Heavy Snow in California, November 30, 2019

Area Forecast Discussion
National Weather Service Reno NV
235 AM PST Sat Nov 30 2019

.SYNOPSIS...

The best time to complete travel over the Sierra will be by early
this afternoon; however, chain requirements and road closures
remain possible so check road conditions. Snow develops in the
Sierra late this afternoon and continues into Monday so expect a
very long commute over the Sierra at the end of the holiday
weekend. This storm will also produce a mix of snow, ice, and rain
for Sierra valleys and out across western Nevada. After a breather
Tuesday, more precipitation is on the way mid-week and next
weekend.

&&

.SHORT TERM...

* Bottom line first: A chaotic forecast in terms of winter
  precipitation and large valley-by-valley differences in
  precipitation character/totals can be expected with the next
  storm. Expect difficult travel conditions and long waits
  especially on Sierra roadways.

It should be fairly obvious by now that a winter storm is expected
to bring moderate to acute impacts for much of the region
including the Sierra and valleys along the Sierra Front tonight
through Monday. If you have to travel, it would be best to
conclude it by early this afternoon. That said, Interstate 80 from
the Nevada State line to near Colfax, CA has been closed for many
hours starting Friday evening due to very icy conditions and
issues related to a large volume of trans-Sierra travelers. Our
best suggestion is to check road conditions prior to heading into
the Sierra.

Forecast Changes: Surface flow is trending more out of the south
adding support for a transition to a rain/snow mix for Sierra
Front locations Sunday. Usually this flow direction brings warmer
temperatures, but flow will not be very strong and there is long
fetch of colder temperatures southward. Deterministic NAM and GFS
have been favoring colder temperatures aloft with a moderate
depth isothermal layer. The result is not much change to the
previous forecast in absolute terms, but some movement on error
bounds, boom-bust potential, for certain locations.

After canvassing snow total reports and latest model trends,
locations that have a higher chance of exceeding the current
forecast snow totals include the Carson Valley, Virginia City,
and the Susanville. These locations have received the most snow
over the last few days, 5-10 inches, and will start colder than
other locations after strong radiational cooling this morning.

Meanwhile, the Reno Metro area has had a shift to lower end of the
forecast range and may under-perform since surface flow has
trended more out of the south and snow has largely melted from
many surfaces after yesterday's sunny skies. Precipitation
intensity and mechanical mixing will have larger impacts here.
Sierra locations remain largely unchanged, meaning that there is
an equal chance of over/under performance in terms of snow
accumulations. Precipitation intensity and actual warming aloft
will have the most influence on determining how much snow falls
for the Tahoe Basin and Truckee.

The mention of freezing rain was left in place since conditions
are being forecast right at the envelope of phase differences.
While current trends in deterministic runs favor rain, snow, or a
rain/snow mix, only a couple degrees of warming in warm-nose would
make all the difference and allow for freezing rain to occur for
western Nevada and the Susanville area.

Similarly, subtle changes in precipitation rates, depth of warm
layers, and temperatures will have large implications on phase
characteristics for each valley. It is quite possible that
precipitation will change from snow to rain, back to snow, to a
mix, and back again (with a little freezing precipitation)
through early Sunday before warming aloft is sufficient to allow
for more stable rain in valley locations Monday night and Monday.
One thing that can be said, when snowing, the character of the
snow will be very wet and should limit accumulations while
allowing for lousy traction where it does accumulate.

Conditions begin to improve slowly Monday and more rapidly over
Monday night as the circulation in the eastern Pacific retrogrades
and digs southward. Expect precipitation to taper down. GFS does
show an increase in precipitation chances for Mono County by
Tuesday morning over the previous model runs, but the forecast
only nudged very slightly in that direction since most model
suites favored a lowering trend. Boyd

Snow Forecast:

Sierra Front: Snow totals for Sierra Front valleys could run in
the 1 to 4 range, with perhaps a 40% chance for more than 6
inches, mainly in the Carson Valley and Virginia City, if the
colder, wetter scenario pans out. Some ice accumulation cannot be
ruled out at this time. This would probably be more of an issue on
elevated surfaces and areas that have been cleared of snow.

