Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201013

Area Forecast Discussion
National Weather Service Cheyenne WY
413 AM MDT Mon Aug 20 2018

.SHORT TERM...(Today and tonight)
Issued at 331 AM MDT Mon Aug 20 2018

Main weather highlight for the next 24 hours will be on the
initial dry and clear conditions this morning transitioning to
building clouds and scattered showers from the west this afternoon
and through the overnight. Elevated fire weather concerns will
also be in place across Carbon County before higher moisture arrives
mid- to late-afternoon. See more details in the fire weather
discussion below.

H5 shortwave ridging is occurring over much of WY and the NE
Panhandle this morning with light patchy fog and smoke being
noted on regional satellite channels. Dewpoint depressions
continue to fall this morning and more widespread fog, some
possibly becoming locally dense, will be likely through the 9am
time frame for portions of the High Plains.

Farther upstream, a positively tilted trough over WA/OR is noted
and this system will aid moisture and positive theta-e advection
across WY through the late afternoon and over the next several
days. Mid- and upper-level moisture will increase and coupled
with weak elevated instability late this afternoon will support
shower and embedded thunderstorms across Carbon County. This
activity will shift east through the evening and overnight hours
into the High Plains with greater focus towards the WY/CO/NE
borders as H5 shortwave trough vorticity advection and slightly
stronger upper-level divergence persist.

.LONG TERM...(Tuesday through Sunday)
Issued at 331 AM MDT Mon Aug 20 2018

Continued unsettled pattern Tuesday into Thursday as multiple
rounds of rain showers and thunderstorms will be probable during
this time frame. Isolated western facing higher terrain locations
could receive upwards of 1-1.5 inches total through Thursday.
Main concentration of 0.5" total or greater probability will be
along and south of a Cheyenne to Seminole Dam line. Despite the
greater cloud cover and rain likelihood, temperatures will slowly
warm as southwest flow aloft occurs through the week. Temperatures
back in the 80s are expected late week and weekend.

The aforementioned H5 positively tilted trough over the PAC NW
will remain in that position as embedded shortwave impulses ride
over the region Tuesday and Wednesday. With PWATs increasing
upwards of 0.8-1.1", moderate rain showers will be likely at times
when greater divergence shifts overhead. Some training of shower
could occur but the spatio-temporal overlap should be low enough
to limit any flash flood risk outside of the Badger Creek burn
scar area. There could be some low potential for strong storms
Wednesday afternoon as greater surface based instability occurs
adjacent to the Laramie Range across the High Plains. The trough
will finally open up and shift across north WY/MT Thursday. A pre-
frontal trough axis continues to appear in model data and this
feature shifts much of the boundary layer moisture farther east
before the best height falls and forcing reach the region. Much
drier and warmer conditions will ensue Friday into the weekend
behind this boundary. A quick moving shortwave could help generate
some showers Saturday for NE Panhandle but confidence is low at
this stage.


.AVIATION...(For the 06Z TAFS through Monday evening)

Skies have cleared across the region for the most part. However,
smoke/haze from fires well to the northwest of Wyoming have settled
across the eastern plains through the night.  Do not expect these
MVFR visibilities to subside any time soon, with the Nebraska
panhandle likely seeing VFR conditions by sunrise Monday as winds
begin to shift into the northeast and east.  The only major change
was to KLAR, which will probably see some fog over the Laramie
Valley beginning over the next 2 to 4 hours.  Dense fog is also
possible just before sunrise.


Issued at 331 AM MDT Mon Aug 20 2018

Clear skies and dry conditions initially this morning and early
afternoon but shower and isolated thunderstorms are expected to
develop across Carbon County near 5pm and shift east through the
evening and overnight into the High Plains of WY and NE.

Prior to these showers and storms forming however, will be a window
across Carbon County for elevated to near-critical fire weather
concerns from early afternoon to just before the probable scattered
wetting rains move into the area. Humidity values will fall to near 20
percent with some isolated areas reaching the teens. Surface winds
will generally be near 15-20mph but some gusts to 25mph can not be
fully ruled out and thus the marginal fire weather concerns for the
day. Will need to watch for very temporary red flag conditions but
confidence is low at the moment.

A few rounds of wetting rain showers with a few thunderstorms will
occur Tuesday and into Wednesday with the greater focus across the
higher terrain and for southern half of the forecast area. Some
lucky locations could see upwards of 1 inch of rainfall through
Thursday. Drier conditions expected Thursday evening into the
weekend. Will need to monitor fire weather conditions late in the
week as drier air and warmer temperatures look to be present.




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