Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 200530 AAA

Area Forecast Discussion...Updated Aviation
National Weather Service Cheyenne WY
1030 PM MST Thu Jan 19 2017

.SHORT TERM...(This evening through Saturday)
Issued at 113 PM MST Thu Jan 19 2017

Tonight...Initial shortwave trough aloft rotates negatively tilted
across southern Wyoming and northern Colorado with low and mid level
moisture increasing across our western counties, where we have
painted a chance of snow at lower elevations west of a Douglas to
Laramie line, with widespread orographic and dynamic snow over our
Snowy and Sierra Madre Ranges. Dry elsewhere. Relatively mild due to
cloud cover.

Friday...Not quite as mild in the wake of a weak cold frontal
passage and with more cloud cover in the form of mostly cloudy
skies. Lift is negligible, so expect a chance of snow only over our
Snowy, Sierra Madre and northern Laramie Ranges with adequate low
and mid level moisture.

Friday night...Low amplitude shortwave ridging aloft will make for a
mostly dry night though cooler than Thursday night with less cloud
cover and more radiational cooling.

Saturday...Slightly cooler in most locations based on cloud cover
and cooler 700 mb temperatures. Scattered orographic snow showers
anticipated over our Snowy and Sierra Madre Ranges on moist
northwest flow aloft with dry conditions elsewhere.

.LONG TERM...(Saturday night through Thursday)
Issued at 245 PM MST Thu Jan 19 2017

A longwave upper level trough will remain in place through late
next week resulting in near normal/slightly cooler than normal
temperatures. Models are hinting towards a shift in the longwave
pattern by later next week...but first there is potential for a
major snow event on Monday and Tuesday before a drier weather
pattern develops.

A progressive ridge aloft will push across the forecast area on
Sunday ahead of the next storm system digging southeast along the
Washington/Oregon coastline. Temperatures near or slightly above
normal are expected due to southwest flow aloft...although most
areas north and west will remain in the 20`s to mid 30`s for
highs. Snow will develop over the mountains Sunday night as a
strong 160 kt jet lifts into Colorado and Wyoming.

Difficult forecast shaping up for late Monday through Tuesday.
Models have shown poor run to run consistency with this storm
system...although they are in better agreement overall compared to
24 hours ago. GFS and ECMWF now show the Pacific storm weakening
as it moves onshore...but then redeveloping further north and east
across the plains compared to before. These models now place the
upper level low over Nebraska as it rapidly moves eastward. The
12z Canadian is somewhat it shows the southern
energy considerably stronger compared to the GFS/ECMWF solutions
as well as further south and east with the development of the
upper level and surface low. The GFS/ECMWF solutions would suggest
higher snow amounts further east across Nebraska with lighter snow
across most of southeast Wyoming...especially further south along
the I-80 corridor. There is still quite a bit of uncertainty as
the ensemble means and most of the individual members are slower
and further south. With all this in mind...did not make many
changes to this portion of the forecast since confidence in any
particular scenario is low at this time. It will be rather cold
however with highs in the 20`s Wednesday and Thursday and lows in
the single digits and low teens.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1030 PM MST Thu Jan 19 2017

Clouds will increase and lower from west to east overnight and
Friday. VFR cigs will prevail east of the Laramie Range, with
periods of MVFR to IFR cigs/vis to the west as snow develops.
Airfields affected will be RWL and SAA. Light snow spreads east
to LAR late tonight with a period of MVFR cig/vis between 14Z and
18Z Friday. West to northwest winds 10 to 15 kt will prevail, with
gusts to around 20 kt at CYS Friday.


Issued at 126 PM MST Thu Jan 19 2017

No concerns based on projected winds and humidities.




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