Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 271524

Area Forecast Discussion
National Weather Service Las Vegas NV
825 AM PDT THU OCT 27 2016

.SYNOPSIS...Nice weather will continue today before the next storm
system moves through the area tonight and Friday.  A good chance of
showers exists for areas north of Interstate 40 with some high-
elevation snow likely in the southern Sierra.  After a brief break
on Saturday, another system will bring windy conditions and a chance
of precipitation to the Sierra and southern Great Basin on Sunday.
Much cooler weather is in store next week.

.UPDATE...Widespread high cloudiness was evident on satellite loops
this morning. The GFS indicates this layer of moisture will be
fairly thick throughout the day. I updated sky grids to account for
this trend of increased cloud cover today. -Harrison-

307 AM PDT THU OCT 27 2016

.SHORT TERM...through Friday night.

The ingredients are coming together for a widespread, although
generally light to moderate, rainfall event across much of our
forecast area late tonight through Friday. Impacts are expected to
be minor with the primary concerns being rain-slick roads, gusty
winds to 35 mph, and some high elevation snow in the southern Sierra.
The two main features to note are an eastern Pacific low spinning
near 30N/135W and moisture being pulled northward from rapidly
weakening Hurricane Seymour...which was centered near 20N/127W or
about 800 miles WSW of the southern tip of Baja California. The
latest GFS and ECMWF models are in pretty good agreement now with
the timing of the inland progression of the Pacific low and the
moisture streaming up ahead of it. A substantial amount of low to
mid level moisture will be stripped from Seymour, though the
majority of the low level moisture will head across northern Baja.
Indications are that the fairly wide band of precipitation will be a
little slower than was depicted 24 hours ago with precipitation
focusing over the southern Sierra and western Inyo/San Bernardino
Counties from late this evening through early Friday morning...with
perhaps light rain spilling into west central Nevada. The swath of
moisture will then spread across southern Nevada Friday morning then
transition over southeast Nevada and northwest Arizona Friday
afternoon before tapering off Friday evening. QPF amounts are
expected to be generally less than 0.10" across the Mojave Desert
zones and 0.10-0.30" for the Spring Mountains and zones to the north.
The southern Sierra should see up to 1" of liquid equivalent
precipitation and several inches of snow on the peaks. It looks like
snow levels will hold above 11,000 feet when the brunt of the
precipitation falls. There may be isolated thunderstorms across
south central Nevada and northwest Arizona Friday afternoon due to
the dynamic forcing associated with the shortwave lifting over
central Nevada, but CAPE and LI values are marginal. Temperatures
will peak today then drop several degrees Friday with the cloud
cover and showers.

.LONG TERM...Saturday through Thursday.

Lots of forecast concerns in the long term period, including
wind/precipitation associated with a storm system moving through on
Sunday, temperatures next week, and sensible weather effects from a
low-predictability system moving through on or around Wednesday.

Saturday looks to be mostly dry across the area as southwest
midlevel flow briefly takes hold.  However, operational models are
subtly hinting at residual light showers in the higher terrain of
Lincoln/Mohave Counties from the departing system affecting the
region during the short term as well as precipitation breaking out
in the southern Sierra as the next system from the Pacific
approaches the region Saturday night.  Operational models continue
to show considerable spread for the medium range with this
precipitation, with localized lift from topographic/mesoscale
effects likely inhibiting stronger consensus at this forecast range.
As such, felt it was wise to continue slight chance PoPs in the
higher terrain of Inyo County Saturday night to account for onset
timing uncertainties.  Saturday looks to be breezy across the
southern Great Basin and Inyo County as the surface pressure
gradient increases in advance of the system.

