Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 230909

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
309 AM MDT Fri Aug 23 2019

.DISCUSSION...(Today Through Thursday)
Issued at 302 AM MDT Fri Aug 23 2019

Upper air analysis and water vapor imagery depict a trough
currently translating across western MT/WY and eastern ID with SW
flow across the region ahead of the circulation centered in SW
MT. At the surface there are two areas of low pressure in eastern
MT and south- central WY with winds out of the SE advecting plenty
of theta-e rich air into central and western SD.

As the aforementioned upper trough moves through the area Friday
into Saturday chances of showers and thunderstorms will increase
throughout the day Friday. Convection initiation will likely
be favored across the higher terrain of NE WY and the Black
Hills during the afternoon. Additionally, a slow-moving surface
trough/cold front located across NE WY will provide further low
level lift for thunderstorm development. Steep mid-level lapse rates
(greater than 7-8 C/km) atop a very moist air mass will result in
ample instability across the region with MLCAPE in excess of
1000-2500 J/kg. However, a weak wind profile will limit storm
organization, especially across SD, with the strongest shear
further west along the front in Wyoming. The juxtaposition of the
strongest shear and sufficient instability in NE WY and far SW SD
will promote the greatest threat for storm organization. Cannot
rule out a severe thunderstorm along and near the Black Hills,
particularly the northern BHs where low-level convergence will be
greatest. However, weaker shear (0-6 km bulk shear less than 30
knots) and lack of appreciable forcing, especially east of the
Black Hills, will limit the overall threat this afternoon. Despite
some capping concerns across WY and SD resulting from cloud cover
during the day, the convective evolution most likely would be for
a few transient supercells/multi-cellular clusters to form across
NE WY and SW SD during the afternoon. While uncertainty remains
regarding the eastward extent of the severe threat, CAM guidance
does show the potential for some upscale growth as the initial
convection moves into the eastern portion of the forecast area
with gusty winds the primary threat. Heavy rain and flooding is
also possible as precipitable water values will be 150-200 percent
of normal this afternoon and evening.

Unsettled, northwest flow will reign over the northern half of
the country this weekend through much of next week as cooler weather
settles in. A cold front will move through late Sunday into Monday
with breezy and much cooler, drier air behind it. Most of the
forecast area will see highs around 70 to the upper 60s for much of
next week while overnight lows in the Black Hills and portions of NE
will drop into the 30s much of this upcoming week.


.AVIATION...(For the 06Z TAFS Through 06Z Friday Night)
Issued At 1113 PM MDT Thu Aug 22 2019

Thunderstorm chance will increase this evening across northeastern
Wyoming and far western South Dakota. Widespread MVFR/Local IFR
ceilings are expected to develop across much of the area later
tonight. Conditions will improve to VFR again on Friday afternoon.
Scattered to numerous showers and thunderstorms are expected from
late Friday afternoon through Friday evening.




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