Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 161718

Area Forecast Discussion
National Weather Service Cheyenne WY
1118 AM MDT Mon Apr 16 2018

.SHORT TERM...(Today through Tuesday night)
Issued at 432 AM MDT Mon Apr 16 2018

Quiet weather expected for today with dry conditions and
increasing winds in the afternoon west of I-25. High temperatures
in the mid 60`s to mid 70`s are expected across most of southeast
wyoming with 700mb temperatures increasing over 5c. The coolest
temperatures will actually be found over western Nebraska this
afternoon due to a shallow stationary cold front along the
Wyoming/Nebraska border. To the east of this front, winds will be
east or southeast resulting in some cooler upslope flow and highs
generally in the mid to upper 50`s. With warmer air aloft, expect
mostly stable conditions with some fair weather cu across
southeast Wyoming.

Winds will be the primary forecast concern Monday night and
Tuesday as the next Pacific trough is forecast to push across the
area by Tuesday afternoon. The progressive nature of this system
will limit any significant precipitation amounts, but the frontal
passage should have plenty of wind. There is the potential for
High Wind headlines beginning as early as late this evening, and
more so for Tuesday. However, models remain in poor agreement with
surface winds and winds aloft. Most models, which are not named the
GFS, are a solid 15 to 20 knots lower than the GFS and have
consistently been on the low side for days. Although surface
gradients are strong as the front moves through the area, models
disagree with wind speeds between 800 to 600mb aloft in addition
to the position of the surface lee-side trough. Another issue is
the apparent weakening of the jet stream aloft as the trough axis
becomes negatively tilted as it moves through the area, and the
high probability of significant cloud cover, which may greatly
impact mixing heights and mixing of upper level winds. With all
this in mind, wind forecast tonight and Tuesday are low
confidence. Regardless, did nudge winds towards the GFS since the
GFS tends to do a little better with gusty winds this time of the
year. Otherwise, all models show precipitation quickly spreading
west to east through the day. Believe there is a pretty good
chance for thunder north of I-80 just ahead of the front as most
model soundings show a shallow unstable layer between 700mb and
450mb, which should be enough for isolated thunder. However, if
the GFS winds at these level verify, there may be too much speed
shear. Added some thunder to the forecast, but limited the area
coverage to mainly the panhandle. Some snow is expected this
evening and tonight as the colder air moves into the region, but
accumulations outside of the mountains will likely be less than
one inch if anything accumulates at all.

.LONG TERM...(Wednesday through Sunday)
Issued at 332 AM MDT Mon Apr 16 2018

A quiet start to the extended forecast period on Wed/Thu as upper-
level ridging becomes established over the central CONUS. The main
concern; however, is the potential for a significant precipitation
event beginning Thu night and continuing through Sat. The GFS & EC
remain in excellent agreement with a closed upper low & associated
surface low tracking across southeast Colorado late in the week. A
bit of a trend toward warmer thermal profiles in the last 24 hours
or so, leading to questions regarding how much precipitation could
be in the form of snow. Our confidence in widespread, and possibly
heavy precipitation, is high. Some snow is likely w/ 700 hpa temps
of -3 to -6 deg C, but the timing of the change over will be a big
factor in accumulations. Rain and/or snow should come to an end by
late Saturday as dry air pushes in from the west.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday morning)
Issued at 1118 AM MDT Mon Apr 16 2018

VFR predominating over the area through 09Z, then some hint that
there could be some MVFR cigs over parts of the Panhandle after 09Z.
 Otherwise some gusty winds affecting the area through this evening
with gusts over 30kts mainly from KRWL to KCYS.


Issued at 352 AM MDT Mon Apr 16 2018

Fire Weather concerns will be low across most of the area this
week, and perhaps moderate west of I-25 today due to low
humidities and very windy conditions in the afternoon. However,
fuel moisture is non critical over most of these areas with green-
up underway and remnant snowpack over the higher terrain. Lighter
winds and higher humidities are expected further east. Another
dry day is expected on Tuesday along the I-80 corridor with
scattered rain/snow showers further north. However, humidities are
expected to be over 25 percent. No other concerns for the
remainder of the week as a strong Pacific storm impacts the area
late Thursday and Friday.




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