Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 210945
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
245 AM PDT Sun May 21 2017

.SYNOPSIS...

Warm temperatures will increase snow melt, 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 next week, with record levels possible.
Some 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 the middle of next
week; now is the time to act! See the latest flood products and
the hydrology section at the bottom of the forecast discussion
for updates.

No major changes were made to the existing short term forecast.
TS was added to the White Mountains in Mono County for this
afternoon where convergence and a passing trough support slight
chances of thunder.

Otherwise, expect some thunderstorms for Mono County mainly in the
Sierra and over other terrain as a weak trough moves through the
region. The upper ridge strengthens Monday with stability
factors becoming too high to support upward development of clouds;
expect cloud cover over terrain with more flattened elements.
There could be sufficient depth to allow for a few sprinkles in
the Sierra, but the extent is too low to warrant much of a
mention. Better chances of thunderstorms will occur Tuesday as
heat builds under the ridge as the axis shifts into a more
favorable location to allow for vertical development of cloud
elements. As a result, locations from the southern shores of Lake
Tahoe through Mono County will have the possibility of some
thunderstorms. The zephyr should kick in later in the afternoon
spreading chances of thunderstorms into southern Douglas County to
the Pine Nuts and east of Mono County into the Sweetwater Range.
Boyd

.LONG TERM...Wednesday through Saturday...

A trough will be moving through the Pacific Northwest at the
beginning of period with a cold front approaching far northern areas
late Wednesday. Wednesday will be another unseasonably warm day as
high temperatures push well into the 80s across western NV with mid
70s Sierra valleys. As winds aloft increase and surface gradients
tighten gusty ridge winds will develop Wednesday with breezy to
windy conditions at lower elevations during the afternoon. As a
result of this increased westerly flow, instability is going to be
suppressed southward and we have lowered coverage as well as limited
storms to mainly Mono-Mineral Counties. There may also be a storm or
two far northern areas ahead of the front although confidence is
lower there.

Upper trough then drops southward into the northern Great Basin for
the remainder of the week which will bring cooler temperatures.
Highs will return closer to normal by Friday-Saturday with highs
generally ranging from 60s Sierra to 70s lower elevations. Another
day of gusty winds can be expected ahead of the front on Thursday as
it sags southward. The only chance for showers and maybe a storm or
two will be close to the upper low axis or near the Oregon border.
Otherwise it will remain dry across the region. Hohmann

&&

.Aviation...

Warming temperatures will persist into Monday with VFR conditions
and light winds to prevail. There may be an isolated shower or storm
along the Sierra crest near KMMH today. Otherwise, conditions will
be too stable for more than some afternoon cumulus buildups. A
trough is becoming more likely for mid-late week which would
decrease the storm threat but bring increasing winds for Wed-Thu.
Hohmann

&&

.HYDROLOGY...

Warming temperatures this weekend through the middle of next week
will accelerate snowmelt in the Sierra. Overnight low
temperatures in the higher elevations are likely to remain above
freezing as well and this will enhance snowmelt further.
Temperatures will likely decrease towards the end of next week,
but flows will remain elevated as water moves downstream.

This increased snowmelt will cause significant rises in many area
small creeks and rivers from southern Mono County through the
eastern Sierra and into Sierra and Plumas counties. These higher
flows are likely to produce overbank flows in some of the smaller
streams and rivers, although flooding may mainly be restricted to
pasture lands, campgrounds, and trails in and near the Sierra.
Farther downstream, flooding possibilities will increase for some
of the major basins although the worst conditions are still
likely on the Walker River.

Major to near-record flooding is likely for the Walker River
through Mason Valley. On the East and West forks of the Walker
River minor to moderate flooding is likely for areas including
the Antelope and Smith valleys. Higher flows are also likely below
Weber Reservoir but the expected extent is unknown at this time.
Current projections show similarities to the flooding that
occurred in 1983 and 1986, but some locations may near record
levels.

ONE OF THE MAIN CONSIDERATIONS IS WHETHER THE WALKER BREACHES ITS
BANKS in Mason Valley near and upstream from Yerington. IF IT
DOES, FLOODING IMPACTS COULD BE MUCH MORE WIDESPREAD AND SEVERE IN
YERINGTON. Now is the time to act; it may be too late once the
waters rise above moderate flood stage early in the week!

Elsewhere, minor flooding is possible by early next week for the
Carson Valley area. Some minor overbank flow is also possible
along the Truckee River in California. Fortunately, reservoirs on
the Truckee can be managed to limit the extent of flooding.

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.

&&

.REV Watches/Warnings/Advisories...
NV...Flood Watch from 11 AM PDT this morning through Tuesday evening
     NVZ002-003.

CA...Flood Watch from 11 AM PDT this morning through Tuesday evening
     CAZ071>073.

&&

$$

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



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