Area Forecast Discussion

KEY MESSAGES

- Risk for excessive rainfall this afternoon into Sunday across eastern Wyoming and the Black Hills.

- Areas of fog possible tomorrow morning across much of western South Dakota.

- Gradual warming with drier conditions this week.

DISCUSSION

(This Evening Through Saturday) Issued at 122 PM MDT Sat Sep 19 2026

Current analysis shows the closed low continuing to spin and move slowly east into western Wyoming this afternoon. Downstream of this area of low pressure much of eastern Wyoming and western South Dakota sit under weak flow and widespread low clouds. These clouds have allowed the temperatures to remain steady, not changing much from the overnight low temperatures as most areas are still sitting in the low 60s with a few pockets of mid to upper 60s.

Heading through the rest of today, we will watch the Black Hills and areas to the west and south for potential thunderstorm initiation this afternoon ahead of the main area of low pressure as it will continue to move east toward eastern Wyoming. Limited heating today may suppress more robust thunderstorm cores from developing today but we are still expecting some thunderstorms to develop as the better dynamics associated with the main area of low pressure continues to approach the area. Window for initiation looks to be now (1pm MDT) through the next hour or so. The coverage and strength of the thunderstorms will alter both the marginal severe risk and excessive rainfall risk at least through Saturday. Still any storm that does develop will produce moderate to rainfall as they will be slow moving and will be able to tap into the modest PWAT values sitting across western SD.

Come Sunday we expect the coverage of showers with embedded thunderstorms to increase as the area of low pressure moves into the region. Nearly everyone should see rainfall but totals will depend on your proximity to the axis of where the clustering of thunderstorms are expected to be. Probabilities continue to of seeing rainfall amounts over 1" over the Black Hills and adjacent areas. Outside of these areas we could still rainfall totals close to 0.50". Sunday will begin cloudy and foggy in most areas with mist/drizzle occurring before transitioning into rain. This rain will persist through much of the day before we begin to slowly dry out from west to east come Sunday night before we finally get rid of the unsettled weather by Monday. This will begin the gradual warm and dry pattern that will develop as a ridge of high pressure begins to expand across the Intermountain West. Temperatures will climb back to or slightly over seasonal normals by Wednesday.

AVIATION

(For the 18Z TAFS Through 18Z Sunday) Issued At 1108 AM MDT Sat Sep 19 2026

Low clouds are expected to stick around through much of the TAF period. Ceilings could see some gradual improvement through the afternoon, however, they should still be limited to no higher than MVFR, especially as showers and thunderstorms redevelop during the afternoon (~19/19-21z) and evening hours. Expect gusty/erratic winds associated with any stronger thunderstorms or heavier precipitation cores. Hail cannot be ruled out with any of the stronger thunderstorms this afternoon/evening. Mist is expected to mix in with the precipitation late tonight into tomorrow reducing the visibility to IFR (possibly LIFR) at KRAP and KGCC.

UNR WATCHES / WARNINGS / ADVISORIES

SD...None. WY...None.