Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 222225
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
325 PM PDT Mon May 22 2017

.SYNOPSIS...

Warm temperatures will increase snow melt this week, leading to
flooding near some creeks and streams flowing out of the Sierra.
The Walker River through the Mason Valley and Yerington is expected
to reach minor flood stage Wednesday then cresting just below
moderate flood on Friday. Some minor flooding is also likely for
the upper portions of the Carson and Truckee River systems.

&&

.SHORT TERM...

There has been a significant change this morning in the forecast
for the mainstem Walker River, affecting the Yerington and Mason
Valley area. The forecast has now been adjusted down from major
flooding down to minor/moderate flooding. Please see our updated
hydrology section below for more details.

Otherwise, there have been only minor changes to the forecast. We
did increase temperatures a bit for tomorrow, as the ridge axis
pushes over, tomorrow will be the hottest day across the region.
This will push some locations in western Nevada to over 90
degrees. With the well above normal temperatures across the
region, this will also get some good instability and convergence
over the Sierra for come afternoon buildups. There are some
cumulus buildups over the Sierra today, but with subsidence
inversion capping, we do not expect any thunderstorms for today.

By tomorrow, we will see slight cooling aloft as weak low
pressure off the CA coast pushes slowly inland. Thunderstorms were
largely removed last night from the forecast for Tuesday, but
there is still a slight chance we could see a few isolated
thunderstorms along the Sierra Crest in the late afternoon and
evening, along with buildups as far north as South Lake Tahoe.
Thunderstorm chances increase even more on Wednesday as a weak
vorticity maxima moves over the region, helping to destabilize the
atmosphere. Isolated thunderstorms will be possible on Wednesday
in Mono-Mineral counties, as well as to the north over the Lassen
convergence zone.

Winds will increase Wednesday afternoon with gusts up to 30-40 mph
in the afternoon ahead of the front. Choppy conditions are likely
on area lakes Wednesday afternoon, especially for Pyramid Lake
late in the day. Hoon

.LONG TERM...Thursday through Sunday...

Another weak low pressure wave will brush by the region on Friday
which will help moderate temperatures for the end of the week.
However, the moderation in temperatures may only bring high
temperatures back to around average for a day or two before another
warm up this weekend.

The ridge is expected to build back into the region over the
weekend, potentially amplifying significantly for a few days as it
moves over the area. This would allow temperatures to again quickly
rebound to 5-10 degrees above average by the weekend, and maybe even
higher going into early next week. Low temperatures in the upper
elevations may come close to freezing Friday night, but this might
be the only day upper elevation temperatures approach freezing in
the near future. There will also be some possibilities of
thunderstorms in the long term, but it`s a little too far out to
nail down which day may have the highest chances.

As the ridge amplifies early next week it will weaken allowing a
split flow or potentially a trough to return to the west coast.
While it does look like the ridge of high pressure will not get
stuck over us for an extended period of time, sustained above
average temperatures over the weekend and into next week will again
raise concerns for snowmelt flooding. -Zach


&&

.AVIATION...

VFR conditions with light winds today and a light late day zephyr
wind on Tuesday. A few flat cumulus have developed near the
mountains this afternoon. Increased instability tomorrow will allow
more significant cumulus development with a few thunderstorms
possible along the Sierra crest, with the best chances around Mono
and Alpine counties. -Zach

&&

.HYDROLOGY...

Warm temperatures through the middle of the week will accelerate
snowmelt in the Sierra. The increased snowmelt will produce varying
levels of impacts next week. Anything from minor and nuisance
flooding of small creeks and streams to minor/moderate flooding
for the mainstem Walker River through the Mason Valley.

Make sure to check the most recent forecasts and observations if you
live near a stream or river or plan to hike or camp near any streams
or rivers. Remember that snowmelt flows can be highest in the
evening and overnight in the smaller streams.

WALKER RIVER:

* Walker River forecast has been lowered to minor/moderate
  flooding. Major flooding is no longer expected. Current
  projections have a peak flow just below moderate levels, near
  10.8 feet or near 2500 CFS. Minor flood stage will be reached
  Wednesday morning and flow will peak out late Friday.

* There are several reasons for the updated forecast: less
  response form the snowmelt over the last couple days, new
  emergency gages that have been installed near Topaz allowing for
  more precise management and forecasting of the West Walker
  River, change in anticipated releases from Bridgeport Reservoir,
  and slightly cooler temperatures expected for the weekend.

* Minor flooding is possible on the East and West forks of the
  Walker River. Minor flooding is already occurring on the West
  Walker River just downstream of Wilson Canyon. Areas potentially
  affected include Antelope and Smith valleys. Higher flows are
  also eventually likely below Weber Reservoir but the expected
  extent is unknown at this time.

TRUCKEE RIVER:

* Minor flooding impacts possible from Lake Tahoe to Truckee. Most
  of the minor flooding impacts will be confined to areas right
  along the river which may impact some riverfront yards, basements,
  campgrounds, and bike paths.

* The flows along the remainder of the Truckee River will remain
  high, swift, and cold! These conditions can always be dangerous
  and it is recommend to keep a safe distance from the river.

CARSON RIVER:

* Broad minor flooding in the the southwestern Carson Valley between
  Centerville and Genoa.

* Minor Flood stage as early as Tuesday night with daily peaks
  occurring each night through the week. Flows will remain cold and
  swift on all portions of the river.

EASTERN SIERRA CREEKS AND STREAMS:

* Best estimates for flows this week coming out of unregulated
  eastern Sierra creeks and streams are likely to be 2-2.5 times
  current flows with smaller creeks likely to see 3 times current
  flows.

* Creeks and streams will run fast and very cold, bringing the risk
  of hypothermia for those without protective gear. Flood water will
  likely inundate pasture land, some campgrounds, cover hiking and
  biking trails and roads leading into the high country.

&&

.REV Watches/Warnings/Advisories...
NV...Flood Watch through Wednesday evening NVZ002-003.

CA...Flood Watch through Wednesday evening CAZ071>073.

&&

$$

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


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