Area Forecast Discussion
Issued by NWS Reno, NV

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958
FXUS65 KREV 242040
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
140 PM PDT Tue Apr 24 2018

.SYNOPSIS...

Warm afternoon temperatures are expected through Thursday, with a
cool down to below average temperatures by the weekend. A few late
day showers or thunderstorms are possible near the Sierra through
Wednesday, with increasing chances spreading into western Nevada
on Thursday. Conditions become breezy for Friday through Sunday
with a chance for showers as low pressure moves into the region.

&&

.SHORT TERM...

Some adjustments were made in the short term with reduced chances
for thunder through Wednesday, then overall thunderstorm chances
look most favorable for Thursday. By Friday, the best chances for
storms favor northeast CA and far northwest NV, while a drier air
mass pushing across the Sierra will reduce chances for shower or
thunderstorm activity across the remainder of the region.

Outdoor plans should include a method of getting to safety if
thunderstorms begin to form - When thunder roars, go indoors.
Forecast details follow.

For the rest of today, weak convergence along the Sierra during
the peak heating should lead to some cumulus buildups, but
temperatures aloft are relatively warm so we are not expecting
much in the way of thunderstorms. There is a small potential for
a couple of short lived cells popping up along and just east of
the crest in Alpine and Mono counties through sunset.

For Wednesday, the ridge axis shifts a bit farther east into
central NV with a bit more daytime heating (highs into the lower
80s for western NV valleys and lower 70s near the Sierra) and
slightly cooler temperatures aloft along the Sierra. This will
lead to more favorable chances for isolated afternoon
thunderstorms across the Sierra including the Tahoe basin. The
development of west winds is trending later in the day and looks
more modest, reducing the risk of cells developing or spreading
into far western NV. Most cells that form are likely to be short
lived and relatively weak, producing brief moderate precipitation
and some outflow gusts of 30-40 mph.

By Thursday, forcing mechanisms for thunderstorm formation
improve as an upper low slowly approaches northern CA. This low
will produce increasing upper level divergence east of the Sierra
into western NV, with instability already in place as temperatures
at the surface and aloft remain similar to Wednesday. Isolated to
scattered storms are most likely to develop near the Sierra
during the afternoon, then spread into northeast CA and western NV
through Thursday evening. Thunderstorms have a better chance of
having a longer life cycle with stronger cells capable of
producing surface gusts from 40-50 mph, and hail up to 1/2 inch
diameter. The threat for fire starts can`t be completely ruled
out, but is not a significant threat as we are still in a green-
up period with cell movement slow enough (generally 15 mph or
less) allowing for wetting rains to reach the ground. Isolated
showers could linger into the overnight hours mainly north of I-80
as weak upper level shortwave swings through.

On Friday, the upper low makes a slow progression to the
northwest CA coast, with southwest to west winds increasing
across the region. Wind gusts around 40 mph are possible in lower
elevations, with Sierra ridge gusts up to 75 mph. It also appears
that a dry slot will push across the Sierra crest into western NV
and reduce the threat for thunder for Reno-Tahoe southward to
Mono County. We could see a few late day cells producing some
gusty outflow winds along the Pine Nut Mountains and into the
US-95A and US-50 corridors (Fernley-Yerington-Fallon) with
afternoon heating and surface convergence, but the more favorable
location for thunderstorms will be north of Susanville-Gerlach
where stronger upper forcing and greater moisture is projected,
with storms in these areas capable of producing locally heavy
rainfall and small hail, and more frequent lightning.

This low then moves inland Friday night, with showers continuing
across northeast CA and northwest NV. A colder air mass also
spreads across the region, with snow levels dropping to near
5500-6000 feet from Tahoe northward by early Saturday morning.

.LONG TERM...Saturday onward...

The synoptic scale pattern is finally taking shape for this
weekend as both the operational and ensemble models are coming
into better agreement. A cooling trend along with chances for
showers and thunderstorms is expected Saturday through Sunday as a
closed low approaches the region. The details of the locations
for convective development are still uncertain. Moreover,
confidence in the forecast remains low for what will happen during
the early part of next week as model solutions differ quite a bit
past Sunday.

The closed low will be near the CA/OR border 12Z Saturday, slowly
lifting northeastward across the intermountain west through Sunday.
Breezy conditions can be expected Saturday and Sunday afternoon as
the thermal gradient tightens between the Sierra and lower valleys
due to cooling temperatures aloft.

The GFS solution progresses the low much faster across the
intermountain west than the ECMWF solution does. Therefore, the
trough axis could vary by several hundred miles. This leads to
moderate-to-high uncertainty as to where showers and thunderstorms
will be most likely. Both solutions have a secondary shortwave
rounding the base of the parent longwave trough by Monday, where
again the GFS solution is much faster with this progression than
the ECMWF.

The main takeaway from all of this is to expect a much cooler
weekend compared to this week`s warm temperatures. High
temperatures could cool as much as 5-10 degrees below normal
Saturday and Sunday. There will be chances for rain and mountain
snow showers above 6500-7500 feet. Thunderstorms also remain a
possibility which may bring gusty and erratic winds along with
small hail. These cool and unsettled conditions could continue
into the early portion of next week. If you have outdoor plans,
pay close attention to the forecast and ongoing weather so you are
not caught off guard. -Johnston

&&

.AVIATION...

VFR conditions will continue for much of the week outside of a
chance of morning fog at KTRK/Martis Valley. Expect isolated
afternoon thunderstorms along the Sierra crest through Wednesday.
Coverage will increase Thursday as an area of low pressure makes
its way towards the west coast. Storms may bring brief periods of
reduced ceilings and visibility, gusty and erratic outflow winds,
and small hail. It is too soon to say if storms will directly
impact any of the terminal sites, however, cumulus/stratocumulus
clouds could be near any thunderstorms especially in the late
afternoon and early evening. -Johnston/Dawn

&&

.REV Watches/Warnings/Advisories...
NV...None.
CA...None.
&&

$$

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



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