Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201743

Area Forecast Discussion
National Weather Service Cheyenne WY
1143 AM MDT Mon Mar 20 2017

.SHORT TERM...(Today through Wednesday)
Issued at 400 AM MDT Mon Mar 20 2017

Predawn water vapor loop depicted an moist energetic west-southwest
flow aloft from the plains westward. A quasi-stationary surface
front extended from the lower Missouri Valley across southern
Nebraska into north central Wyoming. Regional radars detected weak
returns, mainly aloft north of a Rawlins-Alliance line. Otherwise,
skies were mostly cloudy with light winds. Temperatures ranged
from the mid 30s to mid 50s.

Short range numerical models are in good agreement with a cooler
and unsettled start to the first day of spring. A short wave and
Pacific cold front will push through the CWA today, bringing chances
for showers and even a few afternoon thunderstorms to areas mainly
west of Interstate 25. High temperatures today will continue mild
with 40s-50s west to 60s-low 70s east. Showers will become more
widespread tonight as the short wave passes through the area. Rain
will mix with and change to snow for elevations above 8000 ft.
Accumulations will be light. The showers will end from southwest
to northeast Tuesday morning with the departure of the short wave
and arrival of a weak upper ridge. A backdoor front will push
into the eastern Wyoming plains and western Nebraska panhandle
during the day, with highs in the 40s, while areas to the west
will be milder with 50s-low 60s. Widely scattered afternoon
showers will be possible Tuesday afternoon along the Interstate 25
corridor westward due to frontal convergence and approach of next
short wave from southwest. Southeast winds will increase Tuesday
afternoon for the plains, gradually decreasing Tuesday night. Rain
and snow showers will become more widespread east of Interstate
25 Tuesday evening, ending by Wednesday morning. Wednesday will be
milder and windier ahead of a stronger Pacific system approaching
the Four Corners. Expect highs in the 50s-60s with 40s mountains.
Isolated showers will be possible over the southeast Wyoming
mountains, mainly during the afternoon.

.LONG TERM...(Wednesday night through Sunday)
Issued at 330 AM MDT Mon Mar 20 2017

A potential winter storm continues to be the main forecast challenge
in the extended period. The GFS/ECM are in excellent agreement w/ an
intense upper-level low tracking across the Four Corners on Thursday
w/ significant surface cyclogenesis occurring somewhere over eastern
Colorado. Meanwhile, a strong surface cold front is expected to push
southeast across the CWA on Thursday afternoon/evening. Strong upper
level divergence courtesy of the favorable low track, coupled with a
90+ knot H25 SW-NE oriented jet over eastern Colorado suggests wide-
spread precipitation, some of which may be heavy from Thursday night
through early Friday as the surface low ejects out. The ECM is still
the more classic solution with a pronounced TROWAL over western and/
or central Nebraska, but both models show fairly widespread QPF over
0.5 inch. That said, the bigger question mark with this storm should
be the precipitation type. The progged timing of the cold front does
not push H7 temperatures below 0C until almost 06z Friday, which may
cut into potential snowfall amounts depending on how quickly thermal
profiles cool. We trended colder on Thursday night/Friday, and opted
to keep precipitation as rain outside of the mountains for now. This
will most certainly need to be watched closely over the next several
days, however. Upper-level ridging will build into the central Rocky
mountains for Saturday, offering a brief drier period before another
mid-level wave traverses the region late in the weekend.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1139 AM MDT Mon Mar 20 2017

Some gusty conditions over the WY TAF sites this afternoon with
VFR prevailing area-wide. Showers will develop over the northern
areas along a southward moving front late this afternoon and
spread eastward through this evening. As a warm advection regime
sets up over the plains late tonight, could see a period of MVFR
ceilings over the NE sites with some IFR possible in the CDR area.


Issued at 300 AM MDT Mon Mar 20 2017

Cooler and unsettled weather is on tap this week as a couple of
Pacific cold fronts and upper level disturbances move across the
districts. Lowest humidities will fall to 15 to 20 percent near
the Colorado border between Cheyenne and Sidney this afternoon,
however wind gusts will be less than 25 mph. It will be breezy
along and west of the Laramie Range. Scattered showers and
isolated afternoon thunderstorms will develop, mainly west of
Interstate 25. Showers will become more widespread tonight,
tapering off Tuesday. Periods of rain and snow will return for
the latter part of the week.




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