Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 240812
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
212 AM MDT Thu Aug 24 2017

.DISCUSSION...(Today Through Wednesday)
Issued at 210 AM MDT Thu Aug 24 2017

An upper level ridge covers the western 2/3 of the CONUS, with
the next wave to affect the region pushing into western WY. Skies
are clear across the northern half of the CWA, while the southern
half is partly cloudy. At the surface, a trough continues to
stretch across eastern MT/WY. Winds are generally southeasterly
around 10 kts, and temperatures are in the lower 60s.

The upper ridge axis will shift over the central states today as the
Canadian trough moves through the Pacific Northwest. Aforementioned
wave will approach the region this afternoon, while instability
increases along and east of the surface trough. Showers and
thunderstorms will develop this afternoon across the Black Hills,
northeastern WY, and southwestern SD, where the cap weakens. Models
are showing slightly less CAPE and shear than yesterday`s runs, but
an isolated stronger storm will be possible. Weak steering winds
of 5 to 10 kts and PWATs 150% of normal may result in some slow-
moving storms with heavy rain. Showers and thunderstorms will
diminish in the evening, although a few may move into northwestern
SD after midnight.

A secondary shortwave will move through the region on Friday,
sparking another round of showers and thunderstorms. Instability
will increase across the SD plains ahead of the surface low, which
will gradually move eastward along southern SD. MLCAPE up to 2000
J/kg, combined with 0-6 km shear around 30 kts, may result in
some strong to severe thunderstorms Friday afternoon and early
evening. Best forcing will shift into eastern SD late in the
evening.

On Saturday, one more wave will cross the region before another
upper ridge builds over the western CONUS. Decent instability and
shear may result in some strong to severe storms Saturday afternoon
and evening. Several days of dry weather will begin on Sunday.
High temperatures for the next week will be in the 80s.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Thursday Night)
Issued At 1120 PM MDT Wed Aug 23 2017

Isolated -TSRA over southwest SD will dissipate overnight. Isolated
to scattered -TSRA with local MVFR conditions will develop by
Thursday afternoon, especially near the higher terrain and move
east into the evening hours. Outside of storms, VFR conditions
will prevail through the period.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION...Pojorlie
AVIATION...Helgeson


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