Weather Service Report

FXUS65 KVEF 261033

Area Forecast Discussion
National Weather Service Las Vegas NV
333 AM PDT WED OCT 26 2016

.SYNOPSIS...Tranquil weather is expected through Thursday before the
next system moves through the area Thursday night and Friday, with
chances of precipitation increasing especially for northern parts of
the region.  After a brief break on Saturday, another system is
likely to affect the area late this weekend.

.SHORT through Friday night.

Dry and mild conditions under a ridge of high pressure today and
Thursday will deteriorate Thursday night and Friday as a Pacific
system brings another good shot of precipitation to much of the
region. A Pacific trough digging along 140W has a decent plume of
subtropical moisture rounding its base. However, forecast models are
in good agreement that substantial moisture from Hurricane Seymour
will be entrained in the southwest flow ahead of the system.
This will spread across Inyo County and west central Nevada Thursday
night then quickly over southern Nevada and northwest Arizona
Friday. There is some uncertainty on how the moisture band and
precipitation will evolve as the Pacific system opens up and moves
inland. The body of the upper low will move across central/northern
California and northern Nevada while the base of the trough pushes
the brunt of the moisture over southern Nevada Friday morning then
off into Arizona Friday afternoon. The ECMWF is still a little
faster than the GFS but both show a similar trend of the best window
of precipitation generally in a midnight to 10 AM window Friday
morning across southeast California and southern Nevada then 10 AM
to 4 PM Friday afternoon across northwest Arizona. Precipitation
amounts are not as expected to be as high as the most recent system
due to the quick transit of the moisture. However, the southern
Sierra should intercept a lot of precipitation with significant snow
likely above 10,000 feet. A warming trend today and Thursday will be
knocked down several degrees Friday as clouds and showers move over
the region.

.LONG TERM...Saturday through Wednesday.

The midrange forecast looks active, with a storm system affecting
the area Sunday and Sunday night followed by another system Tuesday
and Wednesday.

The long range starts with a transition day Saturday.  Southwest
flow will be in place as the system affecting our area late this
week progresses rapidly northeast, amplifying the longwave ridge
across the Rockies and adjacent Central Plains.  An upstream and
rather deep trough moves onto the Pacific Coast by 12Z Sunday.  This
system features a strong 500-mb jet streak (near 100 kts) that will
be positioned nearly orthogonal to the southern Sierra Sunday.  High-
resolution models are unsurprisingly aggressive in generating very
strong winds in the high terrain of Inyo County.  Local high-
resolution simulations suggest the presence of an inversion near
ridgetop level Sunday afternoon with at least some tendency for
strong winds to descend to the midslopes by this time.  With the
favorable synoptic pattern and dynamic/thermodynamic environment
present, this will require monitoring as the storm system nears.
Potential fly in the ointment is the onset of precipitation, which
would likely shut down the winds in a hurry.

The 00Z operational runs continue to show increased consensus in
timing of the system and in its potential impacts to the area.  The
00Z ECMWF is much more aggressive in developing precipitation in the
southern Sierra and downstream into the southern Great Basin.  The
GFS depicts a similar scenario, albeit with a stronger shadowing
signal and a noticeable displacement of the stronger lift and more
extensive precipitation southeast of the ECMWF.  Nevertheless, the
decent agreement among these models (with at least some improvement
in ensemble spread) prompted me to increase PoPs quite a bit for
Sunday northwest of I-15.  A rather strong indication of
precipitation in Lincoln County, possibly aided by right entrance
region jet dynamics, called for boosting PoPs here in particular and
lingering these chances through Sunday night.  Snow levels remain
high with this system (9000+ feet), so any snow will be confined to
the highest terrain of Inyo County.  Models are noting at least some
moisture tap from the remains of Tropical Cyclone Seymour, which may
explain the general increased aggressiveness in QPF with tonight's
model runs.  If such trends continue, this may become a more
substantial storm than has been suggested by the models to this
point.  Stay tuned.

After this system moves out Monday, broad southwesterly flow
reestablishes itself across the region early next week.  With
ridging remaining to our east, temperatures will be near to slightly
below average for this time of year.  Another system is expected to
approach the region Tuesday night.  So far, models are not
particularly aggressive in producing precipitation in our area
(owing to a more favorable orientation of the flow for substantial
precip shadowing from the Sierra).  Nevertheless, there is at least
some indication from the 00Z GFS/ECMWF of precipitation breaking out
in northwest/north-central Arizona in a region with some jet
dynamical support (250-mb coupling) and some low-level warm air
advection.  For now, kept PoPs unmentionable Tuesday, as most of
this large-scale lift appears to occur to our east, but if models
trend slower/stronger with the trough, would not be surprised to see
Mohave/Lincoln Counties experiencing some additional precipitation
with this system.  Breezy conditions look to be likely on Tuesday as
the system approaches the area.

Models diverge strongly after Tuesday, with quiet zonal flow (GFS)
versus a high-over-low synoptic pattern (ECMWF/CMC) indicated. Large-
scale pattern in the Pacific makes me lean toward a more
blocky/amplified pattern, but with such a stable/progressive pattern
in place so far this autumn, do not have much confidence in that

.AVIATION...For McCarran...Light, diurnally-driven winds are
expected for the next 24 hours.  VFR conditions are forecast with a
general, gradual increase in high clouds today.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Have seen some indications of patchy fog in some valley
locations this morning, including near KDAG.  This should dissipate
rapidly after sunrise.  South breezes will commence in the Owens
Valley and southern Great Basin today (speeds 10-20 kts with gusts
to 25 kts or so).  Otherwise, light/diurnally-driven flow expected
across the area with VFR conditions.

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any areas of fog, including visibility estimates early this morning.
Otherwise quiet conditions are expected today and tonight.


Short Term:  Adair
Long Term/Aviation:  Shafer

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Western Regional Climate Center,