Flash Flood Warning 20, NWS Reno, NV

Area Forecast Discussion
National Weather Service Reno NV
256 AM PDT Tue Aug 11 2020

.SYNOPSIS...

Thunderstorm coverage and intensity increases today. Dry and
breezy conditions will develop for Wednesday. The rest of the week
will feature a rather typical summer time pattern, with light
afternoon winds and a few thunderstorms possible over the weekend.

&&

.SHORT TERM (through Thursday Night)...

No major changes to the forecast thinking. A trough moves inland
today, which will provide forcing for enhanced thunderstorm
development. PWATs remain high based off of model soundings in
addition to ensemble data, so storms have the potential to produce
heavy rain. Pay special attention to recent burn scars which have
enhanced flash flood potential. Storm motions increase by late
afternoon as the flow aloft strengthens. This could help mitigate
the heavy rain potential and also increase chances for dry
lightning strikes. Please see the fire weather section below for
additional details.

Storms today have the potential to be strong with gusty outflow
winds in excess of 50 mph, especially given the modeled DCAPE
values of ~1000 J/kg. Hail also remains a threat along with
blowing dust from any outflows. If you have plans in the Sierra
today, it is best to be off the mountains and/or lakes by no later
than noon.

Based on the latest high resolution guidance and HREF, storms are
likely to build along the Sierra/Sierra Front today by noon-1 pm,
shifting eastward into the late afternoon as the westerly winds
increase. This means storms could move off the Sierra and be out
of the Tahoe Basin by mid-late afternoon. HREF guidance continues
to show high probabilities of storms continuing into northeast CA
(near/E of US-395) and far northwest NV through 9 pm. Outflow
interactions, however, could cause storms to fire off in various
locations.

Much drier air works into the region behind this trough tonight
into Wednesday with PWATs bottoming out. This will keep skies
clear Wednesday with cooler nights anticipated the middle into the
end of the week. Winds will be enhanced (zephyr+ if you will)
Wednesday with gusts of 30-35 mph possible north of I-80
Wednesday, 20-30 mph south of there. The wind/low RH combination
brings enhanced fire concerns, which are discussed in the fire
section below. -Dawn

.LONG TERM...Friday onward...

A strong ridge of high pressure is projected to lift northward
from the southwest US to the Great Basin from Friday through
early next week. This will bring a warming trend, especially from
Saturday onward with highs well into the 90s to near 100 degrees
for lower elevations, and mid-upper 80s near the Sierra. If this
ridge expands farther west, we could end up with with the warmest
air mass of this summer, with highs surpassing 100 degrees across
more areas in western NV.

In addition to the warming trend, this pattern would lead to
increasing moisture and instability across the eastern Sierra and
portions of western NV. Some of this moisture may be from decaying
tropical systems but regardless of the source, conditions would
become favorable for mostly isolated afternoon/evening showers and
thunderstorms. Timing and location may vary, but currently the
first moisture push appears to favor areas south of US-50 Friday
and Saturday. A second round of moisture arriving early next week
could bring shower/thunder chances farther north, including the
Reno-Tahoe area and possibly parts of northeast CA. MJD

&&

.AVIATION...

Very light showers are contributing to random strong and erratic
wind gusts early this morning, potentially impacting all terminals
along the Sierra Front at times through 5 am. Otherwise, stronger
and more widespread thunderstorms are expected today with the
potential for 40-50+ kt erratic outflows, heavy rain obscuring
terrain, hail, and blowing dust. Storms could start as early as
19z, lasting through 05z.

Thunderstorm chances and rough timing within 10 miles of terminals
for today are:

KRNO/KMEV/KCXP: 40% 20z-02z
KTRK/KTVL: 20% 19z-23z
KMMH: 25% 19z-00z

Winds are likely to be gusty on Wednesday, especially near/north
of I-80. Gusts of 25-30 kts are possible.

There is a new wildfire, R-1, located north of KSVE, which may
bring haze/smoke impacts to the region. The severity is dependent
on fire activity today. -Dawn

&&

.FIRE WEATHER...

Main concerns continue to be thunderstorms today followed by dry
and breezy conditions on Wednesday.

Tough call today with how likely new fire starts will be from
thunderstorms. Storms are going to be wet, with plenty of
atmospheric moisture available, but we'll also be seeing an
increase in storm motion by late afternoon and abundant
lightning. Dry strikes outside of wet cores are certainly
possible, and with the current state of the fuels, new fire starts
from lightning seem to be rather efficient lately. That being
said, we are not expecting widespread dry lightning by any stretch
of the imagination and these storms are more likely to put down
local areas of heavy rain. With that in mind, will keep the fire
watch intact today to keep the "heads up" in place, but am not
going to upgrade to a Red Flag Warning. Gusty and erratic outflow
winds, potentially in excess of 50 mph, also remain a concern.

Storms should kick off in the Sierra and northeast CA by early
afternoon, then push into western NV for mid afternoon and
evening hours. High resolution ensemble guidance continues to hint
at high probabilities for storms focusing in NE CA (near/east of
US-395) into far NW NV this afternoon and into the evening. This
could be an area to watch for increased lightning potential.

Conditions rapidly dry for Wednesday with slightly enhanced
afternoon breezes. We aren't looking at a big wind event, but
30-35 mph gusts are possible for the northern Sierra Front,
northeast CA, and northern Washoe/Pershing Counties. This along
with the dry conditions could yield critical fire conditions,
which is especially concerning following an active thunderstorm
day. A Fire Weather Watch remains in effect Wednesday for this.
-Dawn

&&

.REV Watches/Warnings/Advisories...
NV...Fire Weather Watch from noon PDT today through Wednesday evening
     NVZ420-423-458.

CA...Fire Weather Watch from noon PDT today through Wednesday evening
     CAZ270-271-278.

&&

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

Special Weather Statement
National Weather Service RENO NV
1123 AM PDT Tue Aug 11 2020

CAZ070>073-NVZ001>005-120000-
Surprise Valley California-Lassen-Eastern Plumas-
Eastern Sierra Counties-Greater Lake Tahoe Area-Mono County-
Mineral and Southern Lyon Counties-Greater Reno-Carson City-
Minden Area-Western Nevada Basin and Range including Pyramid Lake-
Northern Washoe County-
Including the cities of Cedarville, Eagleville, Fort Bidwell,
Portola, Susanville, Westwood, Sierraville, Loyalton,
South Lake Tahoe, Tahoe City, Truckee, Markleeville, Bridgeport,
Coleville, Lee Vining, Mammoth Lakes, Hawthorne, Yerington,
Smith Valley, Mina, Schurz, Stateline, Glenbrook,
Incline Village, Sparks, Verdi, Gardnerville, Virginia City,
Fernley, Fallon, Lovelock, Silver Springs, Nixon, Imlay, Empire,
and Gerlach
1123 AM PDT Tue Aug 11 2020

...Thunderstorms to impact the region through 9 PM...

Showers and thunderstorms are building over the region and will
impact eastern California and western Nevada through 9 PM this
evening.

Strong to severe thunderstorms are possible with erratic outflow
winds over 50 mph, hail, periods of moderate to heavy rain and
dangerous lightning. Outflow winds may damage trees and utility
poles, and generate areas of blowing dust across western Nevada.
Heavy rain may lead to rock slides, debris flows on recently
burned slopes, and cause ponding of water on roads and poorly
drained urban areas.

Now is the time to consider alternate plans for outdoor
activities the remainder of today. Lightning poses an immediate
risk for all outdoor activities. If you have outdoor plans, make
sure you have quick and easy access to shelter inside a building
or vehicle.

http://weather.gov/reno