Flash Flood and Dust Storm in Nevada, August 29 to 31, 2017

Area Forecast Discussion
National Weather Service Las Vegas NV
920 AM PDT Tue Aug 29 2017

.SYNOPSIS...A very hot weather pattern is expected across the Mojave
Desert Region much of this week as high pressure parks over southern
Nevada and surrounding states. Limited moisture circulating around
the area of high pressure will lead to isolated showers or
thunderstorms over the mountains each afternoon.
&&

.UPDATE...A few virga showers moving across the southwest Las Vegas
valley and across southeast California near Essex this morning.
Cloud cover and associated virga will continue to thin out through
the morning hours. With slightly deeper mid-level moisture and
weaker capping aloft today compared to yesterday, afternoon
thunderstorms may be a little more intense today. Storms will likely
initiate over the mountains this afternoon and remain focused over
the high terrain through the late afternoon and early evening.
Current forecast looks to be in good shape. -Boothe-

.PREV DISCUSSION...
250 AM PDT Tue Aug 29 2017

.SHORT TERM...through Thursday night. Main concerns are still heat
and thunderstorm chances. Excessive Heat Warning will remain in
place through Wednesday as forecast highs for Monday verified well
and not much change is expected for today and Wednesday. It is
possible that the warning may need to be extended into Thursday for
San Bernardino County and the Colorado River Valley, but will hold
off on this decision for now. Thunderstorm coverage was a bit
greater than expected yesterday, and as noted by previous shift, an
increase is expected in the northwest corner of the CWA today.
Raised PoPs just a bit in the southeast half of the CWA for today
and again for Thursday.

.LONG TERM...Friday through Monday.

The ridge overhead will break on Friday, rebuild off the western US
coast, then, migrate over northern Nevada into Utah over the
weekend. This will set up a period of lower heights and east to
southeasterly flow over most of the area. This will open the door up
for moisture and thunderstorm complexes to move into the area,
especially along/south of the I-40 corridor. The ridge then wobbles
and amplifies over the area next week.

The GFS and ECMWF are both in agreement in breaking off a piece of
energy from northern plains trough, then rotating it around the
upper-level high across the desert southwest on Saturday. With some
moisture already currently in place, and expected increases in
moisture from easterly flow, expect thunderstorm chances to be on
the increase for Saturday and Saturday night. Would not be surprised
to see a complex or two with flow coming off the Mogollon Rim,
especially along and south of I-40. Thus, have increased PoPs there,
though they may need further increases if current trends hold.

By sometime Friday or Saturday, a tropical system is likely to be
churning near the southern tip of the Baja Peninsula, well within
the window to induce a gulf surge. It gets rather complicated after
that. While a surge of low level moisture appears to be a good bet
at this point, upper-level ridge will begin amplifying over the area
by Sunday. This will act to divert most mid and upper level moisture
out to the Pacific, as well as the tropical system itself. Questions
about warm air aloft, subsidence, and depth of the low level
moisture surge preclude much of any confidence in the forecast
beyond Saturday.
&&

.AVIATION...For McCarran...There is a very low chance of a
thunderstorm near the terminal today, but there is a better chance
of one or more outflow boundaries affecting the terminal this
afternoon and evening with sudden wind shifts. Otherwise, expect a
typical diurnal wind pattern, with easterly winds less than 10 knots
this afternoon becoming south southwest around 10 knots after
sunset.

For the rest of southern Nevada, northwest Arizona and southeast
California...Spotty thunderstorms are expected again today, mainly
over the higher terrain, with a few storms possibly persisting into
the night. The main threats with storms will be erratic gusty winds,
briefly heavy rain, and mountain obscuration. Away from storms,
expect typical summertime winds of 5 to 15 mph, along with hot
temperatures and high density altitudes.
&&

.CLIMATE...

Climate Records for Las Vegas

                 Forecast Max Temp       Daily Record Max Temp
August 29               109                    110 (1948)
August 30               108                    112 (1948)
&&

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating
procedures.
&&

Morgan/Steele

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