Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 191028
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
328 AM MST Fri Jan 19 2018

.SHORT TERM...(Today through Saturday)
Issued at 328 AM MST Fri Jan 19 2018

Overnight water vapor imagery showed the upper ridge axis from
the Upper Midwest to the Desert Southwest. The flow aloft has
shifted to the southwest, ahead of a vigorous eastern Pacific
upper trough. A quasi-stationary surface boundary extended from
the southern NE Panhandle to a low over southern MT, then across
eastern ID and northern NV. The pressure gradient between this
feature and a surface ridge over eastern UT/western CO was tight
enough to generate gusty westerly winds of 40 to 50 mph through
the wind corridors between Laramie and Rawlins, and near Bordeaux.
Unseasonably mild nighttime temperatures for January prevailed
with 30s and 40s.

This morning`s forecast challenge will focus on high winds for
parts of our wind prone areas. Have not seen sustained winds of
40 mph or higher nor peak gusts of 58 mph or higher yet. Models
depict downward momentum of 50-55 kt 700mb winds and 850/700mb
gradients of 50-60m peaking just before sunrise. For these
reasons, will maintain the High Wind Warning for the Arlington,
Elk Mountain and Bordeaux areas through late this morning. The
day shift may cancel the Warning early should these high winds
not materialize.

Downslope westerly low level winds will once again promote very
mild temperatures today. Record or near record highs will be
possible at Cheyenne and Sidney with highs in the upper 50s and
lower 60s. The Pacific trough will move ashore by this afternoon,
then track east across the Great Basin toward the Four Corners
Saturday afternoon. A strong cold front extending from the western
Dakotas into southwest WY tonight will progress southeast through
the day Saturday. Snow will develop beginning late tonight across
western Carbon County, spreading as far east as Cheyenne, Scottsbluff
and Chadron by Saturday evening. It will turn much colder behind
the front Saturday, with high temperatures in the 30s to lower
40s.

.LONG TERM...(Saturday night through Thursday)
Issued at 328 AM MST Fri Jan 19 2018

As the low starts to deepen over eastern Colorado Saturday
evening, snowfall rates will increase in a wide swath extending
from central Colorado northeastward across southeastern Wyoming
and the Nebraska Panhandle Saturday night into Sunday morning.
Snowfall rates could be heavy at times as the northern 2/3rds of
our forecast area come under the region of maximum lift, deepest
moisture and coldest midlevel air on the northwest side of the
upper low. For the early morning package, we expanded the coverage
of the Winter Storm Watch eastward to include our remaining
counties in southeast Wyoming as well as the northern 2/3rds of
the western Nebraska Panhandle. Timing of the onset of heavier
snowfall and travel impacts look similar to what we were
advertising yesterday, starting Saturday morning into the
afternoon for the western counties, to along I-25 by late
afternoon, then into the Nebraska Panhandle through the evening
into the early morning on Sunday. Confidence still a bit higher of
4+ inches of snowfall for areas north of a Cheyenne to Alliance
line, with localized amounts of 8 to 10 inches possible some lower
elevation areas and upwards of 1 to 2 feet expected in the
mountains. Latest model guidance suggests we could see an uptick
of stronger winds on the backside of the system Sunday across the
Plains, so we upped wind speeds for the eastern Plains which would
suggest potential for blowing snow issues during the day Sunday
along and east of I-25...something we will need to keep an eye on
for additional refinements to messaging as details become more
clear.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 328 AM MST Fri Jan 19 2018

VFR. Sky conditions SCT-BKN between 15000 and 25000 ft AGL, except
at KRWL where CIGS lower to 5000-10000 ft after 00Z. Wind gusts 25
to 40 kt will occur at the southeast WY terminals, especially west
west of the Laramie Range.

&&

.FIRE WEATHER...
Issued at 328 AM MST Fri Jan 19 2018

It will be another dry and unseasonably warm day today before drastic
changes take place this weekend. Lowest afternoon humidities will
range between 20 and 25 percent. Very windy conditions will persist
this morning, especially in and near Arlington and Bordeaux. A vigorous
Pacific storm system will track from the Four Corners Saturday across
southeastern Colorado Sunday. A strong cold front will push through
the area Saturday night. Snow will spread east across southeast Wyoming
and western Nebraska Saturday afternoon through Sunday morning.
Significant accumulations are expected, especially over the mountains
where one to two feet may fall. Lower elevations may receive five
to ten inches of snow by Sunday afternoon. Gusty north to northeast
winds will cause blowing and drifting for the plains Sunday. Snow
will taper off by Sunday evening. Much of next week will be seasonably
cool, with occasional snow showers over the Snowy and Sierra Madre
mountains.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for WYZ102-107-108-117>119.

     High Wind Warning until noon MST today for WYZ106-110.

     Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for WYZ101-106-115-116.

     Winter Storm Watch from Saturday morning through Sunday
     afternoon for WYZ103>105-109>114.

NE...Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for NEZ002-003-019-020-095-096.

&&

$$

SHORT TERM...MAJ
LONG TERM...JG
AVIATION...MAJ
FIRE WEATHER...MAJ



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