Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 220948
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
248 AM 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 major flood stage early this week. Some minor flooding
is also likely for the upper portions of the Carson and Truckee
River systems.

&&

.SHORT TERM...

The primary concern for the near future continues to be the
expected flooding for main stem rivers, mainly the Walker River
in and downstream of the Mason Valley. Some locations along the
Walker River may experience record flooding by Tuesday; now is
the time to act! See the latest flood products and the hydrology
section at the bottom of the forecast discussion for updates.

There were some modest changes made to the forecast mainly
concerning thunderstorms for Tuesday and Wednesday. Essentially,
thunderstorms were largely removed for Tuesday limiting formation
to the Sierra crest of Mono County. Still, there could be a
build-up or two as far north as Meyers, but chances are less than
10%. This was done to account for the thermal trough placement and
general under-performance of convergence zones over the last
couple of days. Wednesday looks more promising with a more
favorable thermal trough position and added instability from an
approaching wave. Expect isolated convection for Mono/Mineral
Counties as heating aids in surface instability.

Winds increase ahead of the approaching wave on Wednesday with
breezy conditions from northern Mono/Mineral Counties to the
Oregon border. Expect gusts in the 30-40mph range with highest
gusts along the frontal boundary. Precipitation will be limited,
so no widespread rain, but sufficient instability for isolated
thunderstorms north of Interstate 80. Chances of thunderstorms
were expanded from Lassen County since westerly flow will aid in
development of isolated thunderstorms along the Lassen convergence
zone. The one foil for thunderstorms north of Interstate 80 could
be the amount of shear; it may be difficult for initial updrafts
to become self-sustaining. Boyd

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

A low pressure trough will be dropping into the northern Great Basin
Thursday and Friday and then moving slowly east out of the region
over the weekend. Temperatures will cool back closer to normal
Friday before rebounding over the weekend. Gusty west winds will
accompany the front for one more day on Thursday, especially for
areas along and south of I-80. The dry flow around the southwest
periphery of upper low will bring dry and stable conditions to most
of the area. There may be a few showers far northern areas as upper
low swings by Thursday night into Friday. Beyond this weekend, there
is low confidence. However there are some signs for increasing
thunderstorm potential with temperatures 5-10 degrees above normal.
A lot will depend on location of developing upper low off the
west coast. Hohmann

&&

.AVIATION...

Light winds and VFR conditions through Tuesday. The only risk of
thunderstorms will be late Tuesday along the Sierra Crest in Mono
County. Coverage will be isolated and probably not affect KMMH. A
front will be dropping south into the region Wed-Thu for increasing
winds and potential turbulence. Isolated showers and thunderstorms
will be limited mainly to Mono-Mineral County and north of I-80
Wednesday afternoon/evening. Hohmann

&&

.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 major 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:

* Major flooding is likely for the mainstem Walker River through
  Mason Valley. Current projections show similarities to the
  flooding that occurred in 1983 and 1986. The Walker River could
  begin reaching into minor flood stage as early as Tuesday with an
  estimated short range crest Fri-Sat with continued flows in excess
  of Major Flood stage.

* Impacts to Mason and Yerington are largely dependent upon
  unpredictable channel breaches along the river. If a breach does
  occur, IMPACTS COULD BE MUCH MORE WIDESPREAD AND SEVERE IN MASON
  AND YERINGTON. In any event, now is the time to act; it may be too
  late once the waters rise above moderate flood stage.

* Minor/Moderate flooding is possible on the East and West forks of
  the Walker River. This includes Antelope and Smith valleys. Higher
  flows are also 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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