Flash Flood and Debris Flows in Nevada, July 20 to 22, 2018

Area Forecast Discussion
National Weather Service Reno NV
220 PM PDT Fri Jul 20 2018

.SYNOPSIS...

High temperatures will drop by a few degrees this weekend but
still remain near warm July normals. Thunderstorm coverage will be
greatest over the weekend and early next week, bringing heavy
rainfall and increased flash flooding potential. Hot weather
returns from the middle to the end of next week with a drying
trend looking likely.

&&

.SHORT TERM...

The strong ridge of high pressure that had been situated over the
west has shifted eastward over the four corners region. The
Sierra and western Nevada will be between this feature and the low
off the southern California coast for the weekend, which will
help to advect modified monsoonal moisture into the region. PWATs
are forecast to reach 0.75-1.2", which climatologically is above
the 95th percentile and would likely lead to heavy rainfall under
thunderstorm cores, especially given the slow storm motions. The
ample moisture and instability will lead to an increased flash
flood threat, especially in steep terrain and over recent burn
scars. Despite all this, dry lightning strikes outside of the rain
core could potentially ignite new fires. However, based on
activity and how wet storms were yesterday, the lightning caused
fire threat is on the lower end of the spectrum.

Storms today are likely to reach near the I-80 corridor with a few
storms possible a bit farther north along the Sierra crest. Coverage
will further increase into Saturday and especially Sunday with
isolated nocturnal thunderstorms possible both Saturday and Sunday
nights. Activity could be fairly widespread Sunday, but the one
question mark is how dense the remnant cloud cover from the activity
Saturday/Saturday night will be. If cloud cover is too dense in some
locations, it will inhibit the surface instability. While the main
storm threat is heavy rain, hail and gusty outflow winds up to
40-50 mph are also possible.

There are many outdoor festivals and sporting events occurring this
weekend, so please pay close attention to the weather surrounding
you. Some things to keep in mind -- if you can hear thunder, you are
close enough to be struck by lightning. Also, heavy rain can drop
temperatures by 30 degrees in a short period of time. This can
easily catch campers/backpackers off guard, so prepare for wet
weather.

Smoke and haze mainly from the Ferguson Fire in California will
continue to impact the region, with the density dependent on how
activity the fire burns. Today the smoke has proven to be
especially thick and latest HRRR-Smoke runs indicate some
improvement late this afternoon before smoke fills back in again.
The rain from anticipated thunderstorms should help to decrease
particulate matter in the air and improve air quality. -Dawn

..LONG TERM...Monday through Friday...

Very moist and unstable air mass remains in place on Monday with
another round of thunderstorm activity likely in the afternoon
and evening, with potential for strong storms producing heavy
rain and flooding (especially if floods occur from the Friday-
Sunday storms). There appears to be another upper disturbance
rotating into the Sierra around the periphery of the strong ridge
over the southwest US Monday night, keeping the threat for showers
and thunder going through the overnight hours and into Tuesday
morning.

Tuesday remains moist and unstable, but there may be more
widespread cloud cover if the Monday night convection occurs,
leading to a lull in activity through much of the afternoon. At
this time, areas near the Sierra and far western NV will have the
best chance of seeing some clearing and a re-activation of storms
by late afternoon-evening. There does not appear to be any upper
disturbance for Tuesday night, so most storm activity should
diminish before midnight.

From Wednesday through Friday, the upper ridge is projected to
retreat farther to the south with drier southwest flow returning
to eastern CA-western NV. The thunderstorm chances decrease
significantly and look to be limited to areas south of US-50 by
Wednesday, and southern Mono-Mineral counties Thursday. As we get
to Friday, a more pronounced east-west ridge axis develops across
southern CA, which would leave all of our region in an environment
unfavorable for thunderstorms due to warm mid level temperatures.
Temperatures will likely remain above average for next week with
triple digit heat in some valleys especially around midweek, but
is less likely that we we'll be as hot as it was this week. MJD

&&

.AVIATION...

Areas of smoke and haze from the Ferguson Fire have produced MVFR
surface vsby today but slantwise vsby is likely even lower.
There could be more smoke return late tonight into Saturday,
although its trajectories and concentration will be affected by
afternoon-evening TSRA and outflows, as well as activity on the
fire itself. TSRA will continue to be the main issue for the next
several days, with most storms today south of I-80. For the
weekend, the chance of storms spreads north of I-80. At the main
terminals, KMMH has the highest overall TSRA potential each day
(greater than 50%), with 30-40% chance at the other terminals.

All thunderstorms will be capable of producing periods of heavy
rain with reduced vsby/cigs and mountain obscurations, with
stronger storms capable of producing flooding, hail and outflow
gusts greater than 35 KT. MJD

&&

.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
1131 AM PDT FRI JUL 20 2018

CAZ072-073-NVZ001-003-202130-
Greater Lake Tahoe Area CA-Mono CA-Greater Reno-Carson City-
Minden Area NV-Mineral and Southern Lyon Counties NV-
1131 AM PDT FRI JUL 20 2018

...SCATTERED THUNDERSTORMS DEVELOPING THIS AFTERNOON...

Thunderstorms are developing early this afternoon in Mono, Mineral
and southern Lyon counties. Storms are expected to become stronger
through the afternoon and spread north towards I-80.

Nickel size hail, frequent lightning, and wind gusts up to 50 mph
will be possible with these storms. Localized flash flooding will
also be possible, especially in steep terrain and over wildfire burn
scars.

Locations impacted include...
Mammoth Lakes, Bodie, Chalfant, Crowley Lake, Sonora Pass, Benton,
June Lake, Bridgeport, Lee Vining, Twin Lakes, Toms Place, Mono Lake,
Coleville, Topaz Lake, Hawthorne, Yerington, Wellington, Tioga Pass,
Smith and Oasis.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

During a thunderstorm, seek shelter inside a sturdy structure and
stay away from windows. Lightning can strike 10 miles away from a
thunderstorm. Hikers and campers should seek shelter in a vehicle if
possible.

&&

LAT...LON 3748 11885 3759 11903 3774 11913 3771 11923
      3775 11926 3779 11920 3788 11920 3795 11932
      3804 11931 3820 11963 3845 11968 3873 11999
      3909 11911 3810 11811 3789 11835 3789 11842
      3747 11783 3746 11784
TIME...MOT...LOC 1824Z 240DEG 0KT 3887 11906

ZT