Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 202350 AAA
AFDCYS

Area Forecast Discussion...Updated Aviation
National Weather Service Cheyenne WY
450 PM MST Fri Jan 20 2017

.SHORT TERM...(This evening through Sunday)
Issued at 219 PM MST Fri Jan 20 2017

Tonight...Relatively tranquil night with shortwave ridging aloft
moving overhead. Winds will decrease this evening.

Saturday...Primary shortwave trough works across Colorado, though
with enough moisture in the flow, expect an increase in orographic
snows over the Snowy and Sierra Madre Ranges in the afternoon. High
temperatures from the mid 30s to mid 40s based on projected 700 mb
temperatures.

Saturday night...Flow aloft turns northwest. Low and mid level
moisture decreases in the flow with areal coverage of snow showers
decreasing across our Snowy and Sierra Madre Ranges.

Sunday...Dry day with decent ridging aloft overhead. Similar
temperatures to Saturday based on thicknesses and 700 mb
temperatures.

.LONG TERM...(Sunday night through Friday)
Issued at 330 PM MST Fri Jan 20 2017

A longwave upper level trough will remain in place through late
next week resulting in cooler than normal temperatures through
Friday. Extended models still show a shift in the longwave pattern
which will result in a drier pattern for most of the western
United States. but first there is potential for a winter weather
event on Monday and Tuesday.

Difficult forecast remains for late Monday through Tuesday night.
Models have shown poor run to run consistency with the Pacific
storm system...and the ECMWF is now showing a solution which is
further south compared to the 12Z Canadian and GFS and is
different compared to the 12Z ECMWF from yesterday. A low
confidence forecast remains for this time period...but confidence
is beginning to increase that areas north of the North Platte
River valley and the Dakotas have the best chance to see moderate
to potentially heavy snow accumulations. Most of this snowfall
will have to come from the rapid development of the surface low
pressure system in addition to llvl instability since most of the
dynamic forcing aloft will move south of the forecast area in
Arizona and New Mexico with the southern disturbance. Unless the
northern system slows down...this energy will remain south of
Wyoming and western Nebraska which will limit precip amounts. Most
models show the surface low associated with the northern system
moving southeast from Wyoming or northern Colorado into the
central plains somewhere over Nebraska or Kansas. This track is
not favorable for a widespread moderate/heavy snowfall across the
area as we would like to see redevelopment across eastern Colorado
with the low moving northeast instead of southeast. Models
indicate the storm will eventually take a northeast track...but
this may be too late in the event. Areas near Douglas north and
eastward towards Lusk...Chadron...and Alliance have the best
potential for significant impacts from snow and wind.
However...There is still quite a bit of uncertainty as the
ensemble means and most of the individual members are slower and
several are further south. Believe most locations will see at
least some snow...so increased POP above 60 percent especially
north and east.

Models then show this system rapidly moving eastward into the
Mississippi river valley by early Wednesday with some snow showers
activity remaining over the mountains and eastern plains . It
will be on the cold side with highs in the 20`s Wednesday and
Thursday and lows in the single digits and low teens. Will need to
monitor which locations see decent snow amounts as this will
impact overnight low temperatures due to radiational cooling. Cold
northerly flow will continue through the end of the week as a high
amplitude ridge of high pressure builds west of the continental
divide.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday afternoon)
Issued at 450 PM MST Fri Jan 20 2017

A few light rain or snow showers will end by 02Z across western
Nebraska. Otherwise, VFR conditions prevail outside the mountains,
which will be obscured due to snow shower activity. Westerly winds
around 10 kt are forecast, except lighter and variable for western
Nebraska.

&&

.FIRE WEATHER...
Issued at 228 PM MST Fri Jan 20 2017

No concerns based on projected winds and humidities.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

SHORT TERM...RUBIN
LONG TERM...TJT
AVIATION...MAJ
FIRE WEATHER...RUBIN


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