Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 210339
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
939 PM MDT Sat May 20 2017

.UPDATE...
Issued at 939 PM MDT Sat May 20 2017

Only significant update this evening was to bring isolated shower
coverage a bit farther east to along I-25 and minor adjustments
elsewhere (cloudcover, etc).  Rest of forecast on track.

&&

.SHORT TERM...(Late this afternoon through Monday night)
Issued at 300 PM MDT Sat May 20 2017

The early afternoon GOES water vapor loop showed the potent upper
low that brought the heavy rain and snow to the CWA the past few
days centered over central NE. The occluded surface low was located
over western IA. Plenty of wrap-around moisture, along with weak
instability and lift aloft has developed isolated rain showers over
Carbon County. Also, cannot rule out a few light showers east of a
Alliance-Sidney line through late afternoon. Cool temperatures
prevailed with mostly cloudy skies and gusty northwesterly winds.

An active northwest flow aloft will dominate the next 24 to 36 hours
with periodic shortwaves passing through the region. One shortwave
will keep isolated to scattered rain and snow showers over much
of Carbon County through tonight. Undercut MOS low temperatures a
couple of degrees tonight for areas that still have a snowpack
from this week/s storm.

Another shortwave will produce a better coverage of showers Sunday
afternoon, with isolated thunderstorms as CAPES peak between 300-500
j/kg. The Snowy Range could measure 1 to 3 inches of snow. High
temperatures Sunday will be 5-10 degrees warmer than Saturday with
mid 50s to mid 60s. Showers will end from northwest to southeast
Sunday evening.

The last in a series of shortwaves will be more robust, with added
lift from a cold front pushing south across the CWA Monday. More
widespread showers and isolated afternoon thunderstorms are forecast.
High temperatures Monday will be a tad cooler, especially for the
northern CWA. Gusty north to northwest winds are expected in the
wake of the cold frontal passage. Showers will taper off Monday
evening.

.LONG TERM...(Tuesday through Saturday)
Issued at 215 PM MDT Sat May 20 2017

Mainly dry weather is expected across the CWA Tuesday into Thursday
as an upper ridge moves across the region. Cannot rule out a few
showers mainly over the NE Panhandle Tuesday as a weak lobe of
vorticity moves out of that area. Temperatures will warm back to
around seasonal averages by Weds. The Thursday through Saturday time
period will see the return of mainly afternoon and evening convection
over parts of the CWA as weak impulses ride across the region on the
front side of an upper trough/low that will drop into the Pacific
northwest late Weds then gradually shear out into the northern Rockys
by Saturday. Temperatures remaining warm Thursday and Friday before
turning cooler Saturday behind a cool front.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 510 PM MDT Sat May 20 2017

VFR ceilings and visibility expected overnight into Sunday with
higher based cumulus over the Nebraska Panhandle airports this
evening diminishing. Will have some scattered light showers in the
vicinity of Rawlins and Laramie overnight but should not see
significant lowerings of visibilities and ceilings.


&&

.FIRE WEATHER...
Issued at 215 PM MDT Sat May 20 2017

No fire weather issues this weekend with wet fuels from recent heavy
precipitation, cool temperatures and high relative humidities.
Temperatures will gradually moderate through next week. Scattered
showers and isolated thunderstorms are forecast Sunday and Monday.

&&

.HYDROLOGY...
Issued at 215 PM MDT Sat May 20 2017

Concerns this weekend into early next week will focus on runoff from
recent heavy rain and snow melt into area creeks, streams, and mainstem
rivers east of the Laramie Range. Although flooding is not expected,
many of the streams and creeks may reach bankfull by early next week.
Fortunately, cooler than normal temperatures will continue for the
next week, which should help slow the mountain snow melt and resultant
runoff into the mainstem rivers. Will continue to monitor these water
levels very closely over the next few weeks.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...JG
SHORT TERM...MAJ
LONG TERM...RE
AVIATION...JG
FIRE WEATHER...MAJ
HYDROLOGY...RJM/MAJ



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