Area Forecast Discussion
Issued by NWS Reno, NV

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923
FXUS65 KREV 231027
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
327 AM PDT Sun Jul 23 2017

.SYNOPSIS...

Areas of haze from the Detwiler Fire could return early this week,
mainly near the Sierra. Afternoon thunderstorms are expected each
afternoon through Tuesday or Wednesday for northeast California
and western Nevada. Some storms today will produce little rainfall.
More moisture arrives Monday and Tuesday for stronger thunderstorms,
with heavier rain and localized flash flooding possible.

&&

.SHORT TERM...

Divergent flow and a slightly unstable mid-level atmosphere over
northern California out ahead of an upper low off the coast is
producing some altocumulus (possibly ACCAS) this morning. So far
no lightning has been noted; however, there is a 5-10% chance for
a lightning strike in Plumas (near/west of crest) and western
Lassen Counties this morning given the weak instability. If any
thunderstorms develop, they are nearly certain to be dry given
moderate movement and a dry lower atmosphere.

An approaching low will lead to increasingly moist southerly flow
along with steep upper-level lapse rates around 8.0 C/km this
afternoon and evening. Thunderstorm motions 10-20 mph, high cloud
bases, and smallish rain cores are expected with most storms.
Thus, lightning strikes outside of rain cores are likely to start
new fires and thunderstorm outflows could produce gusts in the
45-60 mph range. A Red Flag Warning for the east Sierra and
western NV is in effect for this afternoon/evening. See the Fire
Discussion below for additional details.

Divergent flow aloft over the region with low pressure near the
northern California coast and simulations indicating some mid-
level CAPE brings the potential for nocturnal convection (showers
and/or thunderstorms) for the next couple nights. There are
discrepancies among simulations (NAM vs GFS/EC) as to how far west
the potential exists, but most agree on some potential for at
least western or west-central NV. It may wind up just being mainly
mid-level clouds; however, a low chance for showers and
thunderstorms is warranted for much of the region.

Turning back to afternoon and evening thunderstorms, simulated
soundings show moderate, unidirectional mid-level flow with
increasing speeds between 500 and 300 mb for Monday and Tuesday.
Although instability is fairly modest (less than 1000 J/Kg) in
soundings, the increasing flow aloft could support stronger
thunderstorms which last a bit longer than is typical...and bring
some threat for training given the unidirectional flow aloft. With
PWATs rising to between 0.75" and 1" and possible training, the
threat for localized flash flooding will rise Monday and Tuesday.
Finally, as far as location of thunderstorms early next week, they
could start a bit east the northern Sierra crest by Tuesday as
flow aloft turns more southwesterly and pushes the focus for
afternoon convection more into western Nevada.

Temperatures will be on the downtrend over the next few days as
clouds increase and temperatures aloft cool with the upper low
edging into northern California. -Snyder

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

Low pressure begins to lift out of the region on Wednesday with
southwest flow returning to our area. This will help to dry out the
region and reduce thunderstorms chances to mainly south of highway
50 in Mono-Mineral Counties. Four Corners ridge remains in place
through the end of the week and into next weekend. Warming
temperatures aloft will help to increase temperatures once again to
above normal with high temperatures in the upper 90s across western
NV and 80s in the Sierra. We will need to watch how far east/west
the ridge aligns over the Sierra, as we could see some isolated
thunderstorm chances increase if the ridge holds off further east
and allows for monsoonal moisture to move up from the south. Hoon

&&

.AVIATION...

Thunderstorm chances increase today through Tuesday with scattered
showers and thunderstorms each afternoon and evening. Main threats
will include lightning, gusty outflow winds, small hail and
localized heavy rainfall. Thunderstorms will initiate over the
higher mountains and Sierra, then push off the north and east into
the western Nevada Sierra Front. 20-30% chance of thunderstorms
are expected for all area terminals this afternoon and evening.
Additional chances for thunderstorms Monday and Tuesday. Hoon

&&

.FIRE WEATHER...

Hot and dry conditions will give way to increasing moisture through
Tuesday. Mid and high level moisture are increasing this morning
with lower level moisture lagging a bit. Thunderstorms are expected
to begin later today and continue into Tuesday and possibly
Wednesday.

With the moisture more limited at lower levels, the Red Flag
Warnings still look good. Above normal temperatures over the last
couple days have made fuels susceptible to new starts, especially
for lightning strikes outside of rain storm cores. The storms will
start dry and begin to transition to wetter storms by this evening.
Still, ignition efficiency is likely to be high with numerous new
starts likely. Outflow gusts to 50 mph, and possibly 60 mph in the
deserts will be capable of rapidly spreading any starts.

Monday will see a transition to wetter storms for most of the area
with localized flash flooding becoming possible, especially over
recent burn scars. The question is up north where the models are now
slower to increase the moisture. In fact, they show drying Monday
over Sunday. This looks unreasonable as moisture typically pools
south of the deformation zone. All areas will be south of the area
of deformation so expect moisture to increase all areas. Tuesday
will see moisture increase even further with very wet storms as PWAT
nears 1 inch. Wallmann/Hoon

&&

.REV Watches/Warnings/Advisories...
NV...Red Flag Warning from 3 PM this afternoon to midnight PDT
     tonight NVZ450-453-458.

     Red Flag Warning from 1 PM this afternoon to 9 PM PDT this
     evening NVZ459.

CA...Red Flag Warning from 3 PM this afternoon to midnight PDT
     tonight CAZ270>272-278.

     Red Flag Warning from 1 PM this afternoon to 9 PM PDT this
     evening CAZ273.

&&

$$

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



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