Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 202138

Area Forecast Discussion
National Weather Service Reno NV
238 PM PDT Tue Mar 20 2018


A period of light valley rain and higher elevation snow is expected
this evening and tonight. A much stronger atmospheric river type
storm pushes into the region Wednesday night through early Friday
morning. This will bring periods of heavy valley rain and heavy
Sierra snow along with gusty winds. The weekend will be colder with
snow showers possible down to all valley floors.



The first system to deal with tonight continues to indicate
enough warm air advection to generate light precipitation from the
Tahoe Basin northward this evening and tonight. There is a swath
of heavier precipitation indicated north of Highway 44 to about
the Lassen/Modoc County line. While soundings from the area do not
support snow much below 6000 feet north of is not
out of the question that higher intensity precipitation could
briefly drag the snow levels lower and create some minor
accumulations north of a line from Susanville to Gerlach. In the
Tahoe area snow levels should remain mostly above 7500 feet.

The other part of tonight`s weather is the initial push of the
deep moisture plume into Mono/Alpine counties. This will generate
mainly upslope flow as upper level forcing is minimal. Snow levels
will be around 8000 feet or higher with QPF generally around 0.20
inches or snow accumulations are not expected to be
significant except near the Sierra Crest.

There may be some very light warm air advection precipitation in
western Nevada tonight...but the main event for there will be

The models have continued trending rather dry for areas east of
the Sierra Wednesday as a defined break develops. Continued
upslope flow in the Sierra and increasing forcing from an
approaching trough across northeast California will mean continued
chances for light rain and high elevation snow through
Wednesday...but accumulations are expected to be light.

Gusty winds develop Wednesday as well...but they are not as strong
as we earlier thought. This is mainly due to decreased flow aloft
and the jet retreating to the north a bit more.

The jet retreating north has two other effects: decreased chances
for spill over in western Nevada and increased QPF potential in
the Sierra and northeast California.

Much like yesterday...the model solutions are displaying a
coupled jet structure by Wednesday evening into Thursday that
stretches in a south-southwest to north-northeast orientation
through the Sierra into northeast California. That feature...and
its orientation...means increased potential for higher QPF as well
as slightly higher snow levels. We could see 2 to 3 inches of
rain in western Lassen...central Plumas and Sierra counties...
and below 7500 feet in the Tahoe Basin. In excess of 3 inches is
possible in Mono County below 7500 feet. That leads to increased
runoff potential so we have issued Flood Watches from northeast
California south through the Tahoe Basin into Mono County.

The jet orientation also leads us to believe spill over potential
will be a bit more limited so QPF was reduced slightly in the
valleys of western Nevada. Make no mistake...western Nevada will
still get rain Thursday...just maybe not as much as we initially

As the upper low edges closer Thursday evening and overnight...
cold air filters into the region. Snow levels will fall by early
Friday morning...but by then most of the moisture will have pushed
to the south. The best snowfall potential remains above 7000-7500
feet in the Tahoe Basin and above 7500-8000 feet in Mono County.
Multiple feet of heavy wet snow is likely in both locations so
winter storm watches were upgraded to warnings.

.LONG TERM...Friday through next week...

By Friday, a brief break in precipitation is expected although
breezy conditions are likely in advance of the next storm system.
This system is weaker than the Wednesday-Thursday storm, but air
mass will be cold enough for precipitation to fall as snow in lower
elevations Friday night, with the best snow chances in western NV
along the US-395 corridor and north of I-80, and in eastern CA from
Tahoe northward. Preliminary snowfall projections for Friday night:
2-6" for lower elevations of northeast CA/far northwest NV/Tahoe
basin, with up to 10" possible near the Sierra crest. For far
western NV, up to 1" or so could fall around Reno-Carson-Virginia
City by early Saturday morning.

For this weekend, yet another shortwave rotates around the main low
as it moves inland across northern parts of CA/NV, keeping a chance
for snow showers with temperatures remaining well below average
(highs mainly in the 40s in lower elevations/mid-upper 30s near the
Sierra). While snow accumulations will generally be limited and
sparse (especially during the daytime), increased instability
associated with the upper low/cold pool aloft could lead to bands of
heavier snow showers Saturday evening/night in parts of northeast
CA, the Tahoe basin, and even into the urban areas of far western

From Sunday night through much of next week, upper level ridge
building off the west coast will likely shut down the storm door for
the Sierra and western NV. Temperatures will begin a gradual warming
trend, although light northeast flow with limited mixing will keep
highs below average at least through Tuesday. MJD



Flying conditions deteriorate tonight through Thursday as a
significant storm makes its way into the forecast area. Ceilings
lower tonight as light precipitation develops north of the Tahoe
Basin and also south into Mono County. Some lowering of
ceilings...but still possible east of the Sierra tonight.

Winds increase ahead of the main system Wednesday...especially
across western Nevada. Gusty winds over the ridges should create
turbulence while southerly low level winds increase for the
terminals east of the Sierra. Low level wind shear is likely at
KRNO by Wednesday afternoon due to directional shear.

Ceilings and visibilities lower further Wednesday evening and
overnight as the main brunt of the rain associated with this
storm begins pushing into the Sierra and northeast California;
MVFR and occasional IFR conditions are likely through Thursday
afternoon before snow levels start to fall. Lower ceilings are also
likely in western Nevada Thursday as spill over rain develops.

Falling snow levels will mean widespread IFR conditions in the
Sierra late Thursday night into early Friday with some slushy
accumulations on the runways of the Sierra terminals. A weak
system Friday night into Saturday could bring snow showers to the
western Nevada valley floors.


.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory from 8 AM to 5 PM PDT Wednesday for Lake
     Tahoe in NVZ002.

     Winter Storm Warning from 5 PM Wednesday to 5 AM PDT Friday
     above 7000 feet in NVZ002.

     Flood Watch from Wednesday evening through late Thursday night
     below 7000 feet in NVZ002.

     Lake Wind Advisory from 11 AM Wednesday to 8 PM PDT Thursday for
     Pyramid Lake in NVZ004.

CA...Flood Watch from Wednesday evening through late Thursday night

     Flood Watch from Wednesday afternoon through late Thursday night
     below 7500 feet in CAZ073.

     Winter Storm Warning from 5 PM Wednesday to 5 AM PDT Friday
     above 7500 feet in CAZ073.

     Lake Wind Advisory from 8 AM to 5 PM PDT Wednesday for Lake
     Tahoe in CAZ072.

     Winter Storm Warning from 5 PM Wednesday to 5 AM PDT Friday
     above 7000 feet in CAZ072.

     Flood Watch from Wednesday evening through late Thursday night
     below 7000 feet in CAZ072.



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