Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 221821 AAA

Area Forecast Discussion...UPDATED
National Weather Service Reno NV
1121 AM PDT Mon May 22 2017


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


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 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/Moderate 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.


* 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.


* 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.


* 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

* 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.


.PREVIOUS DISCUSSION... /Issued 248 AM PDT Mon May 22 2017/


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.


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


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


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

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



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