Sierra from Tahoe north: 4 to 10 inches of snow forecast below
7000 feet with an eventual transition to rain likely on Sunday. 1
to 3 feet (localized up to 4) is possible above 7000 feet with
winds gusting 35-55 mph (higher over exposed ridges).

Mono County: A few inches of snow are possible for valleys;
however, the main impact looks to be above 7000 feet for passes
along Highway 395. Above 7000 feet near and west of Highway 395, 1
to 3 feet of snowfall is possible through Monday.

Lassen/Plumas Counties: 4 to 8 inches along and east of Highway
395 with up to 18 inches possible west of Highway 395 above 5500
to 6000 feet. Ice accumulation cannot be ruled out for valleys at
this time, especially for the Honey Lake Basin. -Boyd/Snyder

.LONG TERM...Tuesday through Friday...

Changes in the long term part of the forecast were minimal and
mainly focused on the end of the forecast period. Tuesday sees a
brief break in precipitation for most areas as a weak shortwave
ridge tries to develop over the region. Upslope flow along the
western slopes of the Sierra could still produce very light snow in
the higher elevations just east of the Sierra crest.

A weak area of low pressure is still being modeled to pass just to
our south Wednesday. There remain differences in the model solutions
with some a bit farther north. A more northerly trajectory would
open our area up to more widespread precipitation. Given the
continued differences in the model forecast...we have opted to leave
this system much as inherited with a broad brushed area of slight
chance to low end chance pops for Wednesday and Wednesday night.

Now we get into the changes in the forecast. The deterministic model
solutions and some of the ensemble members have slowed the approach
of the next storm system. Thursday may now be a dry day that extends
into much of Thursday night. That would also mean a longer period of
warming which could drive initial snow levels higher with this
system. The GFS remains a little faster with the initial
precipitation Friday than the ECMWF. We have slowed the initial
precipitation east of the Sierra until later Friday and also bumped
up temperatures just a few degrees. This still looks like mainly a
mountain snow/valley rain system into early next weekend. It is a
relative fast mover...but it also has a fairly prominent moisture
tap early in its life cycle. So while total precipitation might not
be overly impressive...there could be about a 12 hour period next
Friday into Saturday with very high precipitation rates in the
Sierra.

.AVIATION...

VFR conditions early today will give way to deteriorating conditions
this afternoon into the overnight hours. Ridge top winds begin to
increase late this morning and reach into the 30-35 kt range by
afternoon with gusts topping 50 kt at times. This should lead to
increased turbulence above and downwind of most mountain ranges.

Snow moves into the Sierra this afternoon then into northeast
California this evening. Precipitation should spill into areas east
of the Sierra by late evening. IFR conditions develop in the Sierra
late in the afternoon with mountain obscurations. MVFR conditions...
and possibly some IFR conditions...move into areas east of the
Sierra through the evening and overnight hours.

The storm continues through Sunday and Monday before winding down
late Monday night. Snow levels may rise Sunday and briefly limit the
accumulations at the Sierra terminals...but overall accumulations of
6 to 12 inches are likely at KTRK/KTVL/KMMH and KMMH may see
significantly higher accumulations.

In the valleys east and northeast of the Sierra...a brief period of
freezing rain is possible Sunday...increasing the likelihood of low
level clear icing. Snow accumulations at the valley terminals should
be in the 1 to 5 inch range...except for areas across northeast
California where accumulations in the valleys may reach 4 to 10
inches.

There is a brief break Tuesday before another system passes just
south of the forecast area Wednesday. Another...larger...system is
lining up in the model forecast for the end of next week.

&&

.REV Watches/Warnings/Advisories...
NV...Winter Storm Warning from 4 PM this afternoon to 10 AM PST
     Monday NVZ002.

     Winter Weather Advisory from 7 PM this evening to 4 PM PST
     Sunday NVZ003.

CA...Winter Storm Warning from 4 PM this afternoon to 10 AM PST
     Monday CAZ071.

     Winter Storm Warning from 7 PM this evening to 10 AM PST Monday
     above 7000 feet in CAZ073.

     Winter Storm Warning from 4 PM this afternoon to 10 AM PST
     Monday CAZ072.

&&

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