Special Weather Statement, NWS Reno, NV

Area Forecast Discussion...UPDATED
National Weather Service Reno NV
643 AM PDT Sat Aug 5 2017

.UPDATE...

We will increase precip chances for this afternoon and evening for
eastern CA and parts of western NV (generally west of a Gerlach-
Hawthorne line) as latest high resolution guidance indicates more
widespread coverage of showers and thunderstorms. However, there
isn't high enough confidence that the storms will be strong enough
to produce a significant flash flooding threat, which is reflected
in the high resolution guidance which reduces overall intensity
of the precip compared to earlier runs. The 12z Reno sounding
shows plenty of moisture especially between 550-350 mb, but there
is a notable cap above this moist layer up to near 300 mb.
Assuming this cap is not fully eroded, it would limit the strength
of the storms and reduce the risk for severe storms as well as
flash flooding. However, some of the stronger cells will still be
capable of producing frequent lightning, small hail and a short
period of moderate to locally heavy rainfall, and recent burn
scars in steep terrain could still be at risk for flash flooding
if a cell of sufficient strength persists overhead. MJD

&&

.PREVIOUS DISCUSSION... /Issued 242 AM PDT Sat Aug 5 2017/

SYNOPSIS...
For this weekend and into early next week, low pressure stalling
over northwest California will bring increased potential for
heavy rainfall and strong thunderstorms each day. Drier weather
then returns later next week. Temperatures will cool to near or
slightly below average today through early next week, with some
warming possible later next week.

SHORT TERM...

We are anticipating an active weekend for thunderstorms, which
could continue into Monday. Low pressure reaching the northwest CA
coast is expected to stall for the next few days, with ample
moisture and instability in place. It is strongly recommended
that anyone planning outdoor activities or attending large outdoor
events across the region prepares for dangerous thunderstorm
potential, by ensuring quick access to a safe shelter and
considering a "Plan B" before conditions become threatening.

For today, there are several favorable ingredients that are
tilting toward at least some scattered thunderstorms this
afternoon and evening. The main features are an upper level 60+
kt jet streak directed toward the central Sierra, with upper level
divergence extending northward across much of western NV by late
afternoon/early evening. Mid level wind profiles are also
favorable for sustaining more organized and longer duration cells,
while surface convergence becomes favorable as a weak zephyr sets
up later today. On the less favorable side, thicker cloud cover
could limit instability, and low level capping also is indicated
on model soundings. However, with the arrival of decent forcing
later today, this cap has a reasonable chance of being overcome,
leading to more intense and rapid development.

The most likely scenario is for storms to develop near the Sierra
especially in Mono County by early-mid afternoon, then extend into
northeast CA with some enhanced coverage possible in northern
Lassen County. By late afternoon-early evening, the best potential
shifts into western NV from Reno-Carson eastward to Fernley-
Yerington and southward across Douglas, Lyon and Mineral
Counties, with showers and isolated storms continuing overnight
mainly over west central NV. Strong cells are probable with a few
severe storms possible, containing hail, strong downdraft/outflow
gusts and very heavy rainfall. Even though most cell motions
should be relatively fast, intense rainfall rates and possible
back-building or terrain anchoring could produce localized flash
flooding.

For Sunday, another round of scattered thunderstorms is expected
as the upper low slowly edges towards the northern Sierra. Shear
profiles continue to support the possibility for more organized,
longer-lived thunderstorms, with strong to severe storms also
possible with overall greater instability compared to today. The
upper level jet streak feature is not expected to be a factor,
but upper level disturbances rotating around the main low will
provide decent forcing. The evolution of the storms looks similar
to today with most cells initially forming near the Sierra and
northeast CA, then spreading into western NV, although activity
may begin a few hours earlier due to less morning cloud cover and
a weaker low level cap. The stronger afternoon cells could then
evolve into a larger cluster of rain and thunderstorms with
frequent lightning during the evening and late night hours, most
likely across parts of northwest and west central NV.

For Monday, the thunderstorm pattern is likely to repeat again.
There is less overall instability and less distinct forcing
mechanisms evident at this time, but some strong cells with heavy
rainfall will remain possible. MJD

LONG TERM...Tuesday through next weekend...
Few changes were made with a gradual drying trend expected under
light southwest flow. The flow with the help of the afternoon
zephyr will act to slowly push the moisture eastward through
Friday.

Another decent day of convection is expected on Tuesday, but
probably not as strong as seen previous days. Any storm though would
be capable of locally heavy rain. As we move into Wednesday,
storms will become more isolated and by Friday the only storms
that remain will likely be over the main convergence zones to the
south (Mono-Mineral) and to the north (Lassen Peak). Temperatures
will also begin to warm as the moisture shifts east with less
cloud cover. By next weekend they will warm back above average by
a few degrees but very hot temperatures aren't expected. Wallmann

AVIATION...
Scattered thunderstorms are expected this afternoon through
Monday. The storms have the potential to be stronger the next two
days, but most will have hail up to 3/4" in diameter and wind
gusts to 40 kts. A couple of the strongest storms may have 1+"
hail and gusts over 50 kts, but these will be very isolated if
they occur at all.

All terminals will have a threat of storms and the peak times will
be 20-23Z for KMMH-KTVL-KTRK and 22-02Z for KCXP and KRNO. A few
storms may last overnight, but they will be much weaker.

Other concerns will be the haze and smoke. Trajectories will keep
the smoke around again through this evening and slantwise visibility
may be an issue at times. Visibility should improve Sunday as the
flow comes from the south where few large fires are burning. Wallmann

&&

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

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

Special Weather Statement
National Weather Service Reno NV
104 PM PDT SAT AUG 5 2017

CAZ072-NVZ002-003-052045-
Greater Lake Tahoe Area CA-Greater Lake Tahoe Area NV-Greater Reno-
Carson City-Minden Area NV-
104 PM PDT SAT AUG 5 2017

...Showers forming over Lake Tahoe and along the Sierra Front...

At 1259 PM PDT, Doppler radar was tracking several showers that are
forming over Lake Tahoe...the Carson Range...and along the Sierra
Front. These showers are moving north at 15 mph. While no
thunderstorms are occurring yet...they are expected to form in the
next couple of hours. Coverage to increase through this afternoon
with some storms capable of very heavy rainfall rates that may
result in flash flooding.

Locations impacted include...
Carson City, South Lake Tahoe, Alpine Meadows, Kings Beach, Squaw
Valley, Tahoe City, Incline Village, Dagget Pass, Spooner Summit,
Glenshire-Devonshire Cdp, Northstar, Glenbrook, Galena, Emerald Bay,
Crystal Bay, Zephyr Cove-Round Hill Village, Sugar Pine Point S,
Bliss State Park, Kingsbury and Incline Village-Crystal Bay.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If you have plans on or near lakes in the Tahoe Basin, have a plan
to quickly get out of the water and move indoors or inside a
vehicle. Don't be caught on the water in a thunderstorm. Strong wind
gusts and lightning will pose a danger to boaters. Gusty winds could
cause a rapid increase in wave heights which may capsize small boats.

&&

LAT...LON 3940 11977 3935 11982 3891 11987 3888 12016
      3920 12025 3935 12033
TIME...MOT...LOC 1959Z 181DEG 11KT 3914 12001

MB