Area Forecast Discussion
Issued by NWS Las Vegas, NV

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000
FXUS65 KVEF 281000
AFDVEF

Area Forecast Discussion
National Weather Service Las Vegas NV
300 AM PDT SUN AUG 28 2016

.SYNOPSIS...Drier and more stable air will work into the region
today with slight chances of storms confined to northern and eastern
Mohave County. Dry conditions along with above normal temperatures
are expected Monday and Tuesday before a weak system brings a slight
chance of showers and storms back into the forecast Wednesday and
Thursday.
&&

.SHORT TERM...through Tuesday night.

Potent vorticity maximum that brought last night`s active weather is
slowly trudging eastward through northern Arizona this morning.
Precipitable water is decreasing slowly but surely as the system
pulls away. This trend should continue today as subtropical ridging
noses its way northeastward into the area. With the increasingly dry
and stable air moving in (along with large-scale descent), chances
of precipitation are very low across the area. There are, however, a
couple of exceptions. The first is in northern and eastern Mohave
County, where orographic effects combined with daytime heating and
only a gradual decrease in atmospheric moisture content may get a
storm or two going. The past few HRRR simulations suggest the
possibility of a stray shower or storm moving into the Pipe Spring
area from southern Utah late this afternoon. However, expect no more
than isolated coverage given the increasingly unfavorable
environment for convective initiation. The second area of concern is
the Sierra, where daytime heating plus subtle lift from a very weak
perturbation in the midlevels may be sufficient to generate isolated
storms in the higher terrain. High-resolution guidance is more
consistent in generating convection here, so I am a bit more
confident that something will get going here this afternoon.

For Monday and Tuesday, subtropical ridging exerts greater influence
in our area as a deep trough becomes established in the northeast
Pacific. This will almost certainly preclude any thunderstorm
development in our area for the rest of the short term. Temperatures
will warm as atmospheric thicknesses increase, allowing for maximum
temperatures to reach slightly above average values early this week.
Think MOS is underdoing the warmup a bit (as it has tended to do
this summer), so increased temps a tad in this morning`s package.

The other noteworthy concern in the short term is an embedded
shortwave trough within the longwave trough in the eastern Pacific
that races northeast into northern California by Tuesday morning.
Model agreement on this vort max is below average, but these
differences do not really change the forecast for our area in a
meaningful way. Whether the faster ECMWF or slower GFS verifies,
Tuesday looks to get a little breezy in the afternoon as this vort
max makes its closest approach to the Southwest. This will be
especially true in the southern Great Basin, owing to its closer
proximity to the shortwave trough. May see a bit of an increase in
high clouds in northern parts of the area as well, so edged sky
grids upward a bit Tuesday and Tuesday night.

.LONG TERM...Wednesday through Saturday.

The second half of the week will see a broad trough along the West
Coast gradually move inland resulting in an increasingly dry
southwest to west flow and a cooling trend with breezy afternoons.
There is good general agreement among the medium range models with
this pattern, but there are some differences in the details with
respect to moisture advection up through northwest
Arizona...particularly Wednesday through Friday.

An inverted trough is forecast to lift up through northwest Mexico
and into Arizona by Thursday. The ECMWF is farther west with the
moisture than the GFS and depicts precipitation over Mohave County
Thursday and Friday before the West Coast trough progresses inland
and the flow becomes dry westerly. Even though the ECMWF indicates
the best potential for thunderstorms over northwest Arizona would be
Thursday, will keep a slight chance in for Wednesday to account for
possibly faster moisture advection, which we have seen occur several
times this summer. Then again, if the GFS solution is more correct,
thunderstorm potential would mainly be confined to far northeast
Mohave county and areas to the east. Temperatures will trend
downward a couple degrees each day under the influence of the trough
and southwest winds should kick up each afternoon with areas gusting
15-25 mph.
&&

.FIRE WEATHER...Conditions will be drying out and warming up through
midweek. As a weak system approaches the area on Tuesday, breezy
conditions will occur across the area, especially in the southern
Great Basin. However, critical fire weather conditions are not
expected at this time. A stronger system will approach the area
Wednesday and Thursday, bringing breezy conditions, elevated
humidities, and slight chances for storms, particularly in southern
Nevada and northwest Arizona.
&&

.AVIATION...For McCarran...Light northerly or northeasterly flow
should gradually become more easterly as the day progresses, with
winds likely remaining below 10 kts. Transition to light nocturnal
drainage flow is expected after sunset. VFR conditions through the
period.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Aside from KBIH, where typical diurnal winds should be
observed, mainly light northerly or northeasterly flow is expected
at the terminals tomorrow with perhaps somewhat stronger winds (10-
15 kts) in Mohave County and the Colorado River Valley. Winds should
favor typical nocturnal flow this evening and tonight. A stray storm
is possible in the Sierra and in N/E Mohave County this afternoon;
otherwise, VFR conditions should prevail.
&&

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

$$

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

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