Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 181955

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
155 PM MDT Wed Jul 18 2018

.DISCUSSION...(This Evening Through Wednesday)
Issued at 155 PM MDT Wed Jul 18 2018

Water vapor imagery depicts a shortwave trough crossing the Dakotas
this afternoon. The main midlevel vort max appears to be over
northwestern SD/southwestern ND, with a secondary vort max over
central SD. An anomalously moist warm sector extends from the
Lower Mississippi Valley northwestward through southern
Manitoba/Saskatchewan, with dew points in the 60s across much of
our area. A wind shift boundary stretches across northeastern
WY/northwestern SD, with southerly/southwesterly flow ahead of the
boundary and northwesterly flow behind.

Thunderstorm development this afternoon should be focused in three
regions. The first will be near the Black Hills, where one or two
storms could develop in a relatively low-to-moderate CAPE
environment with plentiful, albeit largely unidirectional, deep-
layer shear. Severe threat should overall be limited, but
marginally severe hail or wind could be possible here. The second
region is over northwestern SD, where activity is ongoing near the
primary midlevel vort max. CAPE is limited here, as well, but
enhanced low-level SRH could contribute to a larger threat for
persistent supercells capable of damaging winds and marginally
severe hail. A tornado also cannot be ruled out in a region of
locally backed flow near the aforementioned wind shift boundary.
Finally, strong to severe storms are expected to develop over
central/south central SD this afternoon and continue through the
evening. CAPE over 2000 J/kg, deep-layer bulk shear approaching 50
kt, low-level SRH over 300 m2/s2, and abundant low-to-midlevel
moisture will support supercells capable of all severe hazards,
including tornadoes. It is plausible that this development could
occur largely east of our area given recent CAM trends, but still
think a few storms will clip our southeastern zones.

Lower PWATs overspread the area from west to east on Thursday as
upper ridging builds over the region. This should spell the end to
convection for at least a couple of days. As a surface ridge
shifts east of the area late this week into early in the weekend,
moisture return should provide anomalously high PWATs (above 90th
percentile) across much of the area again. Thus, despite limited
large-scale forcing, may still begin to see isolated/scattered
storms across the Black Hills/SD plains on Saturday due to upslope
low-level flow. Some of these storms could produce locally heavy
rainfall, and we will need to keep an eye on the threat for severe
hazards. General model consensus suggests an upper-level shortwave
and attendant surface front will cross the area late weekend,
which will support more widespread showers and storms across
portions of the area. Solutions differ on moisture availability
into early next week as the upper ridge flattens and we transition
to zonal flow aloft.


.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1113 AM MDT Wed Jul 18 2018

TSRA with MVFR/local IFR conditions are expected again across the
SD plains this afternoon and evening...especially toward central
SD. Severe storms with heavy rainfall will be possible.




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