Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 180942

Area Forecast Discussion
National Weather Service Reno NV
242 AM PDT Sun Mar 18 2018


A warming and drying trend will ensue for Sunday through Tuesday.
The next storm system is forecast to bring periods of valley rain
and heavy Sierra snow along with strong winds Wednesday and
Thursday. Conditions will turn cooler late week as a cold front
moves through northeast California and western Nevada.



Patchy freezing fog and/or low stratus clouds have formed through
several valleys this morning leading to highly variable visibility.
If roads look wet, it may be light ice deposits, so between this and
the lower visibility, allow extra time and slow down. Otherwise,
brief shortwave ridging builds over the west today into Monday with
increasing high level clouds and warming temperatures through
midweek. Warm air advection could bring light precipitation over
northeast California and northwest Nevada on Tuesday morning. With
a mild air mass in place, any snow impacts should be limited to
the higher elevations.

Greatest concern in the forecast continues to be the system for the
late Tuesday through Thursday time frame. There are two features of
note, the first being an atmospheric river (AR) moisture tap late
Tuesday into Wednesday and the second being a colder trough
Thursday. AR detection tools continue to indicate the deepest
moisture heading into central and southern California, bringing the
greatest impacts to Mono County south. However, much of the Sierra
and northern California are within the ensemble plume for the
northern edge of the AR moisture. The trend for this AR has been
southward, and how far north the moisture makes it will likely be
dependent on the timing and location of the upstream trough and how
the subtropical jet and polar jet interact.

At this point, feel we will get into the warmer wetter portion of
the storm Wednesday, followed by the colder trough and moisture from
that storm Thursday. The warmer portion of the storm could bring
snow levels up to 6500-7500 feet for the central and northern Sierra
and 7500-8500 feet for the southern and eastern Sierra. This is
about 500 feet higher than previously forecast with the duration
possibly lasting around 18 hours for the higher snow levels. Snow
levels look to fall Thursday as the upper trough moves inland and
pushes a cold front through the region. Snow levels have the
potential to reach valley floors by Thursday night, however it is
still questionable how much moisture will be around at this point.

While there are still uncertainties on how the two parts will
interact, what can be said is periods of heavy mountain snow look
likely along with valley rain late Tuesday through Friday. Mono
County is likely to see the greatest boom or bust snow potential
from subtle shifts in this moisture plume as much of the
precipitation for this region will be dependent on the AR moisture.
For the central and northern Sierra, there is a bit less spread in
the precipitation scenarios, with the biggest question being exactly
what the snow levels will do between the two features. How much
travel will be impacted will be dependent on exact snow levels and
how long of a duration there may be accumulating snow along Sierra
passes. A period of stronger winds also looks probable Wednesday and
especially Thursday which may bring travel restrictions for high
profile vehicles, choppy conditions on area lakes, and turbulence
for aviation interests.

While not a slam dunk, there is better support for a shortwave to
bring additional snow to areas mainly along and north of I-80 late
Friday night into Saturday. With plenty of cold air in place and the
current overnight forecast timing, it could lead to snow
accumulating on valley floors in these areas. Another wave could
keep chances for snow in the forecast going into Sunday, though
there is greater model differences with this feature. Longer range
outlooks are hinting at a warming and drying trend going into the
following week. -Dawn



This rest of this morning, there is a ~30% chance for freezing fog
and/or low stratus clouds to develop in valley areas, including
Martis Valley (including KTRK), the Sierra Valley and out into the
Reno-Sparks area (including KRNO/KRTS). Elsewhere, expect VFR
conditions today with light winds as high pressure begins to build

The next potent storm may arrive as soon as late Tuesday-Tuesday
night. JCM


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