Area Forecast Discussion
Issued by NWS Cheyenne, WY

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402
FXUS65 KCYS 191005
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
405 AM MDT Thu Apr 19 2018

.SHORT TERM...(Today - Friday Night)
Issued at 219 AM MDT Thu Apr 19 2018

No major near-term concerns for today. There is some potential for
critical fire weather conditions in FWZ 306 this afternoon, but we
still have questions regarding the degree of mixing and the impact
this has on wind gusts at Saratoga. We will continue to monitor IR
satellite imagery over the next couple of hours, and hopefully get
a better idea of cloud cover prior to upgrading the inherited Fire
Weather Watch to a Red Flag Warning. The HRRR has been showing 20+
knot wind gusts, so Red Flag conditions are possible, albeit quite
marginal.

A vigorous upper-level low over central California this morning is
expected to track eastward across the Four Corners tonight. Expect
this to result in an increase in shower activity for western areas
overnight with strong jet energy and difluent flow aloft. Not much
moisture to work with yet though, so expect minimal impacts during
the overnight hours. Still a bit of uncertainty w/ snow levels Fri
AM, as NAM forecast soundings are considerably colder than the GFS
soundings. We went with a 75/25 blend of the NAM/GFS on snow level
which brings a rain/snow mix down to about 6000 feet at 12z. Light
precipitation should keep impacts low early on Friday.

Precipitation coverage/intensity will increase across a large part
of the area through the day. Temperatures on Friday afternoon will
likely be warm enough for rain outside of the mountains. Challenge
for Friday night/early Saturday will be snow levels. This scenario
with a cut-off low is preventing any real opportunities for colder
air to filter into the region, so that is a limiting factor in the
potential for a widespread significant winter event. Still believe
snow levels will fall to around 6000 feet or so with at least some
minor accumulations as far east as about Cheyenne. The I-80 summit
may see snowfall amounts up to 5 inches, so there is potential for
some travel impacts there which may necessitate headlines. Snowies
and Sierra Madres should see a solid 6-12 inches of snow, so later
shifts will likely need to consider a Winter Wx Adzy there.

.LONG TERM...(Saturday through Wednesday night)
Issued at 315 AM MDT Thu Apr 19 2018

Models show light rain, with snow above 5500-6000 feet,
continuing across the area Saturday morning as the upper level low
weakens and drifts southeast across the southern plains.
Additional snow accumulations after sunrise should be less than
two inches outside the mountains. Even down to 5500 feet, not
expecting snow to accumulate too much due to the potential for low
snowfall rates and a relative mild storm system. The system
should push east of the region by late Saturday resulting in
cooler than normal temperatures and gradual decreasing cloudiness,
mainly along and west of I-25.

All models then show a low amplitude ridge axis settling over the
northern and central plains for Sunday as a progressive shortwave
trough moves into Wyoming from the Pacific. Expect another round
of rain and snow showers with the potential for some thunder
showers on Monday. A cold front will move through the area Monday
night bringing a quick shot of colder air with another cool day
expected on Tuesday. Forecast confidence becomes low by the middle
of next week as models are out of phase with the overall longwave
pattern Wednesday and Thursday of next week.

&&

.AVIATION...(For the 12Z TAFS through Thursday night)
Issued at 319 AM MDT Thu Apr 19 2018

VFR conditions are expected today and into early tonight. South to
southeast winds will increase between 20 to 25 knots by 18z today
with occasional gusts up to 30 knots mainly along and east of
Interstate 25. These winds are expected to continue through late
this evening as clouds begin to increase ahead of the next Pacific
storm system.

&&

.FIRE WEATHER...
Issued at 219 AM MDT Thu Apr 19 2018

Near-critical fire weather conditions are possible in FWZ 306 this
afternoon due to 10-15 percent RH and wind gusts around 25 MPH. No
concerns after today with widespread rain and/or snow expected.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Fire Weather Watch from noon MDT today through this afternoon
     for WYZ306.

NE...None.
&&

$$

SHORT TERM...CLH
LONG TERM...TJT
AVIATION...TJT
FIRE WEATHER...CLH



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