Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 210415

Area Forecast Discussion
National Weather Service Las Vegas NV
915 PM PDT Sun Aug 20 2017

.SYNOPSIS...An area of low pressure off the southern California
coast will pull moisture northward into the region overnight,
resulting in rain showers and isolated thunderstorms developing late
tonight and lasting through Monday. Daily thunderstorm chances will
persist through Wednesday before conditions gradually dry out late
in the week.

.UPDATE...Mid-level clouds have been filling in about as
expected from Mohave and eastern San Bernardino up through Clark
counties. However, surface dewpoints readings in the upper 30s
between Las Vegas and Lake Havasu indicate little in the way of low
level moisture. The 00Z Las Vegas sounding showed an pronounced
inverted V profile and the light showers and virga associated with
the increasing mid-level cloud layer will lead to some areas of
gusty winds. In fact, an outflow boundary has been sweeping across
central Mohave County and produced a gust to 47 mph at the Kingman
airport. It appears these winds coming from the southeast will have
enough momentum to reach the Las Vegas Valley between 10 PM and 11
PM and will bring some increase in low level moisture. The Kingman
dewpoint was 55 degrees F. Brief thunderstorms have been observed in
Mohave County and more recently near Jean. However, cells are fairly
weak and short lived. The latest HRRR indicates a fairly broad band
southeast-northwest oriented band of showers and thunderstorms will
develop from Kingman up across Las Vegas and Death Valley/Beatty
which supports the going forecast. Will make some tweaks to the
Pop/WX/QPF through 18Z Monday based on the latest guidance, but
overall the forecast trends look okay. -Adair

.AVIATION...For McCarran...Southeast winds coming out of Arizona
will likely push into Las Vegas Valley between 05Z and 06Z and keep
a southeast wind component at the terminal until around 09Z. Gusts to
20 knots can be expected.

Overnight into Monday morning, scattered showers will likely develop
in the vicinity and over the terminal...especially after 12Z.
Scattered showers are expected to linger through Monday
morning/afternoon, with embedded thunderstorms possible during the
afternoon. Winds should stay fairly light through the morning with
an easterly/southeast direction favored, however confidence is not
high as nearby showers/thunderstorms may produce sporadic wind
gusts. Increasing cloud cover expected early this evening with
overcast conditions working in overnight with bases likely between 9-
10 kft.

For the rest of southern Nevada, northwest Arizona and southeast
California...An areawide southwest to southeast wind between 10-20
knots expected through the evening. Winds should favor a diurnal
flow regimes overnight, with the exception of sporadic wind gusts
from nearby showers/virga. Through the rest of the afternoon/early
evening, isolated thunderstorms will be limited to NW Arizona and
southern Sierra. Late tonight and into Monday a chance of isolated
to scattered shower activity is expected for most areas with the
exception of KDAG where shower activity is less likely. Accompanying
the showers will be broken/overcast sky conditions a CIGs down to 9
kft at sites south and east of Interstate 15.

205 PM PDT Sun Aug 20 2017

Initial influx of moisture has been northward through Arizona today.
There has been isolated thunderstorms in northern Mohave County with
scattered coverage along and just east of the county line in
Coconino and Yavapai Counties. Elsewhere, seeing less vertical
extent in the cumulus field over Inyo County/southern Sierra Nevada
then previous days.

In the big picture, a quasi-stationary upper low will remain off the
southern California through Monday. All 12z models agree that the
additional moisture needed to support precipitation and associated
jet dynamics/diffluence east of the upper low that will be providing
the lift for showers/embedded thunderstorms will be delayed until
late tonight. Latest operational HRRR and ESRL experimental HRRR
indicate lines of rain showers and probably some embedded
thunderstorms developing south of Las Vegas between 2-5 am Monday
morning. Those bands along with new development slated to lift north
through Las Vegas Monday morning. Again, viewing of the solar
eclipse likely will be obstructed by passing clouds/precipitation.

Air mass forecast to become more unstable Monday afternoon with
isolated/scattered thunderstorms possible within the entire forecast
area. Time heights suggesting low levels of the atmosphere will
remain somewhat dry meaning any stronger cells would likely produce
gusty winds and be less of a flash flood threat. Some some hail
possible as well with part of the area under 25-35kts 0-6km Bulk

Tuesday/Wednesday...Upper low still to our west with models depicting
a 60-70 kts jet across southern California/southeast Nevada and
northwest Arizona. Moisture parameters not overly impressive but
with less cloud cover surface heating will help destabilize the
atmosphere each day for continue threat of isolated/scattered
thunderstorms mainly across southern Nevada, northwest Arizona and
Inyo County. Gusty wind and hail would be threat with storms.

Beyond Thursday, models continue to advertise drying with
temperatures warming through the weekend.

.SPOTTER INFORMATION STATEMENT...Thunderstorm activity will
increase late tonight through Monday with thunderstorm chances
lingering Tuesday and Wednesday over much of the area. Spotters are
encouraged to report any significant weather or impacts according to
standard operating procedures.



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