Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 161801 CCA

Area Forecast Discussion...Corrected Aviation
National Weather Service Cheyenne WY
1042 AM MDT Sat Sep 16 2017

.SHORT TERM...(Today and tonight)
Issued at 258 AM MDT Sat Sep 16 2017

It has been an active night for the eastern half of the CWA. As of
300 AM, radar imagery shows a large area of showers with embedded
thunderstorms over western Nebraska driven largely by an area of
warm advection and low level frontogenesis on the nose of a
vigorous upper jet streak now nosing into SD. This system was
responsible for several reports of 1 to 1.5 inch hail over our
Panhandle counties between 11 PM and 1 AM.

Shower activity should wind down through the morning hours for
much of the area. A weak disturbance seen on GOES 16 WV imagery
over western WY may spark off a few more showers especially over
SE WY this morning but these should not be as widespread or as
strong as what we saw overnight. As the upper trough swings
through this afternoon, a dry punch will move into our western
counties, and strong cold advection, combined with mixing of
higher momentum air from aloft will produce wind gusts of 40 mph
over Carbon and Albany Counties. A few higher gusts are possible
in our more wind prone areas. Skies will clear over much of the
area by tonight and with light winds after midnight, should see
optimal radiational cooling especially west and north.
Temperatures will fall to near 32 degrees for a few hours roughly
north and west of a line from Douglas to Laramie. Decided against
a freeze warning as these temperatures will be only experienced
for a couple of hours. Added areas of frost and will let the day
shift decide on any headlines.

Return flow sets up Saturday night into Sunday as a low amplitude
disturbance tracks over the area from the west on Sunday into
Sunday night. Although CAPE will be limited, good jet dynamics
from the right rear quad of a 90kt upper jet over the Dakotas and
steepening lapse rates aloft will be enough for convection to fire
initially over the Laramie Range before becoming elevated over the
Panhandle in the evening. Freezing levels remain low and shear
will be fairly strong so may see a couple of stronger storms even
in spite of the weak instability.

.LONG TERM...(Monday through Friday night)
Issued at 418 AM MDT Sat Sep 16 2017

Long Range models continue to show a pretty active weather pattern
across Wyoming and western Nebraska for next week, as a series of
Pacific systems and associated cold fronts moving across the
region. The flow aloft will generally be pretty progressive, so do
not expect these systems to linger around for too long. The first
of these Pacific systems will impact the area on Tuesday bringing
windy conditions and some precipitation, and much cooler temperatures
for the remainder of the week. Models show most of the precipitation
north of I-80 into the Dakotas and west central Nebraska, with
some downslope winds further south along I-80. Kept POP on the low
side due to the large timing differences between models and the
ensembles, although still expect some precipitation sometime
between late Monday and early Wednesday. Once the front moves
across the area on Tuesday, it will be cold enough for
accumulating snow across the Snowy and Sierra Madre Ranges as
700mb temperatures drop below -2c. May even see some snow flurries
down to the valley floors Tuesday evening around Rawlins and
Laramie. Models then show a flat ridge of high pressure aloft
quickly moving across the area on Wednesday with slightly
moderating temperatures but windy across southeast Wyoming where
gusts up to 45 mph are expected.

For late in the week, models indicate a stronger storm system
impacting the region. however, solutions are inconsistent with
timing and evolution of the system as it approaches the Front
Range. The potential exists for locations down to elevations of
5500 feet to see their first snow flakes of the season by Friday
or next weekend with temperatures trending 10 to 15 degrees
below average for this time of the year.


.AVIATION...(For the 18Z TAFS through 18Z Sunday midday)
Issued at 1042 AM MDT Sat Sep 16 2017

MVFR CIGS across the western NE panhandle will gradually improve to
VFR this afternoon. Elsewhere, expect SCT-BKN mid level clouds. VCSH
at Sidney through ear;y afternoon. prevailing winds will be breezy
southwesterly at southeast WY terminals and lighter northwesterly at
western NE terminals. CIGS will lower to MVFR east of the Laramie
Range after 08Z.


Issued at 258 AM MDT Sat Sep 16 2017

Recent precipitation along with cooler temperatures will limit
fire weather concerns through the weekend. Breezy conditions will
develop this afternoon over Carbon and Albany counties but RH
values will remain at or above 25 percent.




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