Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 271648 AAA

Area Forecast Discussion
National Weather Service Las Vegas NV
948 AM PDT WED JUL 27 2016

.SYNOPSIS...High pressure will lead to very hot temperatures for the
region into the end of the week. A shift in the pattern will allow
monsoonal moisture and thunderstorm chances to increase for the
weekend which will also bring cooler temperatures.

.UPDATE...Morning models resolved high pressure centered over the
southern Great Basin at 5 AM shifting the high center east to the
Nevada/Utah state line by 5 PM. Models show an inverted trough lying
across central Arizona this morning with the western extension of
this feature forecast to move into southern Mohave and southeastern
San Bernardino counties this afternoon. It could help initiate
isolated convection as it approaches the richer moisture environment
of southern Mohave and eastern San Bernardino counties. The latest
HRRR and local high resolution models are forecasting convection in
these areas but also over the Sierra this afternoon as well. Went
ahead and added a slight chance of thunderstorms along the Sierra
spine for this afternoon. Otherwise, look for a very hot day once
again with temperatures inching closer to record levels in some
areas. Will continue to monitor and see how things develop over the
next couple of hours.


.SHORT TERM...Through Friday night.

The main focus for the short term is excessive heat. Temperatures
have been flirting with criteria the past number of days and look to
warm a few degrees for today and Thursday with a ridge building
overhead. Given the stretch we are currently in along with a
borderline forecast, have elected to issue an excessive heat warning
for through tomorrow evening for the Mojave Desert and Colorado
River Valley. For Thursday, highs will be 110-115 for much of the
Mojave Desert, 115-120 for the Colorado River Valley and the mid
120s for Death Valley. By Friday, approaching shortwave should begin
to nudge the ridge eastward and may begin to allow some moisture
return, both of which should result in the start of a cool down.

With the ridge of high pressure in control for the short term,
expect even lesser chances of any thunderstorms today and tomorrow,
with subsidence and warming mid-levels limiting chances to very
slight, and only over some of the higher terrain.

.LONG TERM...Saturday through Tuesday.

Still looks like an active period shaping up with monsoon moisture
already in place at the beginning of the period. Raised PoPs again,
especially in Mohave County, through the period. Also lowered high
temperatures somewhat, as new MOS guidance came in with highs down
to around 100F at KLAS Sunday and Monday, which makes sense given
the cloud cover and precip chances. Bias corrected guidance gave
higher temps, but given the prolonged dry pattern of late and the
expected shift toward a wetter pattern, expect the bias corrected
guidance to underperform. Models still agree on the existence of a
trough rolling into the Pacific Northwest Monday or Tuesday, but
have diverged on its strength and timing. Earlier thinking was that
this trough would be strong enough to bring dry southwest flow to
the northwest third or so of our CWA, and left that trend intact for
now. If the trough is deep enough to dry out the northwest zones, it
could also produce enough shear to increase the risk for organized
storms in the central and southeast zones. If the trough is weaker,
then there will be less risk for organized storms, but precip
chances could go all the way to Dyer and Bishop. We shall see.

.FIRE WEATHER...Dry and hot conditions into the end of the work
week. Minimum relative humidities will likely drop to around 10
percent for much of the low elevations/deserts across the region on
Thursday. Winds will remain fairly light with some afternoon breezes
today and tomorrow. Friday begins the weekend changes with
increasing winds, moisture, and thunderstorm chances.

.AVIATION...For McCarran...Light diurnal winds through the period.
Very hot temperatures will lead to high density altitudes, otherwise
no significant impacts expected.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Generally light, mostly diurnally-driven winds are
expected through the period. VFR conditions are expected through the
period, although an isolated thunderstorm cannot be ruled out in
Mohave county.

Spotters are encouraged to report any significant weather
conditions according to standard operating procedures.



Short Term/Aviation/Fire...Steele
Long Term..................Morgan

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