Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 191724

Area Forecast Discussion
National Weather Service Cheyenne WY
1024 AM MST Sun Feb 19 2017

.SHORT TERM...(Today through Tuesday)
Issued at 300 AM MST Sun Feb 19 2017

Main forecast concern will be strong winds and the approaching
upper level low which brought quite a bit of rain and mountain
snow to California and Arizona on Friday. This low has stalled
over central Arizona early this morning but is expected to lift
northeast as an open wave trough as more upper level energy digs
into the Pacific NW coastline. Models show the trough gradually
moving into Colorado later today with scattered rain showers
w/mountain snows developing over the mountains and pushing north
and east into the evening hours. There is some llvl instability as
this feature weakens and slides can not rule out a
few strikes of lightning near the I80 corridor east of Cheyenne
into the southern panhandle of Nebraska this afternoon. Although
the system is weak and will move through relatively quick...did
note strong but brief upper level diffluence associated with the
right entrance region of the jet max. This may result in a period
of heavy rain mixing with snow sometime this afternoon and this
evening. Snow levels will lower to around 6500 feet so expect all
rain east of Interstate 25 as 700mb temperatures will struggle to
fall below -5c.

After the precip band moves east...winds will then become a
concern as the GFS continues to hint towards a Bora event behind
the front tonight through Monday. All other models are struggling
with this wind event while the NAM shows 850 and 700mb gradients
30 to 40 meters lower compared to the GFS. Was hoping this would
be resolved for the 00z and 06z model runs...but considerable
differences remain. Confidence is still too low to upgrade the
High Wind Watch to a Warning at this time. Of greater concern is
the second wind event Monday night and into Tuesday with better
model agreement and the dynamic forcing to support a sustained
high wind event...especially across the wind prone areas. Models
show a 120 knot jet entering Wyoming behind the upper level trough
with stronger llvl subsidence and pressure falls across the High
Plains into early Tuesday morning. Added the Arlington zone to the
High Wind Watch for a later time compared to the Laramie
Range...mainly for late Monday night and Tuesday as gradients
rapidly increase. Any additional precipitation Monday and Tuesday
will be mainly confined to the mountains. High temperatures early
this week will remain pleasant with highs in the 60`s and even low
70`s Tuesday afternoon east of Interstate 25. Daytime highs will
remain in the 40`s to low 50`s west of the Laramie Range with
Rawlins and Saratoga likely the cold spots across southeast
Wyoming. Sustained winds between 25 to 35 MPH with gusts around 50
MPH are expected across southeast Wyoming on Tuesday...possibly
extending as far east as the Nebraska/Wyoming border. Do not
anticipate High Wind headlines outside of the wind prone areas at
this time...but will continue to monitor over the next 24 to 36
hours since these Bora events can be tricky.

.LONG TERM...(Tuesday night through Saturday)
Issued at 230 AM MST Sun Feb 19 2017

Main focus in this period will be with the storm system that will
affect the region primarily from Weds night through Thursday night
with significant snow possible. Looks like it will remain mild for
one last day Weds under west-southwesterly flow aloft. Some
rain/snow showers should occur over the higher mtns while lee
cyclogenesis initiates over southeast Colorado ahead of a large
upper trough over the western CONUS. Colder air will filter in
over the CWA late Weds while the upper trough progresses eastward
to the Rocky mtn area by late Thursday. The lee cyclone will
deepen over far se Colorado into sw Kansas during the day Thursday
resulting in an increasingly moist upslope flow over the CWA.
This should allow rain and snow to become fairly widespread over
the CWA Thursday...likely becoming all snow by late afternoon as
the colder air deepens over the area. Snow should continue
Thursday night as the system moves into the central plains with
snow tapering off Friday as the system wraps up over the upper
Mississippi River area. Will have to watch this system as
accumulating snows looking a good bet though too early to tell
where. Friday night and Saturday looking mainly dry and pretty
cool ahead of the next upper low that will be sinking south along
the west coast.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1016 AM MST Sun Feb 19 2017

Ceilings will lower through the afternoon, but remain VFR. MVFR
conditions develop west of the Laramie Range this evening with
passing rain/snow showers. VCSH for terminals east of the Laramie
Range after 03Z. The mountains will be obscured this afternoon
and tonight. Westerly winds will become gusty this afternoon
across southeast WY and continue tonight. Gusts of 25 to 40 kt
are expected, highest near the mountains.


Issued at 214 AM MST Sun Feb 19 2017

Minimal Fire Weather concerns early this week. Although a period
of windy conditions are expected starting Monday with relative
humidities lowering to around 15 percent...fuels are not critical
due to the time of the year. Colder weather is expected late this
week along with a good chance of accumulating snow.


WY...High Wind Watch from Monday evening through Tuesday morning for

     High Wind Watch from 8 PM MST this evening through Monday
     afternoon for WYZ106-116-117.



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