Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 201001
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
301 AM PDT Sat May 20 2017

.SYNOPSIS...

Very warm temperatures will increase the rate of snow melt this
weekend and early next week leading to flooding near 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. Visit http://weather.gov/reno to see the latest
hydrologic warnings and advisories.

&&

.SHORT TERM...

The primary concern for the near future continues to be the
expected flooding for main stem rivers, especially the Walker
River system. Some locations along the Walker River may experience
record flooding by early next week; now is the time to act!
Please, see the hydrology section at the bottom of the forecast
discussion for particulars concerning the expected flooding.

Otherwise, temperatures will become unseasonably warm this
weekend as highs for western Nevada move into the 80s by this
afternoon; highs in the Sierra will head into the low 70s. Expect
these highs to increase each day through the middle of next week.
These warm temperatures will make heat stress a possibilities for
those headed outdoors to participate in more strenuous activities.
Plan ahead and take plenty of water and consider more frequent
breaks to allow for acclimation to the warmer conditions.

Another consequence of this heating will be a destabilization of
the lower atmosphere. There are some low chances of general
variety thunderstorms for Mono County on Sunday. The extent of
thunderstorms was trimmed back due to models accelerating a
passing wave leaving locations from Alpine northward in an area of
higher stability. Chances for Monday were removed altogether
based on ridge position and the presence of a strong mid-level
cap. Some build-ups will be possible and may result in a few
isolated showers along the Sierra crest, but thunderstorm chances
have greatly diminished. Boyd.

.LONG TERM...Tuesday through Friday...

Weak upper low approaching the west coast and upper ridge over the
Great Basin will allow for some limited moisture to ride up the
Sierra Tue-Wed as afternoon temperatures push 10-15 degrees above
normal. Isolated thunderstorms will develop along the Sierra from
Lake Tahoe southward with storms pushing off into western NV
Wednesday as southwest flow aloft begins to increase. Storms will be
slow moving and capable of locally heavy rain, especially Wednesday
as low level convergence increases along afternoon zephyr across far
western NV from the Virginia Range and Pine Nuts south into Mono
County. Temperatures both days will push well into the 80s with a
few lower 90s across western NV valleys...and mid-upper 70s Sierra
valleys.

By Thursday-Friday, both the GFS/EC deterministic runs have trended
cooler as they drop a trough through the Pacific Northwest and
eventually into the Great Basin. The ECMWF is faster and more
aggressive with front on Thursday while the GFS and a number of
ensembles have a more moderated cool down. For now, we have lowered
temperatures down for the very end of the week but they may need to
be cooled further if models continue to trend in this direction.
While this may slow down the snow melt briefly at the end of the
week, the very warm temperatures early-mid week will be melting a
lot of snow and therefore a lot of water is still going to be coming
down rivers and streams next week. See hydrologic section below. As
far as convection both days, a lot depends on strength of trough.
For now we maintained a low chance for Thursday while drying things
out for Friday. A stronger trough would push drier air into the
region with a resultant decrease in storm potential Thursday. 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 south of KTVL on Sunday. Otherwise,
conditions will be too stable for more than some afternoon cumulus
buildups. There is a slightly better chance of thunderstorms Tue-Wed
as temperatures continue to heat up and a trough off the west coast
gets a bit closer. Hohmann

&&

.HYDROLOGY...

Warming temperatures this weekend through the Memorial Day weekend
will accelerate snowmelt across the Sierra. Overnight low
temperatures in the higher elevations are likely to remain above
freezing as well...and this will enhance snowmelt.

This increased snowmelt will cause significant rises in many area
small streams 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 the smaller streams.
Farther downstream...flooding possibilities will increase for some
of the major basins.

Major to near-record flooding is likely for the Walker River
through Mason Valley. On the East and West Walker rivers 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 1997, but some locations may near record levels. 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 possible along
the Truckee River with bank erosion possible from Truckee
downstream to Reno. The extent of any flooding is far less certain
on the Truckee. Reservoirs on the Truckee can be managed to limit
the extent of any minor 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. Visit
weather.gov/reno for more info.

&&

.REV Watches/Warnings/Advisories...

NV...Flood Watch from Sunday morning through Tuesday evening
     NVZ002-003.

CA...Flood Watch from Sunday morning through Tuesday evening
     CAZ071>073.

&&

$$

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



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