Dust Storm Warning 6, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
959 AM PDT THU SEP 22 2016

.SYNOPSIS...A strong trough moving through the region will bring
widespread strong winds and enhanced fire danger to the region into
tonight. Showers and thunderstorms with heavy rainfall will be
possible across mainly northwest Arizona as well. Cool temperatures
will persist through Friday before returning to near normal over the
weekend.
&&

.UPDATE...
Band of convective showers slowly moved over Mohave County
overnight...producing flash flooding in Colorado City and some
additional county road closures near Wikieup. More showers are
developing in Mohave County in association with an elongated
vorticity maximum in advance of a deep/strong trough moving east
into the Great Basin at this time. WV imagery is showing this trough
continuing to deepen and will likely induce a slight negative tilt
before the jet streak rounds the base of the trough later today.
Strong difluent/divergent flow exists in much of the Interior West
located in the favorable left front quadrant of the jet streak.
Meanwhile, a tropical fetch of moisture continues to advect north in
advance of the system, leading to (1) several rounds of convective
showers in the warm sector of the system (mainly now confined to
northwest Arizona) and (2) efficient, mostly warm-cloud
precipitation processes, in which radar frequently underestimates
rainfall rates (as it did this morning near Colorado City) and there
is a notable lack of lightning. As stronger midlevel winds continue
to advance eastward, cell motions with these showers have been fast,
but the flow is oriented favorably for training, so the flash flood
threat continues in northern Mohave County through the
afternoon/evening.

Latest high-resolution guidance suggests storms will rapidly develop
in northern Mohave County this afternoon with slow system motion
eastward given the nearly perpendicular SSW steering flow.
Environmental conditions are favorable for isolated severe storms,
with wind gusts the main threat. Hail may occur given the cold
midlevel temperatures, but instability is quite limited...so
updrafts will be weak.

Strong synoptic-scale winds will be the primary threat from this
system for a vast majority of the area. Seeing several sites in the
southern Great Basin and Mojave Desert already reaching wind
advisory criteria. Latest guidance suggests winds will exceed
criteria through the afternoon in most of the area. Expanded current
advisory into Death Valley and Lincoln County...given current obs
and recent hi-res guidance. The Spring Mountains may flirt with high
wind warning criteria this afternoon, but not confident enough at
this time to upgrade their advisory. Red flag warning expanded to
the Spring Mountains and Sheep Range with the morning package given
projections of minimum relative humidities reaching criteria. It
will take some drying to get this to verify, but upstream
observations in Inyo County combined with expected downsloping this
afternoon should take care of this.

Busy day with additional updates likely.
&&

.AVIATION...For McCarran...Southwest winds 15-25 kts with gusts to 40
kts will be likely through the afternoon and early evening. Forecast
confidence has lowered with cold frontal passage this evening, with
some high-resolution guidance suggesting winds will struggle to
become northwesterly at all overnight. Speeds should decrease after
sunset, but unclear to what degree. Expect somewhat frequent updates
to the TAF through the day as the details become clearer. Mechanical
turbulence is expected today in the lee of the Spring Mountains.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Chances of showers/storms confined to Mohave and
Lincoln Counties today with strong west to southwest winds the main
concern elsewhere. Cold front will move through the Southern Great
Basin and Inyo County this afternoon/evening with switchover to
northwest directions during/after passage. Winds should diminish
after sunset, but may be slower to occur in the Mojave Desert. VFR
conditions outside of storms. Mechanical turbulence and mountain
wave activity is anticipated through this evening.
&&

.PREVIOUS DISCUSSION...
320 AM PDT THU SEP 22 2016

.SHORT TERM...Through Friday night.

A strong area of low pressure currently over northern California
will continue to move southeast through the day. This will lead to a
rather active weather day across the southern and eastern areas of
the CWA through the afternoon. Remnant moisture from tropical system
Paine remains over the eastern third of the CWA and light to
moderate showers have been occurring overnight across portions of
Mohave County. This trend is expected to continue through the
morning hours before additional forcing is expected to bring an
uptick in shower and thunderstorm coverage. Deep moisture combined
with increasing instability along with increased jet dynamics will
lead to more widespread and possible severe thunderstorms over
Mohave County this afternoon, especially as some drier air starts to
work into the mid levels. Although PW values will decrease as the
afternoon wears on the likelihood of training storms will lead to an
increase flash flood threat for mainly northern Mohave County late
morning and afternoon. The current flash flood watch looks good.
Some storms are also possible for far eastern Lincoln County, but
the bigger threat looks to remain over Mohave Co. No need to expand
the flash flood watch at this time.

The other main weather impact today will be increasing winds.
Current RAWS shows some gusty winds over the eastern Sierra, but so
far little else for areas to the east and south. However, winds are
expected to increase significantly late this morning and continue
into tonight before tapering off behind a cold front that will sweep
across the area. Winds 20 to 30 mph with gusts to 45 mph will be
likely over most areas with some potential gusts to 50 mph possible.
The strongest winds are expected over western San Bernardino County
where gusts to 60 mph are possible. Have gone ahead and upgraded the
wind advisory to a high wind warning for zone 523 which includes
Barstow and Fort Irwin. Although there may be some gusty north winds
initially behind the front, strong persistent north winds are not
expected to develop and at this time will not issue any advisories
for the Colorado River Valley where north winds can sometimes be an
issue.

Precipitation will taper off across Mohave county late this
afternoon as drier air sweeps across the forecast area. However, a
few showers will still be possible overnight. Temperatures behind
the weather system will be quite cool (around 10 degrees below
seasonal average) Friday afternoon. Dry conditions areawide are also
expected Friday.

.LONG TERM...Saturday through Wednesday.

Not much change in thinking for this period. Low pressure which
affected the area through the short term will be to our east by
Saturday morning, with northerly mid level flow over our area
between the departing low and high pressure moving into the West
Coast. The main concern for the weekend will be stiff north winds
down the Colorado River Valley.
MOS numbers have been increasing for the past couple of days, and
Lake Wind Advisories may be needed for the weekend. Otherwise, quiet
weather is likely, with a warming trend commencing on Saturday and
continuing through the period.
&&

.FIRE WEATHER...Still anticipating widespread red flag conditions
today across much of the region with increasing southwest winds
ahead of a cold front and gusty northwest winds behind the front.
RH may be more marginal for the lower Colorado River south of
Needles but still stands the chance to see patchy red flag
conditions. Improving conditions with lighter winds expected Friday
but remaining dry.
&&

.SPOTTER INFORMATION STATEMENT...Spotters in Northwest Arizona are
encouraged to report severe weather or flooding. Spotters elsewhere
can report significant weather (especially blowing dust, wind
damage, or fire starts) according to standard operating procedures.
&&

Update/Aviation...Shafer
Short Term/Fire Weather...Gorelow
Long Term...Morgan

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