Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 180019
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
619 PM MDT Wed May 17 2017

.SHORT TERM...(This evening through Thursday Night)
Issued at 415 PM MDT Wed May 17 2017

Strong spring storm currently on track with only a few slight
differences compared to previous models and forecast thinking.
Initial band of thunderstorms have developed as planned early
this afternoon just east of Interstate 25. small hail and heavy
rainfall are expected with these storms through this evening as
they push into western Nebraska. Models have initialized well
today with shower activity picking up behind this band further to
the west. Radar returns are picking up further south and west as
well, so the event is on track thus far.

All models show the storm system near the Utah/Idaho border
drifting southeast and stalling over western Colorado by Thursday.
The upper level low will then hardly move at all Thursday night as
it becomes stationary over central Colorado. Widespread rainfall,
heavy at times, still looks like a good bet below 5000 feet mainly
east of I-25 into western Nebraska. Locations near 5000 feet in
this area may need to monitor snowfall amounts Thursday night and
Friday. The mountains, including the Laramie Range, will do very
well with this system as a foot or two of snow is expected with
locally higher amounts up to 3 feet. Locations between these two
extremes (elevations between 5000 to 7000 feet) are less certain,
but confidence is increase of a significant snow event. Although
some snow shadowing is possible across the Laramie Valley, model
soundings are now much more favorable for deep moist easterly flow
into Albany county which is a key to get heavy snow in this area.
Upgraded the Winter Storm Watch to a warning for this area for 6
to 12 inches of snow. Further west for the lower elevations of
Carbon county is a bit more tricky since not all models showing
moderate to heavy snowfall rates and still show some shadowing.
After some collaboration, decided it is best to have a Winter
Weather Advisory out for this location for slushy and perhaps some
icy roads. Will keep the Winter Storm Watch going for the I-25
corridor from Cheyenne to Glendo since the snow will likely not
begin until sometime Thursday.

There are some differences compared to previous models. First,
winds are considerably stronger Thursday night into Friday with
this storm as models show a strong pressure gradient and a deeper
surface low across Colorado. Although blowing snow is still not
expected, drifting snow may become a problem. Also, the ECMWF is
still very aggressive with this storm and now shows an even
greater magnitude of frontogenesis Thursday and Thursday along
with slightly colder temperatures. This is concerning, so
increased snow totals a bit above 6000 feet. Otherwise, Converse
and Niobrara counties may need Winter Weather Advisories as we
head towards Thursday night for 3 to 6 inches above 5000 feet and
on the higher ridges around Lusk.

.LONG TERM...(Friday through Wednesday)
Issued at 258 PM MDT Wed May 17 2017

High PoPs continue Friday for Snow and rain across the area, and
that will be winding down Friday night as the upper low lifts
northeast into South Dakota. By Saturday Morning deeper layered
drying will be working in from the northwest with small PoPs for
snow showers in the mountains. Cool pattern continues through the
weekend with a warmup by Wednesday where temps in the lower
elevations climb back into the mid and upper 70s. We also see a
return to scattered thunderstorms by the first half of the week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 605 PM MDT Wed May 17 2017

Rain will increase this evening across sites near the Colorado
Border (CYS/LAR). Some MVFR vis at CYS in moderate rain will be
possible. Northeast winds will be gusty through much of the
period. The precipitation will change over to snow across the
southeast WY sites late tonight into Thursday morning. IFR/MVFR
conditions will become more widespread after 06-12Z with the lower
cigs and increasing precip coverage.

&&

.FIRE WEATHER...
Issued at 258 PM MDT Wed May 17 2017

No concerns due to precipitation and humidities over the next
several days and continuing into the weekend.

&&

.HYDROLOGY...
Issued at 239 PM MDT Tue May 16 2017

Although rivers and streams are on the low side across most of the
area, the potential for persistent moderate to heavy rain across
the high plains Thursday and Friday may result in elevated flows
late this week and this weekend. The most favorable area appears
to be along and just east of I-25 into western Nebraska. Lower
rainfall amounts are expected north of a Lusk to Alliance line.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Warning from 3 AM Thursday to 3 PM MDT Friday for
     WYZ105-106-115.

     Winter Weather Advisory from 3 AM Thursday to 3 PM MDT Friday
     for WYZ104-109-111-113.

     Winter Storm Watch from Thursday evening through Friday
     afternoon for WYZ107-118.

     Winter Storm Warning from midnight tonight to 3 PM MDT Friday
     for WYZ103-110-112-114-116-117.

NE...None.
&&

$$

SHORT TERM...TJT
LONG TERM...JG
AVIATION...ZF
FIRE WEATHER...JG
HYDROLOGY...TJT



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