Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 081741 AAA

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1041 AM MST Thu Dec 8 2016

.SHORT TERM...(Today through Friday night)
Issued at 457 AM MST Thu Dec 8 2016

Brutally cold temperatures and wind chills are widespread across
the forecast area early this morning. Most sfc observation
stations are reading below zero temperatures, with the coldest
temperatures so far (as of 4am) Laramie at minus 21F and Deer
Creek along I-25 at minus 24F. Expect to see these reading drop a
few more degrees as we finish out the morning, with temperatures
today only warming into the teens. Thankfully some sunshine today
might help to somewhat relieve the deep chill in the air.

The deep upper trough will shift east today, with heights and
temperatures aloft rising. A weak disturbance will move across
Wyoming tonight, promoting sfc pressure falls along the spine of
the mountains while sfc high pressure persists in western
Colorado. The sfc pressure gradient will strengthen as a result,
with windy conditions developing over southcentral Wyoming as
this occurs. This pattern isn`t quite ideal to see widespread
strong winds, but WAA aloft could cause trapping of stronger
winds in the gap areas of the north Snowy Range foothills. Gusts
in excess of 55 mph are therefore quite possible as H7 winds are
progged to be 45 to 55 kts, so issued a High Wind Watch for this
area tonight through Friday morning, which includes Arlington and
the I-80 corridor from Elk Mountain to Arlington.

Strong westerly flow combined with increasing moisture from the
west will produce orographic snow over the Snowy and Sierra
Madres, with a few light snow showers perhaps moving off the peak
over the I-80 corridor near Arlington. Therefore, the strong
winds and light snow could make for slippery conditions here late
tonight through Friday morning. The mountains could see moderate
snow at times, with accumulations of 6 to 9 inches or so in this
pattern. Large scale forcing looks to be relatively weak so not
anticipating much more than this. Went ahead with a Winter
Weather Advisory starting this evening and continuing through
early Friday evening for snow and blowing snow. Snow should
diminish later on Friday as winds aloft weaken, but may
not completely taper off as westerly upslope flow will not
completely die off either. With the winds and WAA pattern,
temperatures will steadily rise through the night across the high
country, with highs for Friday back into the 20s and 30s across
the board.

.LONG TERM...(Saturday through Thursday)
Issued at 457 AM MST Thu Dec 8 2016

Persistent zonal jet stream will bisect the lower 48 thru the first
half of next week. Shortwave trofs embedded within this progressive
flow will yield periodic bouts of windy conditions, mountain snows
along with leaving the CWA vulnerable to Canadian cold fronts.
Winds will be the main concern on Saturday. Widespread belt of 50-55
knot 700mb peak during the morning hours, while slowly lessening
during the afternoon. KCAG-KCPR H85/H7 gradients sufficiently exceed
the 60 meter threshold during this time as well. Will likely need a
high wind highlight in time for favored wind prone regions of
southeast WY. These strong westerlies will combine with moisture
associated with embedded shortwave energy to produce mountain
snows beginning as early as late Saturday continuing on thru
Sunday. At this point would expect the need for a low end winter
headline for Snowy/Sierra Madre Ranges. There is some indication
that light snow will spread east onto the plains late Saturday
night and on Sunday morning as well. Have steered snow chances
upward some for areas south of the N Platte River on the SE WY
plains. Snow ends late Sunday as the shortwave trof departs. Aside
from perhaps some light orographic snows, dry conditions are
expected Monday. Both the ECMWF/GFS continue to hint at the next,
albeit brief, cold air mass will arrive Tuesday. Have continued
trending forecast highs lower on Tuesday. These will need to be
dropped over the coming days should model cooling trends continue.
Models become out of phase quickly by the middle of next week as
the GFS attempts to amplify the large- scale pattern. Until
confidence increases, have trended Wed-Thu towards normal
temperatures and drying conditions. High temperatures will
generally be in the 20s and 30s for most of the period, with the
exception of teens in areas Tuesday.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1033 AM MST Thu Dec 8 2016

VFR with cloud ceilings increasing/lowering from west to east
this afternoon and evening. Snow develops tonight west of the
Laramie Range, especially over the mountains and Carbon County.
Expect MVFR to IFR conditions in snow at RWL and SAA airfields
after 06Z, with mountain obscurement. West-southwest winds will
increase with gusts 25 to 35 kt from RWL to LAR late tonight.
Higher wind speeds will be possible over and near the Snowy/Sierra
Madre ranges, as well as at Arlington and Elk Mountain. Light snow
or flurries will be possible late tonight at CDR.


Issued at 447 AM MST Thu Dec 8 2016

Windy conditions will develop across the district this weekend,
with the highest speeds expected in the wind prone areas of
southeast Wyoming. No fire weather concerns are expected though
due to cool temperatures and high humidities.


WY...Winter Weather Advisory from 6 PM this evening to 6 PM MST
     Friday for WYZ112-114.

     High Wind Watch from 10 PM MST this evening through Friday
     morning for WYZ110.



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