Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 281500

Area Forecast Discussion
National Weather Service Las Vegas NV
800 AM PDT FRI OCT 28 2016

.SYNOPSIS...An unsettled weekend is in store for the region as two
systems move through, bringing chances of showers and gusty winds to
much of the area.  Temperatures will cool to near or slightly below
average early next week before gradually warming to slightly above
average by the end of the week.

.UPDATE...Several of the fast moving showers that developed in
the CWA this morning were producing lightning. Although instability
looks rather marginal at best based on lifted index progs and cape,
jet dynamics and moderately steep lapse rates at mid levels appear
to be causing enough lift for thunderstorm development. I updated to
add isolated thunderstorms today everywhere showers were already
mentioned. As expected, precipitation amounts so far have been on
the light side with most sites that have received rain reporting one
to four hundredths of an inch. The exception may be near the Sierra
crest based on satellite, radar and web cams.

.AVIATION...For McCarran...CIGs will continue to lower this morning
as a storm system approaches the area. Mainly light showers are
expected around the terminal however an isolated thunderstorm in the
vicinity of the airport is possible. Amended the TAF to include this
possibility. Winds will begin to increase from the S or SSW this
morning and may become gusty at times this afternoon.  There is at
least a chance for showers to affect the terminal this afternoon
with CIGs briefly lowering to 5-8 kft in their vicinity.  Conditions
will rapidly improve near or after sunset tonight with southwest
winds around 10 kts expected. Mountain obscurations are possible

For the rest of southern Nevada...northwest Arizona...and southeast
California...A storm system will move through the region today
bringing lowered CIGs, areas of rain showers and isolated
thunderstorms (primarily north of Interstate 40), and breezy south
to southwest winds (10-20 kts with slightly higher gusts).  CIGs may
briefly approach MVFR this morning near KBIH. Mountain obscurations
are likely with some mechanical turbulence possible through sunset.

305 AM PDT FRI OCT 28 2016

.SHORT TERM...through Sunday night.

An unsettled weekend is shaping up with the main events being
showers and breezy conditions today and strong winds with more
limited showers developing Sunday. Saturday looks to be fairly
benign though remnant moisture from former Hurricane Seymour will be
pulled across southern Nevada and surrounding areas leading to some
potential for light precipitation late Saturday through Sunday.

A closed low could be seen on satellite loops off the central
California coast this morning, and substantial mid and high level
moisture was streaming over the southwest states with help from
former Hurricane Seymour which is now post-tropical and is now
barely identifiable a few hundred miles west of the Baja Spur. The
combination of low and mid level moisture with the low moving inland
will lead to a southwest-northeast oriented band of precipitation
traversing our forecast area primarily during the daylight hours
today then tapering off over northwest Arizona this evening. The
inherited forecast is generally supported by the latest models and
changes were made to increase PoPs across Inyo County this morning
and across the Spring Mountains and central Nye/Lincoln Counties
today where numerous showers are expected. The rest of the zones
across southern Nevada and the Mojave Desert region will see
scattered showers as the precipitation band moves from west to east
and some sites may end up with only a trace or no precipitation at
all. So, will keep PoPs below 50 percent for Las Vegas and sites
south and east of I-15. Also, as the upper low opens up and moves
across central California and northern Nevada today, we will see a
notable increase in southwest winds with gusts around 35 mph,
especially over central Nye and Lincoln counties. Temperatures will
be similar to those observed Thursday.

As the upper low lifts away to the northeast this evening, attention
will turn to the next Pacific low currently dropping out of the Gulf
of Alaska. Light to moderate southwest flow will develop ahead of
this system Saturday and residual moisture off the Baja coast will
be drawn northward. Broad but weak upward forcing ahead of the
approaching system should be able to generate some precipitation
from this moisture...primarily across southeast California, the
Spring Mountains and central Nye and Lincoln Counties late Saturday
into early Sunday. However, amounts are expected to be quite light.
The most noteworthy impact from the approaching trough Sunday will
be widespread gusty winds and the potential for a strong downslope
event for the southern Sierra and Owens Valley Sunday afternoon and
early evening as the trough axis crosses the Sierra. Snow levels
will also drop to around 7,000-8,000 feet over the Sierra Sunday but
only light accumulations are possible on the eastern Slopes.
Not planning to issued wind headlines for Sunday yet, but wind grids
will indicate widespread gusts over 40 mph, especially over the
southern Sierra and south central Nevada zones. Temperatures are not
forecast to vary much Saturday and Sunday with highs holding a few
degrees above normal.

.LONG TERM...Monday through Friday.

Run-to-run variability remains considerable beyond about Day 3 of
the forecast, which renders very low confidence in the long term.
Operational model simulations tonight look quite dissimilar to last
night overall, and this clearly stems from the evolution of a series
of vort maxes moving rapidly along the polar jet in the northern

As mentioned in last night`s discussion, this model volatility is
associated with the uncertainty surrounding ridge amplification
in/near Alaska from a system well to the southwest of the Aleutian
Islands at this time.  Last night`s simulations produced a high-
amplitude ridge that was then quickly flattened and forced southeast
into southern Canada via a strong vort lobe moving southward through
northern Canada.  Tonight`s runs show completely different behavior.
Instead, they show the ridge stalling in AK/adjacent western Canada
followed by rapid retrogression in the polar latitudes.  This
precludes a much deeper southern-stream system from developing
before reaching our area on Wednesday, keeping most if not all of
the CWA dry during its passage.  In fact, the GFS barely produces a
deeper trough at all but instead develops a weak positively-tilted
trough that becomes almost zonally elongated across the
Intermountain West by the end of next week.  The ECMWF produces a
cutoff low near or just off the coast of northwest Mexico and shears
the vorticity as it moves through our area.  The CMC produces a deep
cutoff near the Four Corners.  The behavior of these solutions is
tied to the strength of the Alaska ridging and its degree of
retrogression thereafter, which will only be known in the next 24-48
hours. As such, it is likely poor model continuity will continue in
the Monday-Friday period until the weekend.

For now, decided a dry forecast with near-average temperatures was
best for the entire period.  Until details of any passing systems
become pinned down once the evolution of the Alaska ridge is
resolved, expect a low-confidence and potentially highly variable
forecast for this period.

.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.


Update:  Harrison
Aviation:  Salmen
Short Term:  Adair
Long Term:  Shafer

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