Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 032319 AAA
AFDCYS

Area Forecast Discussion...Updated Aviation
National Weather Service Cheyenne WY
418 PM MST Sat Dec 3 2016

.SHORT TERM...(This evening through Monday night)
Issued at 255 PM MST Sat Dec 3 2016

Not many forecast concerns over the next 12 to 18 hours since
strong gusty winds from this morning are diminishing a bit this
afternoon. This trend will be brief however as winds are expected
to pick up once again on Sunday. Cancelled the High Wind Warning
for this afternoon a few hours early since gusts have dropped
below 40 to 45 mph over all wind prone areas. Another cold night
tonight as low drop into the upper single digits to upper teens
across the region...coldest across the lower valleys.

Dry weather expected to continue on Sunday as zonal flow aloft
develops over the area. Temperatures will moderate a bit as a
response to this as 700mb temperatures range between -2c and -7c
across southeast Wyoming and western Nebraska. This should result
in high temperatures in the mid 30`s west of I25...and in the 40`s
east of I25 due to warm downslope winds. These winds may become
quite strong across portions of southeast Wyoming as models show
700mb winds above 50 to 55 knots by Sunday evening. It appears the
Arlington area across eastern Carbon county has the best chance to
see High Wind criteria in the late morning and afternoon
hours...so upgraded the High Wind Watch tomorrow to a warning.
Kept the watch going for the Bordeaux area since the llvl gradient
is not all that impressive through Sunday afternoon...but
eventually gets stronger Sunday night. However...models show the
lee side troughing a bit too far west to justify a warning at this
time. Extended the watch to include the southern Laramie Range and
foothills west of Cheyenne...including the I80 summit...since
700mb winds increase between 60 to 70 knots by Sunday night.

For Monday...attention then shifts to the strong cold front
forecast to move over the area Monday morning. This Polar front
will actually be ahead of the arctic front...which may not come
through until late Tuesday. High temperatures will be tricky on
Monday due to the timing of the front...but believe most areas
east of I-25 will see early morning highs in the mid 30s to low
40`s before the colder air rapidly moves into the area. Area
sounding show these areas around 35 degrees in the
morning...dropping into the low to mid 20s by early afternoon with
very cold wind chills. In fact...would not be surprised to see
wind chills near zero by Monday evening. There is some concern
that strong winds will impact the high valleys west of I25...and
high plains just to the east of the Laramie Range on
Monday...including Cheyenne. Only the GFS is showing strong
subsidence behind the front with 700mb-650mb winds around 55
knots. The other models including the NAM are about 10 knots
lighter and do not show such considerable low to midlevel subsidence.
Will hold off on extending the High Wind Watch eastward for
now...but will continue to monitor since a few of our local models
show a decent chance at strong winds over 55 MPH across much of
the eastern plains of southeast Wyoming. In terms of
precipitation...the mountains will likely receive 2 to 6 inches of
snow quickly as the front moves southeast of the area. With dry
downslope winds across the lower elevations...it will be difficult
to get any snowfall from the initial front. However...a brief
heavy snow shower or snow squall is possible with some llvl
instability present along with brief dynamic forcing along the
front. Kept POP around 20 to 30 percent across the high valleys
and adjacent high plains into Monday evening. Overnight low
temperatures will plummet to near zero along and west of
I25...with low to mid teens east of I25 including the Nebraska
panhandle.

.LONG TERM...(Tuesday through Saturday)
Issued at 240 PM MST Sat Dec 3 2016

Colder temperatures for Tuesday behind the initial cold front.
Models in decent agreement in depicting a band of mainly light
snow around the WY/CO border around the left front exit region of
an upper jet that will be digging over the western CONUS. This
area of snow sags southward during the day Tuesday as the upper
jet settles southward but a second surge of even colder arctic air
will drop down the high plains late Tuesday and likely squeeze out
some additional light snow over the plains. Right now amounts not
looking too great as dynamical support looks rather weak with
better energy swinging south of the CWA. Snow looks to end
Wednesday with the GFS a little faster in moving it out. Main
story will be the very cold temperatures that will be over the CWA
Weds and Thursday with mins below zero likely in much of the area
especially Weds night and Thursday morning as skies clear out.
Temperatures then moderate Thursday and Friday as a weakening
upper ridge slides across the region and lee troughing sets up
over the high plains. Next upper shortwave moves into the region
Saturday bringing some snow showers along with the next cold front
at that time.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday afternoon)
Issued at 418 PM MST Sat Dec 3 2016

Light snow showers will impact RWL with brief MVFR vis/cigs thru
02Z, otherwise VFR. Higher mountain obscurement in snow showers
can be expected this evening. Gusty west to southwest winds of 20-30
kt will affect the southeast WY terminals, with higher gusts between
LAR and RWL late tonight and Sunday. For the NE panhandle terminals,
west-northwest winds 10-15 kt will prevail tonight, with breezy
conditions developing mainly at AIA and SNY late Sunday morning.

&&

.FIRE WEATHER...
Issued at 155 PM MST Sat Dec 3 2016

No fire weather concerns through next week as a very cold arctic
airmass settles over the area. Accumulating snow is possible
Tuesday and into Wednesday next week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...High Wind Warning from 8 AM Sunday to 8 AM MST Monday for
WYZ110.

     High Wind Watch from Sunday morning through Monday morning for
     WYZ106.

     High Wind Watch from Sunday afternoon through Monday morning for
     WYZ116-117.

NE...None.
&&

$$

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



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