Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 211156

Area Forecast Discussion
National Weather Service Cheyenne WY
550 AM MDT Wed Apr 21 2021

.SHORT TERM...(Today through Friday)
Issued at 354 AM MDT Wed Apr 21 2021

Latest GOES-E water vapor channel showing an upper level low
moving across the Sierra Nevada Mtns. into Nevada with elevated
moisture advection into SW WY and western CO this morning.
Vorticity advection ahead of the low coupled with decent potential
temperature and isentropic lift will contribute for increased
precipitation chances more so on the CO side of things but model
solutions coupled with early radar returns this morning trending
with a better shot for our forecast area compared to previous
model runs last few nights. Given the solar angle for this time of
year and decent melting yesterday, current thought of high plains
seeing most accumulations on the grassy surfaces with area
roadways again having to wait till after sunset outside of heavy
banding to stick. Icy to slick roadways still possible for evening
commuters today with lingering morning impacts Thursday for the
NE Panhandle. Highest accumulations expected along and south the
I-80 corridor from Elk Mountain east to Pine Bluffs. A few degrees
warmer today with low 40s across the Panhandle and low to middle
30s across SE WY.

Upper low continues eastward Thursday with snow tapering slowly
off for eastern portion of the Panhandle as main trough axis
propagates through the region late Thursday/early Friday. Some
stray rain showers may be possible Thursday afternoon in the
Laramie Valley and southern portions of Albany, Carbon and Laramie
Counties but with these quick propagating waves not heavy
moisture carriers. Some model guidance and available atmospheric
mechanisms are throwing potential solutions of snow squall chances
for Thursday night along the I-80 corridor east of Laramie that
will need to be further investigated as we get closer due to
potential travel impacts in the evening. A cold front in the
trough`s wake expected to pass through the region Friday afternoon
and evening with light snowfall accumulations possible while rain
and perhaps garden variety thunderstorm along the South Laramie
Range into the high plains as temperatures reach into the middle
40s in WY high plains to middle 50s in the NE high plains.

.LONG TERM...(Friday night through Tuesday)
Issued at 230 AM MDT Wed Apr 21 2021

Long range models continue to show a decent warming trend heading
into the weekend, as a flattened upper level ridge axis progresses
east into the Great Plains and amplifies. Southwest winds aloft will
increase through Sunday as the next Pacific upper level trough moves
into the Great Basin. Outside of a few convective showers or a
rumble of thunder, not expecting any precipitation Saturday and
Sunday as a weak disturbance lifts northeast and dissipates late on
Saturday. Coverage and location of any convective showers is too
uncertain to add to the forecast at this time, so kept POP around 5
percent for now. Daytime high temperatures will continue to
increase, with widespread mid 60s to mid 70s expected on Sunday
afternoon. The warmest temperatures will remain east of Interstate

Primary forecast concern will be the positively tilted upper level
trough pushing eastward from the Pacific. By Monday, models are in
poor agreement with the positioning of this trough with the GFS
showing the trough axis east of the Great Basin while the ECMWF and
the Canadian, to some extent, show the trough axis still near the
Pacific coast and over the western Great Basin region. With such a
large timing difference, not going to get into too many details with
the forecast on Monday and Tuesday. There is moderate confidence
that it will cool down a bit early next week, with highs lowering
back into the 40s and 50s. Precipitation chances will depend on the
timing of the Pacific front and upper level trough, so kept
increasing POP into Tuesday with values between 30 to 50 percent,
highest in the mountains. This system does not appear to be
significant with possible cyclogenesis much further east across the
central plains. At this time, looks like mainly rain below 6500
feet, but that may change as we get closer to Monday/Tuesday.


.AVIATION...(For the 12Z TAFS through early Thursday morning)
Issued at 550 AM MDT Wed Apr 21 2021

Low to midlevel clouds will continue to increase over most of the
area, and eventually into western Nebraska later today. This is well
ahead of a weak Pacific disturbance aloft moving into
Colorado/Wyoming late today and into early Thursday morning. Clouds
will lower and thicken by 06z to 09z Thursday for most terminals
with a good chance for light snow.

HAZARDS/WEATHER TRENDS: Removed fog from the 12z KLAR TAF since it
has yet to develop this morning due to a low to midlevel cloudiness.
With the exception of KRWL and KCDR, MVFR and eventually IFR CIGS
are expected after 03z to 07z respectively. The best chance for low
VIS due to snow will be near KCYS and KLAR this evening through
tonight. Winds should remain relatively light, below 15 knots, out
of the south through tonight.


Issued at 319 AM MDT Wed Apr 21 2021

Fire weather concerns into the early half of the weekend will be
minimal with periods of light precipitation in the form of rain
and/or snow for the region. Dry conditions and elevated winds will
create fire weather concerns Sunday and particularly Monday with
min RHs dropping into the upper teens and wind gusts in excess of
25 mph particularly in the NE Panhandle.




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