Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 210528
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1127 PM MDT Sun May 20 2018

.SHORT TERM...(Tonight - Monday)
Issued at 219 PM MDT Sun May 20 2018

Showers & a few thunderstorms are developing along and west of the
Laramie Range this afternoon, in response to a mid-level shortwave
lifting northeast across southwestern CO in advance of a deepening
upper low over the western US. Limited moisture & instability will
preclude a risk for strong storms, but activity may linger through
much of the evening aided by ample dynamic support. Recent runs of
the HRRR suggest at least isolated showers may persist through 06z
or so before diminishing.

Fog is possible again tonight w/ moist low-level upslope remaining
in place along/east of the Laramie Range. Moisture profiles should
not be quite as supportive of widespread dense fog tonight, but we
could see some areas of visibilities as low as 1/4 mile. Expect to
see any fog diminish by mid-morning on Monday.

Convective activity will be possible once again on Mon w/southwest
flow aloft ahead of a closed upper low over southern California. A
bit of an issue with capping/subsidence associated w/ a short-wave
ridge over the central Rockies, so expect the bulk of the activity
to be confined to the higher terrain. Highs will continue to trend
warmer as H7 temperatures climb to +8 to +12 deg C.

.LONG TERM...(Monday Night - Sunday)
Issued at 227 AM MDT Sun May 20 2018

Main highlight in the long term will be the warming temperatures
through the period and afternoon shower and thunderstorm chances.
The more significant coverage and intensity day of storms could be
Wednesday for far east WY and moreso the NE Panhandle.

By Tuesday, the H5 ridge axis will shift east in to the Central
Plains as a large western CONUS closed low pivots into the Great
Basin. Greater southwest flow aloft for western areas in the
mountains should allow for greater spatial shower and storm
coverage but instability and shear combination looks to marginal
at best. Going into mid-week on Wednesday, the closed H5 low over
the great basin will open up and shift northeast across the
Rockies. Stronger H3 diffluent flow aloft along and instability
over the High Plains (especially more so NE Panhandle) should
support stronger thunderstorm development. A negating factor is
wind shear as it continues to look meager in the 25-35 knot range.
Strong pulse to semi- organized storms could be possible
Wednesday afternoon into evening.

By late week, the base of the transiting trough axis will shift
north of the region. Stronger west winds will develop and shunt
deeper moisture farther east into the Central Plains as drying
vertical profiles occur. Friday and into the early weekend, weak
northwest flow develops as the trough axis passes east and H5
ridging shifts in just west of the Rockies. Just enough
impulse/vorticity difluence aid and meager instability could help
scattered showers and isolated thunderstorms over the higher
terrain but overall coverage will be limited and the High Plains
will likely remain rain free.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday evening)
Issued at 1127 PM MDT Sun May 20 2018

VFR expected at all terminals through the period.  Some patchy fog
possible early this morning at KCYS.  A few showers possible mainly
vcty of the mtns in the afternoon.

&&

.FIRE WEATHER...
Issued at 227 AM MDT Sun May 20 2018

Despite the warmer and drier conditions, fire weather concerns will
remain low as ample rain amounts have occurred over the past several
days for many areas. One areas that missed out on rainfall was
Carbon County. However, they should remain in the upper 20 to low 30
percentiles relative humidity wise this afternoon. A few higher
terrain showers and isolated weak thunderstorms will be possible
this afternoon but overall coverage will be low to zero elsewhere.

A few isolated showers/storm could again be possible Monday across
the Laramie Range and into the NE Panhandle. Coverage again is
expected to be low. Relative humidity values will fall near 20
percent but winds should remain below critical thresholds at 15 to
20 mph from the southwest.

Overall long range forecast appears to remain just moist enough
during the afternoons to preclude high fire concerns at this time.

&&

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

$$

SHORT TERM...CLH LONG TERM...JSA/CLH AVIATION...WFOCYS FIRE
WEATHER...JSA/CLH



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