Flash Flood and Damaging Wind in Arizona, August 18 to 20, 2016

Area Forecast Discussion
National Weather Service Las Vegas NV
304 PM PDT THU AUG 18 2016

.SYNOPSIS...Slight chances of storms will continue east of I-15
and in the higher terrain of southern Nevada and southeast
California through Saturday before chances increase across the area
as a weak system moves through the Desert Southwest early next week.
Temperatures will cool gradually, becoming near to slightly below
average early next week.
&&

.SHORT TERM...through Saturday night.

A modest push of moisture thanks to outflow from thunderstorm
activity in Arizona last night...combined with the influence of a
broad trough aloft and a small circulation moving across southern
Nevada has resulted in a rather active afternoon of convection.
Strongest activity has been confined mainly to areas along I-15 and
points eastward. The largely unidirectional southwesterly flow aloft
has also supported some training of thunderstorm cells resulting in
an enhanced potential for localized flash flooding. With the loss of
daytime heating later today suspect much of this activity will
diminish but some isolated convection may linger through 11 PM or so
thanks to the added upper level support brought by the trough moving
through.

Slightly drier conditions are expected Friday with the trough axis
shifting southward and shearing apart. Best moisture and instability
will likely be across northwest Arizona tomorrow, but some isolated
activity will still be possible across southern Clark County and the
Spring Mountains.

Conditions will begin to moisten up again Saturday into Saturday
night as a broad low drifts towards the southwest from Southern
California. While the moisture is not overly impressive...the added
dynamics of a broad low moving in will support increased PoPs and
cloud cover late Saturday and into the extended range.

.LONG TERM...Sunday through Thursday.

Forecast thinking has not changed much since yesterday at this time
as models are beginning to converge on the evolution of a weak
midlevel perturbation moving through the Desert Southwest late this
weekend through early next week. The perturbation will be leftover
vorticity from the trough moving through the Interior West late this
week, settling underneath an amplified but fairly progressive ridge
in the eastern Pacific this weekend. Meanwhile, an upstream trough
will impinge upon this ridge and kick the perturbation eastward into
the Southwest Sunday through Tuesday, gradually becoming the
southern extent of the trough axis swinging through the northern
Rockies.  As this perturbation moves through the area, relatively
cool midlevel temperatures and at least weak large-scale ascent will
foster an environment favorable for scattered convection across the
area, especially Sunday and Monday. Given the overall weakness of
the perturbation and associated lift, suspect that orographic lift
and differential heating will be the key initiators of convection
during this period, with outflow/storm-scale processes regenerating
convection thereafter. Operational guidance appears to favor areas
north of I-40 Sunday and Monday with chances becoming confined to
Lincoln and Mohave Counties Tuesday and Wednesday as the
perturbation begins its push east of the area. However, confidence
remains a bit below average owing to the small-scale processes
likely involved during this period (nearly always badly simulated
this far in advance).

Increasing confidence in a more considerable cooldown Sunday through
Tuesday as MOS continues to suggest sub-triple digit temperatures
for Vegas. Given expected cloud debris and lowering heights as the
weak system moves through the area, temps in the mid to upper 90s
for highs are certainly reasonable. Continued to trend highs lower
for this period, and if MOS guidance has anything to say about, may
not be quite low enough.

Models begin to dry the region out as the weak midlevel perturbation
moves out of the area by Wednesday. Temps should warm as shortwave
ridging moves in, clearing the skies out. However, not expecting
temps to climb to much above average readings Wednesday and
Thursday. Again, somewhat low confidence here given an increased
lack of model continuity (and a tendency for models to swing these
systems through too quickly).
&&

.AVIATION...For McCarran...Scattered thunderstorms will remain
possible through early evening before becoming less likely towards
sunset. Gusty winds will remain possible with gusts to 25 knots.
Otherwise...scattered-broken cloud cover above 10kft with convective
turbulence likely. Winds will be a challenge with thunderstorm
activity around but should favor south or southwesterly directions.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Isolated to scattered thunderstorms will remain
possible through sunset mainly along and east of a KELY to KNXP
line. Areas of convective turbulence likely east of this line along
with gusty surface winds near thunderstorms in excess of 25 knots.
Southwesterly winds of 15 to 20 knots likely this evening across the
western Mojave Desert near KDAG.
&&

.FIRE WEATHER...Isolated to scattered storms will continue during
the afternoon and early evening hours through Saturday, especially
east of I-15. Temperatures will cool to near seasonal values by this
weekend. A weak system will move into the region late this weekend,
increasing the chances for storms across the area Sunday and Monday.
Critical fire weather conditions are not expected through the
period.
&&

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
significant weather, including any flooding and/or wind reports,
according to standard operating procedures.
&&

Short Term/Aviation:  Outler
Long Term/Fire Weather:  Shafer

For more forecast information...see us on our webpage:

http://weather.gov/lasvegas or follow us on Facebook and Twitter