895 
FXUS65 KVEF 070908
AFDVEF

Area Forecast Discussion
National Weather Service Las Vegas NV
108 AM PST Sun Nov 7 2021

.SYNOPSIS...Temperatures will remain above seasonal normals
today as a dry weather pattern remains over the region. Early in 
the week an atmospheric river will move into central California 
and weaken as it moves south. This will bring the chance of high 
elevation snow (>8500ft) to the Sierras, as well as some minor 
low level rainfall in the northern Owens Valley. By the middle of 
the week, broad high pressure will build over the desert southwest
bringing drier conditions and warmer than normal temperatures to 
the area.

&&

.SHORT TERM...Today through Tuesday.
The short term focus continues to be the prospect of an
atmospheric river to impact the western CONUS Monday and Tuesday
and what effect it will have to the Sierra, Owens Valley and
overall Great Basin. 

What looking more certain:
1) Sierra snowfall should be rather substantial but it will be 
almost certainly confined to over 10000ft. 

Why? Because the brunt of the AR will be positioned too far north
and will begin deforming as high pressure builds over the Pacific
quickly after landfall. Snow levels below 8500 ft quickly rise as
the parent low pushes northeast and heights begin to build. 

2) Winds will be quite strong, but again confined to high
elevations. This is due to strong ridgetop flow resulting in a
robust mountain wave set up. This will be maximized overnight
Monday into Tuesday morning as the low takes on a negative tilt
and energy pushes over the Sierra. NBMv4.1 is showing max wind
gust potential of over 60 mph at times as this feature pushes over
but as it stands, conditions are not looking favorable, at least
dynamically, for downslope winds. However, depending on how much
precip is able to blow over, precip drag could bring higher winds
down to the western extent of the Owens Valley. The 12Z ECMWF EFI
highlight CA 519 and 523 as the zones most likely needing a wind
product Tuesday of some kind, but at this time, looks advisory
level. 

Where uncertainty is still high: 
1) Snowfall totals for Aspendell (will be sitting very near the
snow level). NBM 24 hour snowfall percentages show ~25% chance for
greater than 2 inches but 5% of being greater than 6 inches.
Either way this isn't advisory criteria but what is concerning is
overall QPF values are much higher, with ~20% chance of >1" over
24 hours. While confidence is increasing that the snow levels
should stay above the elevation of Aspendell, can't rule out much
higher numbers if the snow level indeed stays below what is
forecast. 

2) QPF totals for the northern Owens Valley. WPC guidance has
consistently kept values generally below a quarter inch for Bishop
throughout the event, but that is falling between the 50th and
75th percentile according to the NBM. Thus, illustrating that
higher values are possible (~25%). Feel the WPC number is quite
fair given the fact that we'd be inheriting a deteriorating AR and
it will be taking a northwest to southeast orientation by the time
it reaches Bishop. 

To summarize, a very interesting and complex system should overall
prove to be minor but chances for greater impacts are non-zero 
for our CWA. It's looking more like our most likely product
issuance will be wind related for the Sierra and possibly San 
Bernardino. 


.LONG TERM...Wednesday through Saturday. The aforementioned trough 
and associated atmospheric river are forecast to be gone Wednesday, 
with decreasing clouds and post-frontal northerly winds expected 
areawide, with gusts of 20 to 30 mph possible down the Colorado 
River Valley. Thereafter, models agree on ridging building somewhere 
near or west of the Desert Southwest, meaning dry weather with above 
normal temperatures areawide. If the ridge builds overhead, winds 
would be light; if it builds farther west, northerly breezes would 
be felt down the Colorado River Valley. Otherwise, barring a major 
change in the expected pattern evolution, low impact weather is 
likely outside of the Colorado River Valley through the period.

&&

.AVIATION...For McCarran...Winds around 8-12 kts will prevail 
into the early morning hours today favoring a south-southeasterly 
direction. Winds are forecast to shift back southerly and become 
gusty up to 25 kts during the afternoon. No operationally 
significant cloud cover forecast. 

For the rest of southern Nevada, northwest Arizona and southeast
California...Another day of gusty southwest winds are expected for
all terminals except KBIH. Cloud cover should remain above 15kft
and remain FEW/SCT.  

&&

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating
procedures.
&&

$$

DISCUSSION...TB3
AVIATION...TB3

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