Flash Flood and Damaging Winds in Nevada, July 29 to 31, 2021

Area Forecast Discussion
National Weather Service Las Vegas NV
240 PM PDT Thu Jul 29 2021

.SYNOPSIS...Another episode of high monsoonal moisture is
expected through the weekend which will lead to an increased threat
of flash flooding and strong outflow winds. A Flash Flood Watch has
been issued for much of the region through Sunday morning. Beginning
late Sunday, drier conditions are expected to move in and result in
several days of drier and warmer conditions through next week.
&&

.SHORT TERM...Today through Sunday.
The focus of the near and short term forecast is the return of
anomalous monsoonal moisture and the prospects for flash flooding
through the weekend.

Beginning today, low level moisture is expected to begin spreading
into the region from the south. This is well evident on GOES17 WV
imagery and already this afternoon, several areas of high terrain
convection have begun. The 12Z KLAS sounding (and continued through
20Z in the RAP) indicated a stout inverted V sounding paired with
increasing moisture. While the better upper level support arrives
tomorrow, today's convection will be mostly rooted on high terrain
areas and push northwestward with the mean flow. That being said,
today will likely be the best opportunity for downburst wind
potential as the column continues to moisten but mid and low level
dry air still exists. Especially if this convection intensifies on
high terrain and downdrafts fall through much drier low desert
areas. Of course, as convection continues, outflows will largely
steer the potential for organized convection, but a lack of
effective shear today should help to keep that from growing too far
upscale. Best potential for impacts to the Las Vegas Valley would
come in the way of wind most likely, either by convection that
initiates over the Black Mountain area or inheriting outflow winds
from convection in Arizona or southwest Utah...or both.

Moving to Friday and over the weekend, PWATs begin to exceed 200% of
normal, which is seasonally our period of highest moisture
climatologically. At the same time, inverted troughing from Mexico
slides northward adding additional upper level lifting to graduate
flash flood risk from isolated to more widespread, hence the
decision to issue a watch. The track of the trough will enhance
lifting across much of the area but especially the high terrain
mountain areas of Clark and Inyo counties. 12Z HREF is really
highlighting enhanced upslope lifting due to this trough passage and
this is also supported in the 12Z GFS. If this comes to fruition,
the flash flood and damaging wind threat could be of particular
concern for these areas.

Yet another disturbance is progged to follow northward, but this
time from the Pacific Saturday, leading to a similar concern
Saturday and Saturday night. PWAT values don't change much
afternoon/evening convection will again produce similar threats. The
highest impact areas are less certain Saturday but any location that
sees significant impacts Friday will be highly vulnerable for
further or exacerbated impacts Saturday.

Sunday is a little less certain as there are some indications that
this second shortwave lingers across the Great Basin, pushes farther
north into Idaho, or northeast and accelerates regional drying.
There may be a need to extend our Flash Flood Watch for part of the
area into Sunday but did not have the confidence yet in this coming
to fruition nor the areas that would be affected.

All this to say that the preparations should be taken to prepare for
flash flooding impacts and strong, possibly damaging outflow winds
through the weekend. Ensuring you are capable of receiving Flash
Flood Warnings and other weather alerts is key and staying weather
aware will be important.

&&

.LONG TERM....Next Week.
A chance in pattern over the long term looks to be on the horizon as
a reprieve from anomalous moisture takes shape in the long range
ensembles. PWAT anomalies actually swing to below average, in both
the EC and GEFS by early next week with high pressure solidly in
place over southern California. This would allow for temperatures to
rise back to around normal values and give a break to the region
that has seen an extended period of monsoonal thunderstorms.
However, moisture gradually increases late week and by next weekend,
precip chances may return to the CWA.

&&

.AVIATION...For McCarran...Scattered thunderstorms are expected to
increase across the area this afternoon and evening. CIGS will drop
to around 8kft in and near any storms. The best chance for storms
impacting the terminal is between 01 and 07z Friday. Erratic winds
can be expected near any storms. After 09Z, storms will decrease in
intensity. When storms are not impacting the terminal, expect light
and diurnal winds with CIGS aoa 15kft.

Thunderstorms are possible through Sunday with the greatest chance
Friday and Saturday.

For the rest of southern Nevada, northwest Arizona and southeast
California...Scattered thunderstorms will impact most terminals as
storms become more widespread through the evening into the overnight
hours. Winds will be erratic with CIGS around 8kft near any
thunderstorms. When storms are not around terminals, expect typical
diurnal winds and periods of CIGS around 15kft.

&&

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant flash flooding impacts or wind gust/damage reports
over the weekend.

&&

DISCUSSIONS...TB3
AVIATION...Berc

For more forecast information...see us on our webpage:
https://weather.gov/lasvegas or follow us on Facebook and Twitter