Heavy Snow and Heavy Rain in Nevada, December 24 to 25, 2018

Area Forecast Discussion
National Weather Service Reno NV
320 AM PST Mon Dec 24 2018

.SYNOPSIS...

A moderate winter storm will move into the region today bringing
travel impacts this evening into Christmas morning due to rain and
snow. Gusty winds are anticipated this afternoon into Christmas as
well, with temperatures dropping to near normal. This will be
followed by much colder temperatures and periods of snow showers
later this week through next weekend.

&&

.SHORT TERM...

* A moderate winter storm will through the region today into
  Tuesday (Christmas Day). Expect travel difficulties in the
  Sierra and for adjacent valleys of western Nevada, especially
  this evening and overnight.

* Gusty winds are possible ahead and along the cold front late
  Monday.

A moderate winter storm moves into the region today with a strong
cold front shifting through the area this evening and overnight.
By Sierra standards, this is not a major storm, with 6-15" likely
for areas above 7000 feet, but the increased holiday traffic will
likely compound the impacts.

The timing of the cold front has sped up slightly in most recent
model runs. It is likely to shift through NE CA between 6 pm and
9 pm, through the greater Reno/Carson City metro areas into the
Central Nevada Basin and Range between 8 pm and 11 pm, and then
through Mono/Mineral counties between 10 pm and 1 am. Models also
still agree on a deformation zone forming setting up through
northeast California into extreme northern Nevada this evening.
This will likely lead to higher snow totals up to 6 inches north
of a Susanville-Gerlach line, especially in extreme northern
Washoe County.

Still, the biggest boom or bust potential in the forecast is snow
amounts in the 5500-6500 foot range, where totals could range
from an inch or two up to six inches. Decent mixing during the day
Monday will boost high temperatures, leading to higher snow
levels, but expect a rapid fall in snow levels behind the cold
front. The other question at this point is wet bulb temperatures
in this fairly moist air mass. With dew points anticipated to
already be in the low to mid 30s ahead of the front, it may lead
to precipitation remaining as rain or falling as a slushy
rain/snow mix. Several models are showing a burst of heavier
precipitation rates with the front (possible an NCFR could form),
which in itself could drag snow levels down faster. With all this
in mind, there may be a sharp gradient in snow amounts in a very
small difference in elevation and geographical location. For
example, we are currently expecting up to 2" for Truckee, but a
possible 6" for South Lake Tahoe with only a few hundred feet of
elevation difference between them.

Most of the precipitation will fall as rain across western Nevada,
with the cold air chasing the exiting moisture. Some higher
resolution models are still hinting at scattered showers forming
in the cold, unstable air mass behind the front which would bring
snow to lower valley areas. Minor accumulations are possible,
especially given the snow will fall overnight, though don't expect
much -- likely an inch or less. Higher elevation sites such as
the north valleys of Reno, foothills along the Carson Range, and
Virginia City could see 1-2" of snow overnight into Christmas
morning.

In addition to the rain and snow, winds will increase today,
though latest guidance is not as strong with wind speeds as
previously progged. Most gusts are likely to peak out in the 30-40
mph range, though a short burst of stronger winds are possible
along the cold front. Winds could bring travel restrictions for
high profile vehicles in addition to turbulence and wind shear for
aviation.

Cold temperatures and lingering snow showers will persist through
Christmas Day with the best chances for snow showers from central
Nevada eastward. Breezy northerly winds will continue during the
day on Christmas making it feel even colder. The cold northerly
flow across the relatively warmer lake waters could produced lake
enhanced snow showers downwind, south to southeast of Pyramid
Lake Tuesday, though confidence is low. Winds will be shifting
from north to east over Lake Tahoe, so the deeper unidirectional
flow needed for lake enhancement is less likely to set up.

Going into Wednesday, it will be drier with less wind, though the
seasonably cold air mass will remain in place. A weak shortwave
dropping out of the north could bring snow to areas near the
Oregon border Wednesday night, with additional snow possible
Thursday. For more details, please see the long term section
below. -Dawn

.LONG TERM...Thursday through New Year's Eve...

