Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 172227

Area Forecast Discussion
National Weather Service Reno NV
327 PM PDT Sat Mar 17 2018


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



Isolated to scattered snow showers have developed as expected this
afternoon as some heating has combined with cold air aloft to
bring instability for shallow convection. So far no lightning has
been detected; however, very isolated lightning (most likely in-
cloud) cannot be ruled out north of Highway 50 near the upper
trough axis through around 5 PM. Only the very heaviest and most
persistent snow showers have the potential to bring brief slushy
accumulations to higher elevation (above ~7000 feet) roads east of
the Sierra crest this afternoon and evening.

Tonight, copious snow cover (adding moisture to air as it melts in
the day) in a few lower valleys (including Reno-Sparks and the
Sierra Valley) of northeast CA and western NV and some clearing
brings an increased threat for fog. While convective clouds
(cumulus) should decrease in coverage tonight as daytime heating
is lost, it is far from certain that skies will clear sufficiently
to allow fog development. If skies clear, the chance for fog over
the snow cover will increase substantially. We have thrown in
patchy fog for now late tonight and Sunday morning to cover the

Sunday through Tuesday morning, dry weather will prevail with a
warming trend under a modest ridge. By later on Tuesday and
Tuesday night, simulations show possible warm air advection (WAA)
precipitation over northeast California and northwest Nevada as
the upper ridge axis shifts east. With some agreement among the
GFS/EC (last two runs) and NAM, we have increased precipitation
chances for northeast CA and northwest NV. However, we have not
gone overly high yet as the target of the WAA could shift some.
As far as impacts from snowfall Tuesday night, at this time they
look limited to the higher elevations with the fairly mild and
moist airmass ahead of the WAA unlikely to bring much wet bulbing
to drag down snow levels. -Snyder

.Long Term...Wednesday through Friday...

Another week and another potentially significant storm on tap for
the Sierra. Similar to the last storm though, there is a large
boom/bust potential in the long term forecast this week (last
storm we boomed). Again we have cold air moving out of the Gulf
of Alaska and potentially phasing with warm moisture from

What is different this time is that the moisture from the
subtropics is forecast to move through southern California as a
moderate atmospheric river, stronger than this last week. As the
jet moves into California, model simulations show a more southerly
flow with the Sierra spending more time on the warm side of the
jet this time, meaning snow levels will likely start higher and
come down slower.

One of the other big differences is that the storm, and
especially the cold front, is forecast to move through in one
shot, at least through Friday. This past storm sent an initial
cold front through the region that allowed the second wave to
start with much colder temperatures aloft, helping to drive down
snow levels and create better instability. The GFS is starting
show a second wave over the weekend that would be very cold again
with better chances for snow down to the valleys.

Recent runs of both the GFS/EC have slowed the onset of cold air
moving into the region and don`t allow the cold air to penetrate
as far south during the storm. But, both still do show another
shot of pretty impressive cold air moving into the region by end
of the week and a good tap of subtropical moisture in southern
California. Impact-wise, another decent shot of snow is expected
for the Sierra mid to late week. For western Nevada, it`s more
likely we`ll see rain, changing to snow by the very end of the
storm, with wind more likely ahead of the system. -Zach



Snow showers with areas of terrain obscuration should diminish
greatly this evening as daytime heating for the showers is lost.
There is a very small chance (5%) for lightning (most likely in-
cloud) with the showers near and north of Highway 50 through 5 PM
or so. Snow showers through this evening are not expected to bring
runway/tarmac accumulations given very warm ground temperatures.

Tonight, there is a ~30% chance for freezing fog after midnight in
lower valley areas that still have a few inches of snow on the
ground, including the Sierra Valley and out into the Reno-Sparks
area (including KRNO/KRTS). This chance will rise substantially if
skies clear completely but confidence in this is low.

Sunday will bring VFR conditions (after ~8 AM in areas that get
fog) with light winds as high pressure begins to build overhead.


.REV Watches/Warnings/Advisories...


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