Flash Flood Watch 5, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
747 PM PDT Tue Aug 21 2018

.SYNOPSIS...Expect decreasing winds overnight with afternoon breezes
on Wednesday. Monsoon moisture will remain in place through Thursday
over Mohave County where the best chance for thunderstorms will
continue. Dry conditions with seasonal temperatures on Friday and
over the weekend.
&&

.UPDATE...A sharp moisture/instability gradient exists generally
along and east of a line from Baker, CA - Boulder City, NV - St.
George, UT. Isolated showers/thunderstorms that developed across
southern Mohave, eastern San Bernardino and southeast Clark Counties
will continue to be on the downward trend over the next hour. Most
active area today and forecast to continue overnight will be across
northern Mohave County. HRRR and other convective allowing models
suggest several narrow lines of convection will continue overnight.
Drier air associated with the Pacific Northwest trough has brought
surface dewpoints down into the teens and 20s across Inyo and
Esmeralda Counties and 30s-low 40s across Nye and western San
Bernardino County. The Las Vegas Valley dipped into the mid 40s this
afternoon. However, dewpoints creeping up a bit with sunset as the
valley lies near the previously mentioned moisture boundary.

For Wednesday. Sharp moisture/instability gradient forecast to
remain intact nearly along a similar line or slightly further
eastward. The current forecast for isolated to scattered
showers/thunderstorms primarily across Mohave County looks fine.
&&

.NEAR TERM...through the afternoon.

Visible satellite imagery shows an obvious counter clockwise spin
near Nogales AZ moving north. SPC mesoanalysis shows moisture axis
extending from the lower Colorado River Valley up through Mohave
County where 850mb moisture transport vectors indicate the best
potential for storms producing heavy rain. RAP has moderate to
strong instability focused across Mohave County with the area of
highest CAPE and lowest LIs lifting north through the afternoon.
Otherwise, there is nothing obvious in current observational data
that would indicate other areas that would be favorable for near
term development. 130 pm wind gusts across the region are in the 25-
35 mph range which remains under advisory levels. Temperatures
overall are running 3-6 degrees cooler than yesterday, in line with
the forecast.

Tonight through Friday...

Once daytime heating is lost at sunset, storm development should
dissolve with no other driver in place to maintain convection into
the overnight hours. The moisture/instability gradient remains over
essentially the same area (from the Mojave Preserve to Las Vegas to
Mesquite) Wednesday and Thursday however, the region of deeper
moisture and better instability is further east. Thunderstorm
chances linger over primarily Mohave County Wednesday and Thursday
afternoon before mainly dry conditions return for Friday.

Temperatures will be near to slightly above normal through Friday.

.LONG TERM....Saturday through Tuesday.

Little change in the long term forecast as southwest flow will bring
in drier air across much of the region and keep monsoonal moisture
confined to northwest AZ through the weekend. By Sunday, the medium
range models bring in a trough across the NW states which will
result in increased southwest winds. By Monday, models are in pretty
good agreement and dig this trough further into the Great Basin
region through Tuesday. This trough will likely scour out the
remaining low grade moisture over northwest AZ and bring gusty to
windy southwest winds across the region. As of now, these winds look
to be below wind advisory criteria. Otherwise, expect mostly clear
skies and near or slightly above normal temperatures through the
period.
&&

.AVIATION...For McCarran...Breezy southeast winds are expected to
continue through the rest of the day, eventually subsiding overnight
tonight. Peak winds gusts of 30-35 kts are possible between the 21-
04Z time frame before beginning to subside and the potential for
gusts up to 25 kts may linger as late as 08Z tonight. Models
continue to produce storms this afternoon but latest runs keep most
activity east of the KLAS terminal. So left VCTS out of the TAF with
this package as confidence is still low for terminal impacts.
However, this is not to say that an isolated cell is out of the
question this evening. If so, erratic outflow winds will be the main
impact.

For the rest of southern Nevada, northwest Arizona and southeast
California...Gusty southerly winds will begin to overspread the
region through the rest of the morning and into the afternoon.
Strongest southwest winds are expected between 20-02Z later this
afternoon and evening with gusts to 25-35 kts. Thereafter, winds
should steadily decrease with gusts dropping off by 06-07Z tonight
and then should return to diurnal winds by the early morning. Models
develop isolated to scattered storms late this evening as far west
as the I-15 corridor and best chances for VCTS should be for KEED
and KIFP.
&&

.FIRE WEATHER...Isolated to scattered thunderstorms over Mohave
County Wednesday and Thursday otherwise southwest winds with gusts
to around 25 mph on Wednesday. Relative humidity under 20 percent
across much of the region on Wednesday except in eastern San
Bernardino, southern Clark and Mohave counties. Drier conditions
push into all areas expect Mohave County on Thursday. Mainly dry
conditions area wide Friday and over the weekend.
&&

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

UPDATE...Pierce

SHORT TERM...Salmen
AVIATION...Boucher

LONG TERM...Kryston

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