Flash Flooding and Hail in Nevada, July 29 to 30, 2018

Area Forecast Discussion
National Weather Service Las Vegas NV
213 PM PDT Sun Jul 29 2018

.SYNOPSIS...Warm conditions expected through the week, though
temperatures will be a few degrees cooler than observed over the
past few days. Increasing thunderstorm chances are expected Monday
through Thursday with gradually decreasing coverage expected by next
weekend.
&&

.SHORT TERM...Tonight through Tuesday.

So far this afternoon, thunderstorms has primarily been confined to
the terrain across Clark and northern Mohave counties. This trend
will continue through the afternoon, with most convection locked in
over the terrain due to a very light steering flow. With fairly
large dewpoint depressions still in place combined with adiabatic
lapse rates, modest DCAPE overlaps areas of greatest instability -
Lincoln, Clark, central/northern Mohave counties. Thus, stronger
thunderstorms that develop may produce isolated flash flooding and
gusty outflow winds. Further south across southern San Bernardino
County and lower Colorado River, cloud cover has limited diurnal
heating and destabilization thus far. Clearing is expected over the
next several hours, however there is question on whether
destabilization can be achieved. This evening, guidance has backed
off on the strength of westward propagating cold pool from central
Arizona into northwestern Arizona. Even though CAMs have backed off,
cold pool propagation would favor an easterly to northeasterly
direction this evening. So, even if convection dissipates along the
cold pool, strong outflow winds may sweep across central/southern
Mohave County and into the Colorado River valley - confidence is
low. Convection should wane this evening with the loss of diurnal
heating.

Thunderstorm coverage will push further north and west Monday and
Tuesday as high pressure itself slightly east of the area.
Confidence is low on which areas will see more thunderstorm coverage
over another as vort lobes are suggested to periodically rotate
around the ridge over the next few days. This dynamic forcing may
allow convective activity to flourish or it may limit it with
blanketing cloud cover. At the moment confidence is too low to make
the call on which day or days will be more active than others. Stay
tuned to the forecast to get the latest details.

Outside of thunderstorm and cloud influences, temperatures will
remain near or slightly above July normals through the week.

.LONG TERM...Wednesday through Saturday.

The main area of high pressure will remain in place over the
southwestern US through the end of the week. With continued high
PWATs in place as well as sufficient afternoon destabilization, kept
most of the area for Wednesday and Thursday, with heavy rain and
flash flooding possible. By Friday, the ridge starts to break down
somewhat as a trough moves through the Pacific Northwest. While the
main trough will stay far to the north of us, the area of high
pressure should weaken along with the south flow, drying things out
slightly and decreasing precipitation chances for the first part of
the weekend. Longer range models show some disagreement in how that
will effect precipitation chances, with the Ecmwf being rather dry
but the GFS still pumping out QPF through out much of the area.
Based on moisture profiles and instability, backed off on
precipitation chances slightly for areas north and west of I-15,
with the best chances for any kind of thunderstorm activity being
over Mohave County.

Looking ahead, the lull in monsoonal moisture should be a short
lived lull as the ridge builds back in starting Sunday and moisture
as well as precipitation chances gradually increases.

Temperatures will remain slightly above normal through the week,
though should be a bit cooler than what we have been experiencing
this past week. The biggest change to temperatures was on Wednesday
per guidance trends. Otherwise, little change was made to the
temperatures forecast for the second half of next week.
&&

.AVIATION...For McCarran...East to southeast winds will become
southwesterly after about 02Z around 10 kt.  A few thunderstorms are
possible around the area, mainly over the mountains this evening.
While storms are not expected in the Las Vegas Valley, outflow winds
could affect the terminal.  Winds on Monday will begin light and
variable than transition to more easterly late morning. Thunderstorm
activity should be more prevalent on Monday.

For the rest of southern Nevada, northwest Arizona and southeast
California...Typical diurnal wind patterns are expected for most of
the regional TAF sites outside of the Colorado River Valley.  Near
KIFP and KEED...breezy south winds with gusts to 20 knots are likely
through the evening. A few thunderstorms will develop across the
higher terrain of southern Nevada and Northwest arizona, potentially
building southwest towards the Colorado River in the late evening
and overnight. Smoke will continue to lower vsby at KBIH overnight
into Monday.  Thunderstorm activity will be more widespread on
Monday.
&&

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

SHORT TERM...Boothe
LONG TERM....Wolf
AVIATION.....Berc

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