Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 131015

Area Forecast Discussion
National Weather Service Cheyenne WY
315 AM MST Sat Jan 13 2018

.SHORT TERM...(Today through Monday)
Issued at 315 AM MST Sat Jan 13 2018

Main forecast concern will be later this weekend as another
arctic airmass intrusion is forecast over the eastern Plains
beginning Sunday night. For Today and Sunday, relatively benign
weather is expected as a ridge of high pressure aloft over the
Great Basin region attempts to nudge eastward. A weak shortwave,
embedded in the mean northwest flow aloft, will rapidly move
southeast over the high plains today. Already seeing some snow
flurry activity across far eastern Wyoming and across the
northern/central Nebraska panhandle early this morning with
visibilities briefly reduced to 2 miles at times. Believe this
trend will continue and expect the coverage of flurries/snow
showers to increase by early this afternoon as this shortwave
brushes the area. A few places may see a dusting to up to a half
inch of snow, especially near the Pine Ridge. Otherwise,
temperatures will be near or slightly below normal today and
generally in the mid 30`s to mid 40`s. Temperatures will be even
warmer on Sunday ahead of the arctic front, with afternoon
readings in the mid 40`s to low 50`s.

By Sunday evening, models show the arctic front moving southwest
near the I-25 corridor between sunset and 9 pm. Temperatures will
quickly lower into the teens behind the front with snow developing
and moving southwest into the eastern plains of Wyoming.
Precipitation will be the result of strong jet energy aloft, llvl
forcing, and some warm air advection just off the surface
providing lift. With a moderate amount of frontogenesis along the
front, there are some concerns of heavy snow bands along and east
of the Laramie Range Sunday night and into early Monday morning.
Expect any banded snow to be brief as the front will move
relatively fast. Models QPF has increased over the last 24 hours
and soundings are showing a more favorable environment for some
snow accumulations along and east of the Laramie Range. For now, 1
to 3 inches looks like a good bet across most of the forecast
area. Winds appear to be minimal with this front so blowing snow
does not appear to be a factor at this time.

.LONG TERM...(Monday night through Friday)
Issued at 138 AM MST Sat Jan 13 2018

Models are in somewhat good agreement with the general upper level
flow pattern for this period.  The best chance for precipitation is
at the end of the week.

Tuesday into Wednesday the upper level short wave ridge moves across
the forecast area.  As the ridge moves through it will push the cold
air mass east of the forecast area.

Wednesday night a clipper system will progress quickly across the
forecast area.  Noticed that models have shifted the track of this
upper level closed low south, which places most of the lift with
this trough south of the forecast area.  Snow will be confined to
the higher elevations as the upper level closed low moves through.
The GFS has a similar position to the ECMWF from last night, with
the latest ECMWF much further south, over the Four Corners Region.
Since the GFS seems to be following the trend of the ECMWF of
shifting the track of the low further south, would not be surprised
if it continues to move the low southward.  If this does happen
precipitation chances will shift south of the forecast area with it.

The best chance for precipitation for the period will start Friday
as a stronger upper level short wave trough moves through.  Models
agree very well considering the time range.  The main discrepancy is
in regard to the speed of the trough.  Both the ECMWF and the GFS
have slowed down the trough and amplified the ridge over the eastern
CONUS.  If the ridge continues to strengthen the trough will
continue to slow down.  A slower trough will mean more precipitation
for the forecast area.

Friday could have very strong winds with the upper level short wave
trough.  700mb winds are 50-55kts over the forecast area.  If winds
of this magnitude do develop, the visibilities will be quite low
from the blowing snow.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1014 PM MST Fri Jan 12 2018

Still have some lingering MVFR conditions near KSNY, but do think
these ceilings will begin to break up next hour or two. VFR
conditions remainder of night with northwest wind flow. Gusty
winds across southeast Wyoming Saturday.


Issued at 246 AM MST Sat Jan 13 2018

No Fire Weather concerns this weekend through early/mid next week
as cool conditions and high relative humidities will prevail. An
arctic cold front will move south across the area for Monday,
resulting in more snowfall and cold temperatures early next week.




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