Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 222300
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
500 PM MDT Sat Apr 22 2017

.SHORT TERM...(This evening through Monday)
Issued at 215 PM MDT Sat Apr 22 2017

An anticyclone is apparent on water vapor imagery over the
north-central plains today. Sfc winds across our CWA are
generally light out of the south-southeast, with east-southeast
flow in the midlevels on the western edge of the anticyclone.
Residual moisture caught in this flow pattern combined with
plentiful insolation is allowing for development of a flat cumulus
deck across much of the forecast area. May see a shower or two
develop in the mountains as an upper level front moves overhead.
Otherwise, shortwave ridging approaching from the west will keep
subsidence across the region and thus should remain a quiet
weather day overall. There may be a return of stratus along the
Cheyenne Ridge tonight but boundary layer moisture progs are
marginal, so it should not be widespread nor very deep. The upper
ridge will move overhead tonight with a shortwave trough
approaching from the west on Sunday which should provide support
for afternoon showers and t-storms. Winds at the surface look to
remain out of the south-southwest as a sfc low develops up in
northeast WY. With minimal llvl convergence to initiate
convection, most of the activity will be forced by the upper wave
and the terrain with coverage across the plains highly dependent
on depth of moisture across this area. Unidirectional flow through
the column with H7 winds of 30-40kts means winds could be quite
gusty especially along and west of the Laramie Range through the
afternoon. The upper level trough will move overhead on Sunday night
with another wave on its heels for Monday. This next system overall
looks to be a bit drier, so coverage of convection may be less.
Winds may be a bit more of a concern as the unidirectional flow
may be a little more out of a westerly direction.

.LONG TERM...(Monday night through Saturday)
Issued at 215 PM MDT Sat Apr 22 2017

Unsettled and cooler-than-average weather pattern likely next
week and potentially into next weekend after a brief period of dry
weather earlier in the week. However, long range models are still
not in good agreement even though each model is indicating good
run-to- run consistency. Regardless, it still looks like daytime
temperatures will be 10 to 20 degrees below average with chilly
overnight lows and daily chances for precipitation.

For early in the week, models in good agreement through Wednesday
night showing a pair of Pacific shortwave troughs moving across
Wyoming and into the northern plains Monday night and Tuesday
night. Temperatures will be warm enough for rainfall below 7000
feet with maybe some rain/snow mix at times as low as 6000 feet.
Bands of rain showers are expected to develop and push east or
southeast. Some isolated thunder is possible but can not specify
when and where this may occur. Probability is pretty low at this
time, so will leave it out of the forecast for now. Will have to
watch late Tuesday and Tuesday night as the GFS and NAM show a
polar cold front interacting with the second upper level trough
as the front stalls along the Front Range. Both models are
hinting towards moderate QPF amounts and the potential for
snowfall across southeast Wyoming, even in the valleys, where the
most favorable lift will be located. Adjusted POP to better
reflect this potential, but did not increase them all that much
quite yet since the Canadian and ECMWF show the shortwave energy
quite a bit further south compared to the GFS and NAM.

Forecast confidence is lower than average Wednesday night through
Saturday so will not go into too many details. A potent Pacific
system near the Washington coastline is expected to influence the
weather across most of the western United States during this
period. The problem is, the GFS is a little more progressive and
shows this system moving southeast as the main storm redevelops
somewhere north of the four corners region. The GFS then has the
storm stalling near the four corners into the weekend, resulting
in much cooler than normal conditions and continuous chance of
light-moderate precipitation with some potential for heavier
precip by the end of the week. The ECMWF is much later with this
development and shows the wave moving into Wyoming/redeveloping
over eastern Colorado late Friday night. The 12z Canadian only
shows progressive open waves embedded in northwest flow aloft
pushing across the area with no evidence of storm redevelopment.
Ensemble forecasts seem to be in between the operational GFS and
ECMWF. For now, kept POP between 15 to 25 percent outside of the
mountains due to timing uncertainty. The only exception is Friday
where POP have been increased between 35 to 50 percent across the
area to account for more favorable dynamics across the forecast
area. The coolest day of the week will likely be on Friday as
well, with highs struggling to reach the low to mid 40`s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 500 PM MDT Sat Apr 22 2017

VFR. Few cumulus clouds will dissipate around sunset, with generally
fair skies from a few passing thin cirrus tonight. Surface winds
will be S-SW around 10 KT, increasing to 10-15 KT across western NE
later this evening. Southwesterly winds increase with gusts from 22-
35 KT Sunday morning. More scattered cirrus will spread west to east
on Sunday.

&&

.FIRE WEATHER...
Issued at 215 PM MDT Sat Apr 22 2017

Minimal fire weather concerns will continue through the near term.
The one exception may be in the lower elevations out west of the
Laramie Range early Sunday afternoon as westerly winds gusting
around 40 mph are likely here. Humidities in these lower elevations
may drop into the upper teens so may see elevated fire weather
concerns for these areas. Wetting rains may develop in the late
afternoon though so that should help to improve humidities. Similar
conditions are expected for Monday although overall humidity values
will be higher.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

SHORT TERM...RJM
LONG TERM...TJT
AVIATION...MAJ
FIRE WEATHER...RJM



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