Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 011507
AFDVEF

Area Forecast Discussion
National Weather Service Las Vegas NV
807 AM PDT SAT OCT 1 2016

.SYNOPSIS...Winds will increase this weekend as a storm system
approaches the area, with the strongest winds occurring Sunday
afternoon and evening. Temperatures will cool sharply early next
week as the system departs, with higher valley locations in the
southern Great Basin approaching the freezing mark Monday night.
Temperatures will warm gradually to near average by the end of the
week with dry weather expected.
&&

.UPDATE...It`s been lovely morning across the region. Today will
feature a slight chance of showers across northern Mohave County and
dry conditions elsewhere. There will also be some southerly breezes
of 10-20 mph by this afternoon. The main focus for today will be on
the forecast (wind in particular) for Sunday and Monday.
&&

.AVIATION...For McCarran...No significant cloud cover is
expected through the weekend. Light south-southeast wind expected
through late morning. Winds will increase and become more south-
southwest after 18z with gusts 20-25 kts. Gusts will will subside a
bit tonight with sustained speeds 10-15 kts. Stronger winds are
expected Sunday with gusts 30-35 kts late morning through Sunday
night.

For the rest of southern Nevada...northwest Arizona...and southeast
California...No significant cloud cover through the weekend except
perhaps northeast Mohave County where a shower or thunderstorm
cannot be ruled out today and Sunday. Light winds favoring diurnal
patterns can be expected through early this morning.  Most areas
will see increasing southwest winds by this afternoon as low
pressure approaches the area. Winds 10 to 15 KT gusting over 20 KT
will be common. Stronger winds are expected Sunday with gusts 30-40
knots.
&&

.PREV DISCUSSION...
308 AM PDT SAT OCT 1 2016

.SHORT TERM...through Sunday night.

Current water vapor loop showing the first of two vort maxes
approaching the Pacific Coast this morning (now located near
42N/127W) with the second making a quick jaunt southward (now near
49N/134W). The first system will rapidly progress northeastward
through the Pacific Northwest today as the second system continues
to deepen off the coast.  This second system will reach the Sierra
around 00Z Monday with the midlevel trough axis swinging through our
region shortly thereafter.  Operational models are in decent, though
by no means stellar, agreement regarding the finer-scale details of
the trough passage, with some minor phasing/timing issues noted
between the GFS/ECMWF and some discrepancies regarding the
orientation of the trough/vort max as it moves through the Sierra.

Though the general details of the short term forecast have not
changed much in the past 12 hours, some of the smaller-scale details
were modified with this forecast package.  For example, models are
more sluggish in scouring out moisture across Mohave County on
Sunday, even as the stronger Pacific trough approaches the area. GFS
(and to some degree the ECMWF) produce some precipitation in
northeast Mohave County on Sunday (in addition to today), so felt
inclined to add a slight chance of showers.  On the other hand, 00Z
runs are not particularly aggressive in producing precipitation on
the east side of the Sierra Sunday night, so decreased PoPs (and the
areal coverage of QPF) in this area.  Snow levels decrease markedly
Sunday night, with a potential dusting near the Sierra crest by
Monday morning.

The winds will be the main concern with this system, but latest
local/national hi-res models are not particularly gung-ho with
speeds/gusts on Sunday.  Though hints of a downslope wind event are
indicated in some of the hi-res simulations in the Sierra and Spring
Mountains, the orientation of the midlevel winds relative to the
ridge axes is not particularly conducive to high-wind events.
Moreover, forecast soundings are not especially favorable for
downstream mountain waves/rotors (lack of inversion near ridgetops,
no negative vertical shear signature, etc.). Forecast speeds at this
point are solidly in the wind advisory category in the Spring
Mountains and more marginal elsewhere.  With all of this in mind,
given the overall uncertainty with the wind speeds/gusts in general,
the lack of sufficient model continuity, and the remaining
discrepancies model to model, I decided to hold off on issuing wind
products with this forecast package.  Hopefully, this morning`s
model simulations will provide sufficient clarity to determine
when/where products should be issued.

Nevertheless, expect breezy to windy conditions across much of the
area on Sunday. Temperatures will cool as the Pacific trough
approaches Sunday and Sunday night, with Saturday`s temps slightly
above average and Sunday`s temps near average.  A cold front moves
through the southern Great Basin Sunday afternoon/night, bringing a
switch to northwest winds and an even bigger cooldown that begins
the long term portion of the forecast.

.LONG TERM...Monday through Friday.

The strong upper trough which is expected to generate gusty winds
over the weekend will be moving out of Nevada into Utah Monday. The
main impact of this system by Monday will be much cooler
temperatures with highs about 10 degrees below normal. Winds will be
lighter than on Sunday, and most areas will see a shift to north or
northwest winds. Despite the decrease in wind speeds, there will be
some locally breezy conditions. Moisture will mostly be pushed east
of the area but a few showers can`t be ruled out for northern
Lincoln County. Expecting a chilly night Monday night but at this
time we are not forecasting freezing temperatures except in some
mountain areas. The area will be under a dry northwest flow Tuesday
and Wednesday, transitioning to a southwest flow by Friday. This
will lead to gradual warming with near normal temperatures by
Friday.
&&

.AVIATION...For McCarran...No significant cloud cover is
expected through the weekend. Light south-southwest winds are
expected through early this morning. Breezy southwesterly winds will
develop after 17Z today with gusts 20-25 kts. South/southwest winds
will drop off a bit tonight but will be stronger Sunday with gusts
around 30 kts.

For the rest of southern Nevada...northwest Arizona...and southeast
California...No significant cloud cover through the weekend except
perhaps northeast Mohave County where a shower or thunderstorm
cannot be ruled out today. Light winds favoring diurnal patterns can
be expected through early this morning.  Most areas will see
increasing southwest winds by this afternoon as low pressure
approaches the area. Winds 10 to 15 KT gusting over 20 KT will be
common. Stronger winds are expected Sunday with gusts 30-40 knots.
&&

.FIRE WEATHER...Breezy southwest winds should begin to develop today
across the area, but will increase markedly on Sunday.  Combined
with low relative humidities, critical fire weather conditions will
exist across much of southern Nevada and southeast California.
Upgraded San Bernardino County to a red flag warning for Sunday
afternoon/evening, with the rest of the red flag warning left as is.
Sharply cooler temperatures and a switch to north/northwest winds
are expected on Monday.
&&

.SPOTTER INFORMATION STATEMENT...Spotter activation will not be
needed. Strong winds are expected to develop Sunday and continue
through Sunday night. Spotters are encouraged to report according to
standard operating procedures.
&&

$$

Short Term/Fire Weather...Shafer
Long Term.................Harrison
Aviation..................Pierce
Update....................Wolcott

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