Models do seem to have increased consensus on the evolution of the
precipitation once it begins.  In general, the orientation of the
midlevel flow is more westerly with this system than with the Friday
system.  As such, substantially stronger shadowing is likely with
this system, and the 00Z ECMWF has come into better agreement with
the more consistent GFS in showing a drier solution downstream of
the Sierra.  Nevertheless, there will be some jet dynamics in play,
particularly Sunday afternoon, across the southern Great Basin.
Moreover, the more aggressive 00Z NAM breaks out more precip in
Lincoln/Mohave Counties in advance of the system, apparently tied to
subtle isentropic lift/warm air advection downstream of the main
trough.  As such, continued slight chance to low chance PoPs in
these areas, but did reduce the southern extent of mentionable PoPs
on Sunday and Sunday night quite a bit.  Snow levels look to start
around 10,000 feet in the Sierra but will decrease to around 8500-
9000 feet by Sunday night.  Accumulations with this system look to
be similar to or a little less than Friday`s system -- around a
trace to a couple inches at elevations of 8500-10,000 feet up to 4-8
inches near ridgetop level.

Winds remain a big concern with this system.  High-resolution
guidance continues to suggest a favorable setup for a downslope
event in the Sierra and White Mountains.  A 90+-kt 500-mb jet
oriented nearly perpendicular to the Sierra/Whites ridge line(s)
will set up for about 24 hours (00Z Sunday to 00Z Monday) with an
isothermal layer or inversion setting up near ridgetop level.  Local
hi-res models continue to suggest about a 6-12 hour period in which
strong winds sweep down to near or over the US 395 corridor Sunday
afternoon. Considered a high wind watch for the Owens Valley and the
Sierra with this package, but think the event is a bit too far out
(especially with an intervening storm late this week) and
uncertainty remains a bit too high to justify it at the moment.
Meanwhile, southwest winds will become strong/gusty across the
deserts/valleys of Inyo County and the southern Great Basin on
Sunday.  Wind advisories will likely be required here as well.

Temperatures this weekend will remain above average, though
confidence in Sunday`s forecast values is low as the effects of
cloud cover will compete with enhanced vertical mixing.  However,
after system passage Sunday night, temperatures will sharply
decrease to near or slightly below average values early next week.
Conditions should dry out Monday and Tuesday.

Operational models are now suggesting a deeper system moving through
the area Tuesday night and Wednesday.  Some runs (e.g., the 12Z
ECMWF) showed a slow, wet progression through the area, whereas
others (e.g., the 00Z GFS) show little precipitation affecting the
region.  Large uncertainty exists at this point.  This is tied to
how a series of storms in the northern Pacific amplify and displace
a longwave ridge in Alaska and western Canada.  The recent trend has
been for the models to amplify the ridge briefly before it gets
flattened and shoved southeastward by a downstream trough dropping
southward through northern Canada.  This evolution displaces the
polar jet (storm track) southeast to the southwestern U.S., which
explains the much deeper systems depicted in tonight`s simulations.
Not completely sold on this outcome yet owing to a common model
tendency to progress longwave ridges too quickly and well-known low
predictability with systems undercutting these ridges.  For now,
introduced low-grade PoPs for the higher terrain of the northwest
CWA and northern Mohave County, where consensus is a bit stronger in
generating precip. Substantial modifications are likely in this time
frame owing to the aforementioned predictability issues, however.

.AVIATION...For McCarran...Light, diurnally-driven winds are
expected today and tonight with considerable high cloudiness.  A
storm system will move through on Friday bringing a chance of
showers, lower CIGs, and breezy southwest winds.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Increasing high cloudiness is expected on Thursday as a
storm system approaches the region.  South winds will begin to
increase (10-20 kts with gusts to 25 kts) in the southern Great
Basin and Mohave County with lighter, more terrain-driven flow
elsewhere.  By evening, the Sierra should begin seeing precipitation
with mountain obscurations likely.  This precipitation will spread
east late tonight and Friday with lowered CIGs anticipated.  Breezy
southwest winds will also occur Friday.

.SPOTTER INFORMATION STATEMENT...Two storm systems will affect the
area through this weekend, which may produce significant
precipitation in the Sierra and strong winds in the southern Great
Basin and Inyo County on Sunday. Spotters should report significant
weather according to standard operating procedures.


Short Term:  Adair
Long Term/Aviation:  Shafer

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