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FXUS63 KUNR 230541 AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 1141 PM MDT Fri Sep 22 2023

UPDATE
Issued at 715 PM MDT Fri Sep 22 2023

The severe thunderstorm watch for northeast WY and far southwest SD has been cancelled. The main risk for severe storms has ended in these areas as current disturbance exits and the threat for stronger storms stays south of the area through the early evening.
Activity across the forecast area has become isolated to widely scattered for the time being, with a stronger storm or two across south central SD right now. However, the next wave wrapping around the upper low over western WY is moving across southern and central WY and will arrive later this evening and overnight.
Showers and storms should pick up in coverage and intensity mid to late evening across eastern WY and then spread across much of western SD toward and after midnight. This activity should be elevated, but there will be enough buoyancy aloft overnight, 500 to near 1000 j/kg MUCAPE, for a few stronger storms with at least small hail across portions of northeast WY into the Black Hills and southwest SD overnight. Some heavier rainfall will accompany any stronger storms as well. The bulk of this activity should push east-northeast out of the area late tonight and early Saturday.

Other than cancelling the watch, not much in the way of adjustments is needed to the forecast right now. Some adjustments to pcpn chances may be made later in the evening as new activity develops over western areas.

DISCUSSION
(This Evening Through Friday)
Issued at 100 PM MDT Fri Sep 22 2023

Water vapor shows upper low redeveloping near SLC, with one ejecting wave over western SD and the next over southeast WY. At surface, elongated trof extends from southeast WY through eastern CO. Convective activity over western SD is quite tame well north of the surface trof, but activity over eastern WY is becoming intense.

For the rest of this afternoon into the evening...the best severe potential in our area extends from southern Campbell county southeastward through Fall River county. This is where the best alignment of MLCAPE (up to 2000J/kg) and deep layer shear exists.
There is also enhanced stretching potential in this area given the very high SRH values and where the best potential for a possible tornado exists through the afternoon (as previously discussed).
The severe potential with this southeastern WY wave will diminish by about 00z. Another round of showers/storms (mainly non-severe)
is then expected later tonight and early Saturday morning as the main upper low crosses WY and pushes into far western SD by Saturday morning.

Upper low will shift into central SD by Saturday afternoon, with on and off showers and breezy northwest winds. Showers will gradually end from west to east Saturday night, with drier conditions expected on Sunday. Latest guidance is now pushing the upper low through a bit faster, so precip chances for Sunday have been diminished.

Beyond the weekend, southwest to northeast oriented upper ridge builds over the region, which should lead to a drier and seasonably warm pattern over the first half of next week.

AVIATION
(For the 06Z TAFS Through 06Z Saturday Night)
Issued At 1141 PM MDT Fri Sep 22 2023

Scattered to numerous showers and thunderstorms will track east- northeast across much of the area overnight, with areas of MVFR/IFR conditions and patchy fog expected into early Saturday morning. Showers will taper off some early Saturday, with mostly isolated to scattered showers expected during the day, and a few thunderstorms possible in the late morning and afternoon to the north and east of the Black Hills.

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

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