Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 182032

Area Forecast Discussion
National Weather Service Reno NV
132 PM PDT Fri May 18 2018

Showers and thunderstorms will be confined mainly to the Sierra
and far western Nevada through this evening. Another low pressure
area will settle over northern California and western Nevada,
bringing an increased chance for storms this weekend through the
middle of next week. Some of these storms will produce locally
heavy rainfall, especially Monday and Tuesday.


.SHORT TERM...through Sunday night...

Overall forecast remains on track as the shower and thunderstorm
threat will continue for the next several days. High temperatures
will generally be above average through the weekend with mid-
upper 60s for Sierra valleys and 70s for lower elevations.
However, any area affected by showers and thunderstorms can
experience rapid cooling of 15-25 degrees, so keep adequate
clothing handy if you have outdoor plans. Have an action plan to
take shelter indoors as lightning will be a hazard to boating,
hiking and other outdoor activities.

So far today, moderate cumulus and isolated showers have
developed near the Sierra and over the Basin and Range, but mid
level cap has prevented thunderstorm formation. The activity over
west central NV is less likely to build into thunderstorms due to
more marginal forcing and instability. There is a better potential
for some thunderstorms initially forming near the Sierra crest
this afternoon where terrain-driven convergence combines with
daytime heating. This activity will slowly move or redevelop to
the east through early this evening, possibly reaching as far east
as Reno-Yerington. Short periods of heavy rainfall and small hail
may occur near the Sierra, mainly in Mono County due to the slow
cell movement. Any thunderstorm activity that does develop should
wind down after sunset, with only a few showers lingering in Mono-
Mineral counties until around midnight.

For this weekend, upper level divergence increases and
temperatures aloft cool ahead of incoming trough which will
increase shower and thunderstorm coverage. The potential for
stronger storm development also increases, especially along
preferred surface convergence zones including the Sierra Front
south into Mono-Mineral Counties as well as northern Lassen
County/far northwest NV. Daytime heating/instability will combine
with increasing shear aloft to support longer-lived updrafts.
Heavy rain could lead to some flash flooding issues where storms
anchor to stationary outflow/terrain induced surface boundaries.

Saturday looks more favorable than Sunday for greater thunderstorm
coverage and potential for stronger storms, as more widespread
cloud cover during the day Sunday could limit overall instability.
A bit more of a southwest push off the Sierra is also anticipated
for Sunday, which would shift the focus for convection to areas
east of Reno-Carson (although a few cells can`t be ruled out
around the urban areas during the mid-afternoon hours).

Areas of rain with embedded thunder could persist through the
evening and late night hours for both days this weekend,
especially across west central NV where the best upper divergence
is projected, with some locations receiving overnight rainfall
amounts in the 0.25-0.50 inch range. MJD

.Long Term...Monday through Friday...

The main message: There is a threat for (mainly minor) flood issues
Monday and Tuesday due to a possible combination of steady rain and
increased thunderstorm coverage.

The forecast is still on track for the center of an upper low to
move south into southern CA Monday. This pattern sets up a wind
pattern that is likely to bring wraparound moisture and upslope
easterly flow into the eastern Sierra, and could create a swath of
steady, moderate precipitation (due to boundary layer
convergence) somewhere across western-to-central Nevada. In
addition, model simulations indicate moderate afternoon/evening
instability, enough for a few stronger and slow moving
thunderstorms to develop. The forecast challenge is that small
changes in this pattern could shift the location of the
aforementioned features, and so while there is potential for at
least minor flooding Monday afternoon/evening, at this time we
can`t pinpoint the locations.

Confidence is even lower Tuesday. The upper low could weaken and
shift east by afternoon which would reduce upslope flow into the
eastern Sierra. However, the air mass won`t dry much and it`s very
possible there will still be enough instability for another round
of thunderstorms with locally heavy rain.

With a cooler air mass aloft Monday and Tuesday, we also can`t rule
out snow showers at times down to 7,000-8,000 feet. Harder to get
snow to stick this time of year, but (along with chances for hail
and heavy rain) there is some potential for snow to impact higher
Sierra passes.

For the Wednesday through Friday forecast, there are still large
differences between model simulations. Overall it looks a little
less active (especially Thursday-Friday), but without a strong
signal for a drying trend. Thus, we maintained chances for showers
and thunderstorms through the work week. JCM



Scattered showers and thunderstorms are possible over the Sierra
this afternoon through early this evening, including at KMMH.
Localized turbulence will be possible near any storms along with
terrain obscuration, small hail and gusty winds. There is a slight
chance through this evening (20%) that a storm will move off the
Sierra and cross KRNO and/or KCXP.

Areas of fog are likely to develop again tonight, especially at KTRK
and in the Martis Valley and at any areas that receive rain this

Another upper wave will bring increased chances for showers and
thunderstorms Saturday...then a deeper low moves overhead late
Sunday. This low will persist into the middle of next week with
periods of showers and thunderstorms. Overall, an active pattern
is expected to continue through at least the middle of next week.


.REV Watches/Warnings/Advisories...


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