Area Forecast Discussion
Issued by NWS Reno, NV

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837
FXUS65 KREV 210341 AAA
AFDREV

Area Forecast Discussion...UPDATED
National Weather Service Reno NV
841 PM PDT Sat May 20 2017

.UPDATE...
A couple light showers developed over Black Mountain west of
Schurz earlier this evening, but that has been the exception. It
has been quiet overall with cumulus and buildups over Western NV
now diminishing. Otherwise, attention remains on area rivers and
the snowmelt expected the next few days. As of this evening,
everything is tracking as expected. However, it is the Monday-
Wednesday period that the concerns really rise. See the hydro
section issued earlier today. Wallmann

&&

.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.


&&

.PREVIOUS DISCUSSION... /Issued 313 PM PDT Sat May 20 2017/


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.

High pressure will remain over eastern California and western
Nevada through the first half of next week, bringing well above
average temperatures and dry conditions for most areas. Sunday, a
slight weakening of the ridge is expected as a weak trough of low
pressure moves over eastern NV and Utah. This could cool temperatures
aloft and destabilize the atmosphere enough to squeeze out a few
afternoon showers or thunderstorms near the Mono County crest;
however, with northerly flow aloft any convection should quickly
push south and west of the crest.

Monday, the upper ridge strengthens again with warming mid-level
temperatures. Simulations indicate that this will be sufficient to
suppress showers or thunderstorms near the crest so all precipitation
mention was removed there. By Tuesday, the NAM and GFS indicate low
potential for showers and thunderstorms once again for the Mono crest.
Unlike Sunday, there will be lighter flow Tuesday so convection
has a better chance to build out into the Pine Nut and Sweetwater
Ranges south of Highway 50 along a weak afternoon and evening zephyr.
Temperatures aloft remain rather warm (~-8C at 500 mb) so activity,
if it occurs, should remain isolated and fairly weak. Snyder

LONG TERM...Wednesday through Saturday...

The large scale pattern in the extended forecast continues to trend
towards a broad troughing pattern. Uncertainties exists on the
magnitude and position of the troughing; however, confidence is
increased that we will enter a cooler pattern with temperatures
closer to season averages for the second half of the week. The main
adjustments to the forecast were to continue a cooler trend into
next weekend along an increase in winds Wednesday and Thursday as
the trough approaches the region. The exact magnitude of the winds
will change as the position of the trough comes into better focus
but nonetheless a good surface gradient with increased ridge level
winds aloft can be expected. This trough does not possess a solid
moisture source but will still have enough instability to fire
isolated showers and storms along the Sierra mainly through Mono
County on Thursday but the trend has been drier and have reduced
coverage for both Wednesday and Thursday. Fuentes

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 KMMH 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/Fuentes

HYDROLOGY...

Warming temperatures this weekend through the latter part 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.

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 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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