Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 212013

Area Forecast Discussion
National Weather Service Reno NV
113 PM PDT Wed Mar 21 2018


An atmospheric river type storm will produce significant
precipitation tonight into Thursday. This will bring periods of
heavy valley rain and heavy high Sierra snow along with gusty
winds. The weekend will be colder with snow showers possible down
to all valley floors. Drier conditions are looking more likely for
next week.



Early Wednesday afternoon saw a bit of minor break between the
precipitation associated with warm air advection Tuesday night
into early Wednesday and the incoming main portion of the storm.
That portion will impact most of our area this evening through
Thursday night...but a few areas are already seeing the initial
precipitation with this part of the storm. The warm air advection
part of the storm performed better than expected in terms of
rainfall production and the break today was not as defined as we
thought it would be. Thus we have already seen more precipitation
than we expected this early.

That brings us to tonight and Thursday...and what to expect going
forward. Model solutions still pull the main upper level jet north
with increased upper level divergence from Mono County north into
northeast California. That leads to increased QPF again with
slightly higher snow levels. These adjustments were made for
tonight and early Thursday. There is still the question of how
much precipitation will spill over with the jet oriented more
south to north tonight. It does start to push east Thursday and
that puts western Nevada in a more favorable area for increased
QPF as instability increases through the day.

We did consider adding slight chance for thunderstorms to Thursday
as instability increases. But there is a lack of cold air aloft
until late in the day. The best potential for thunder looks to be
in the Sierra and northeast California. We will hold off including
it for now...but a few isolated rumbles of thunder are not out of
the question.

So the main points for the next 36 hours are:

- The total amount of liquid precipitation has been raised with
  the highest totals...approaching another 4 1/2 to 5
  inches...along the Sierra Crest. Another 2 1/2 to near 3 inches
  is possible near Lake Tahoe with around 3 inches possible west
  of Highway 395 in Mono County below 7500 feet. Lassen and Plumas
  counties could see from another half inch in the far eastern
  parts of the counties to as much as 2 1/2 inches in the western
  areas. Western Nevada could pick up from around half an inch to
  as much 1.30 inches in the western foothills.

- Snow will be confined to areas above 7000 feet due to high snow
  levels. Snow levels start well above 7500 feet in general and
  remain there until Thursday morning when cold air starts to push
  in from the north. By the time the snow levels fall below 7000
  feet late Thursday most of the precipitation will have ended.
  Totals above 7000 feet in the Tahoe Basin could reach 2 to 4
  feet...near 5 feet at the crest. In Mono County...totals above
  7500 feet should range from 2 to 4 feet with totals in excess
  of 5 feet near the crest.

- Winds will not be as big a factor. Gusts up to 40 mph are still
  possible into early this evening...but once the rain spills
  over in earnest...the winds should decrease in Western Nevada.

Winter Storm Warnings remain in effect for the higher elevations
of the Tahoe area and Mono County with a Lake Wind Advisory into
Thursday for Pyramid Lake. Flood Watches also remain in effect.
The Hydrology section will cover that a bit more.

.LONG TERM...Friday onward...

No big changes with the long term forecast.

Model simulations still show a break in precipitation until Friday
afternoon and evening ahead of a cold front and the next storm.
Winds will increase ahead of the cold front with choppy conditions
on area lakes and possible travel concerns for high profile vehicles
along with turbulence for aviation. Precipitation chances will
remain north of Interstate 80 through Friday afternoon, but will
spread southward as the cold front advances toward the I80 corridor
by Saturday morning. Snow levels will be down to all valley floors
with light snow accumulations possible by sunrise on Saturday,
especially north and along the I80 corridor.

Another shortwave drops through the Sierra and western Nevada for
Saturday evening into Sunday morning. Some timing differences still
exist with this final shortwave, but generally anyone traveling over
the weekend should plan on snow showers and travel slow downs,
especially across the Sierra passes. Considering we are getting into
the latter part of March, most road impacts from these lighter snows
will be limited to the overnight and early morning hours outside of
higher elevation mountain passes.

A ridge begins to build off the west coast next week leading to a
drying trend. Cool, northerly flow will dominate the first portion
of the week until temperatures gradually moderate to near or
slightly above normal by the end of the week. -Edan



IFR conditions should become widespread this evening and overnight
in northeast California and the Sierra as a significant storm
spreads high elevation snow and lower elevation rain across the
forecast area. By late tonight and early Thursday MVFR conditions
are likely in western Nevada with brief IFR possible in the
heaviest rain.

Gusty winds of up to 40 mph are possible early this evening...but
as the heavier rain spills into western Nevada the winds should
subside. Still...there will continued to be strong winds aloft
with turbulence along and east of the mountains.

Snow levels in the storm remain quite high until Thursday evening.
By late Thursday night and early Friday morning...a few inches of
slushy accumulation are possible at the Sierra terminals.

After a brief break Friday...another fast moving system could
bring snow showers down to the valleys floors by Saturday
morning. Stronger winds are possible Friday ahead of this system
with gusts possibly as high as 45 mph in the valleys north of
Highway 50. A more substantial storm is shaping up for Sunday and
Sunday night with snow possible down to the lowest valley floors.



Rain Tuesday night into Wednesday morning was more than expected.
This should aid in ripening the snow pack in the Sierra valleys
and increasing soil moisture in the lower valleys and western
Nevada and northeast California where snow is not present.

Expected heavy rain later tonight and into Thursday...with
relatively high snow levels...should increase the runoff
potential. We have already seen some rapid responses on small
streams in basins that picked up quite a bit of rain last night.

Responses should be magnified tonight and Thursday...especially in
the basins that already received heavy rain. This includes the
Sierra below 7500 feet and northeast California along with
streams that drain the east side of the Carson Range in western
Nevada. The other challenge in this situation is heavy rain
ponding or redirecting runoff in areas where there is sufficient
snow to block storm drains. In these areas...the melting portions
of the snowpack will also increase runoff. Recently burned areas
will be another place to watch if the rain intensity becomes very
heavy for an extended period of time. The only mainstem rivers of
concern right now are the Susan River and Middle Fork Feather
River. Both of these should see mostly rain in their headwaters.

We will keep Flood Watches in effect for northeast California and
parts of the Sierra below the snow line. We will this
time...include western Nevada...but it is not out of the question
to see rises on small streams that cause minor flooding in areas
adjacent to the small streams. We could see some minor ponding in
poor drainages areas around the urban centers and low water
crossings in western Nevada as well.


.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory until 5 PM PDT this afternoon for Lake Tahoe
     in NVZ002.

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

     Flood Watch from 11 PM PDT this evening through late Thursday
     night below 7000 feet in NVZ002.

     Lake Wind Advisory until 8 PM PDT Thursday for Pyramid Lake in

CA...Flood Watch from 11 PM PDT this evening through late Thursday
     night CAZ071.

     Flood Watch from 5 PM PDT this afternoon through late Thursday
     night below 7500 feet in CAZ073.

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

     Lake Wind Advisory until 5 PM PDT this afternoon for Lake Tahoe
     in CAZ072.

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

     Flood Watch from 11 PM PDT this evening through late Thursday
     night below 7000 feet in CAZ072.



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