No significant changes were made to the long term forecast, although
for Thursday night-Friday we included snow shower chances downwind
of Tahoe and Pyramid Lake, when conditions may become favorable for
some lake-effect snow bands at times.

The main theme for the final days of 2018 will be brisk and chilly
conditions with a ridge of high pressure off the west coast opening
up the path for shortwave passages from the north. The first wave
looks to move through the region early Thursday, with a reinforcing
shot of colder air. Forcing along and behind the cold front could be
sufficient for short periods of heavier snowfall, otherwise the most
likely result is isolated-scattered snow showers with minimal
accumulations. As surface winds become north to northeast, and the
temperature difference between the lake water and the 700 mb level
is maximized, the potential for some lake effect snow bands south
and southwest of Tahoe and Pyramid Lake will increase from Thursday
night-Friday. A few inches could accumulate in areas where these
bands persist, but it's far too soon to specify the locations and
amounts for such a small scale event.

Also for Thursday night and Friday, portions of Mono County could
experience some increased snowfall amounts due to moist upslope flow
which may combine with some enhancement off Mono Lake. Gusty ridge
winds over the Sierra and increased east winds on Lake Tahoe are
also possible Thursday night through Friday. The magnitude of these
winds are less certain due to variance with the position of the
upper low and trailing ridge axis on Friday after the initial
shortwave passage.

After a break in snow shower activity Saturday, a second wave could
dive south across similar areas Sunday into Monday, but confidence
is lower for this possibility as there are also dry scenarios in
some of the medium range guidance. Since a mention of light snow
showers was already introduced for Sunday afternoon and night, we
won't change it for now, and this would most likely be followed by
another reinforcement of cooler temps for New Year's Eve. MJD

&&

.AVIATION...

South to southwest winds will be on the increase today, peaking
this afternoon and evening and turning west to northwest with a
cold frontal passage. This will bring turbulence, mountain wave
activity, and wind shear to the region. Wind gusts will primarily
be in the 20-30 kt range, but could briefly reach 35-40 kts just
ahead of the cold front this evening. This system will also bring
rain and snow along with potentially significant reductions to
ceilings and visibility. Widespread mountain obscuration should be
expected late this morning through Christmas Day.

The best chances for snow accumulation at KTRK/KTVL will be 02Z-06Z
Tuesday, but there is a high boom or bust potential for amounts at
both airports with possible totals ranging from 1-6". The most
likely scenario would put KTRK on the lower end of that range with
KTVL on the higher end of the range. For KMMH, snow accumulation
is most likely between 06-10z with 2-5" possible.

While 0.25" inches or so of precipitation is likely for
KRNO/KCXP/KRTS/KMEV, there is less confidence in snow accumulation
as it may wind up being mainly rain or a mix of rain and snow
until after 06-08Z Tuesday. By that time, precipitation may cut off
abruptly behind the front with the best shot for a light snowfall
(up to 1 inch) at terminals after midnight with any leftover snow
showers.

Lingering snow showers are likely on Christmas Day, but additional
accumulations are expected to remain minimal for most areas. There
could be areas that see more organized bands of snow showers but
confidence in where is low. At this time, the west-central NV Basin
and Range (including KLOL/KNFL/KHTH) appears to have the highest
chance. Winds will remain breezy and from the N-NE on Tuesday with
peak gusts reaching 15-25 kts. -Dawn

&&

.REV Watches/Warnings/Advisories...
NV...Winter Weather Advisory from 10 AM this morning to 7 AM PST
     Tuesday NVZ005.

     Winter Weather Advisory from 10 AM this morning to 7 AM PST
     Tuesday NVZ002.

CA...Winter Weather Advisory from 10 AM this morning to 7 AM PST
     Tuesday CAZ071.

     Winter Weather Advisory from 10 AM this morning to 7 AM PST
     Tuesday CAZ070.

     Winter Weather Advisory from 10 AM this morning to 7 AM PST
     Tuesday CAZ072.

&&

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