for the end of January
impacting the Pacific Northwest with the storm track continuing to
move north after that. In this pattern we can still get systems to
slide in from the north but these kind of storms generally don`t
bring significant amounts of moisture to the region.

Another factor that could come into play is a building Madden-
Julian Oscillation event. A currently active MJO currently over
southeast Asia is forecast to move into the West Pacific in early
February. If this occurs, and signal remains strong it would
provide an additional teleconnection and support with dry
conditions along the west coast and potentially. This could
reinforce the ridge along the west coast and extend the period of
dryness past the first week of February.

It`s impossible to say how long this pattern will last, but for
now all signs are pointing to at least a dry start for February.
For now, we`ll just have to wait for winter to return. -Zach

* The Week 2 Outlook discussion is an experimental part of the Area
  Forecast Discussion. This product is designed to highlight the
  potential for significant pattern changes beyond day 7 and will
  not be issued on a daily basis.

AVIATION...

VFR conditions with light winds today. A moderate, but cold,
winter storm will impact the region Wednesday morning through
Thursday morning.

Winds will start to pick up late tonight along Sierra ridges and
mix to the surface Wednesday morning through the evening. Ridge
winds 100-120 mph will likely create turbulence in the lee of the
Sierra and could create a period of low level wind shear. The most
likely period for wind shear near the surface will be early
Wednesday morning when surface winds will be light from the south
and more southwesterly ridge winds start to mix down.

Snowfall is expected at Sierra terminals with a relatively short,
2 to 4 hour period of moderate to heavy snowfall between 3-9Z
Thursday. Total accumulation for KTVL/KTRK of 3-8" are possible, with
up to 2" for KMMH. For KRNO/KCXP the most likely scenario is for
0-1/2" of snowfall. The latest simulation show the possibility for
a short period of rain Wednesday evening that could flash freeze
on runway surfaces Wednesday night as colder air moves in. These
events are challenging to forecast so forecast confidence is
fairly low, putting the probability of a light, 0.05" of rain
freezing on the ground is about 30%. Light snow accumulations of
1/2" or less will remain possible through Thursday afternoon.
-Zach

&&

.REV Watches/Warnings/Advisories...
NV...Wind Advisory from 10 AM to 10 PM PST Wednesday NVZ003.

     Lake Wind Advisory from 7 AM to 4 PM PST Wednesday for Lake
     Tahoe in NVZ002.

     Winter Weather Advisory from 4 PM Wednesday to 4 AM PST Friday
     NVZ002.

     Winter Weather Advisory from 7 PM Wednesday to 4 AM PST Thursday
     NVZ003.

     Lake Wind Advisory from noon to 10 PM PST Wednesday for Pyramid
     Lake in NVZ004.

CA...Winter Weather Advisory from noon Wednesday to 4 AM PST Friday
     CAZ071.

     Wind Advisory from 10 AM to 10 PM PST Wednesday CAZ073.

     Lake Wind Advisory from 7 AM to 4 PM PST Wednesday for Lake
     Tahoe in CAZ072.

     Winter Weather Advisory from 4 PM Wednesday to 4 AM PST Friday
     CAZ072.

&&

$$

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



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