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424 FXUS63 KUNR 190340 AFDUNR Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 940 PM MDT Wed Jun 18 2025 .KEY MESSAGES... - Hot and mostly dry conditions build into the region beginning tomorrow, with record heat possible to probable for portions of the area Friday and Saturday. - Unsettled conditions with more seasonable temperatures expected early next week. && .DISCUSSION...(This Evening Through Wednesday) Issued at 158 PM MDT Wed Jun 18 2025 Water vapor imagery this afternoon shows a mostly dry air mass over the High Plains, with ridging building over the Great Basin/Four Corners. Along the eastern periphery of the ridge, lingering midlevel to upper-level moisture in the wake of a midlevel shortwave trof is supporting isolated convection from eastern MT through central/eastern SD, with more widespread thunderstorm activity closer to the parent wave over the Midwest/Great Lakes and atop the ridge in southern BC/AB/SK. Fair weather cu is evident via visible satellite imagery over much of our area, with pockets of more congested cu over south central SD and the southern Black Hills. In each of these areas, some CAMs have reflected the potential for a few showers/storms later this afternoon; however, for now, convective initiation does not appear imminent. Given observational trends, have removed any mention of precipitation today. Fairly innocuous surface pattern is leading to variable wind directions, with a heavy terrain-drive component near the Black Hills. Afternoon temperatures are generally in the 70s to lower 80s--pleasant compared to what's to come over the next few days. High confidence in the midlevel ridge continuing to build and, more importantly, shift eastward over the next 24-36 hours. Ridge axis will likely be overhead by late tomorrow afternoon/evening. Low- level thermal ridging slightly trails the midlevel height ridge, becoming more focused over our region by midday Friday. By this time, midlevel/upper-level flow transitions to predominantly westerly/southwesterly, which will facilitate the advection of weak convective-driven PV anomalies over the region beginning as early as Friday afternoon. Hot/very dry air mass will preclude thunderstorms over much of the area, whereas stout elevated mixed layer across our far northern tier should limit potential for convection. However, it is worth noting a very low probability but high impact scenario could play out across far northwestern SD on Friday afternoon/evening. One of the aforementioned PV anomalies/midlevel shortwave trofs moving through broader southwesterly flow aloft may prompt cyclogenesis over the northern Rockies/High Plains Friday morning into the afternoon, with an associated baroclinic zone/warm front feature plausibly developing over the western/northwestern SD plains. If any convection can overcome the cap, an explosive CAPE/shear combination could lead to high-end but localized severe potential. Ensembles suggest that 850 mb temps of 25 C or greater are near certain by as early as 18z Thursday over our western zones, expanding to cover most of the area by Friday. NAEFS 700 mb and 850 mb temperatures are accordingly above the 90th percentile--and approaching maxima over our southeast--from Thursday afternoon through Sunday morning. Downtown Rapid City's record highs of 98 degrees on both 6/20 and 6/21 are certainly at risk, particularly on Friday (which now looks like the hotter of the two days), when probabilities of highs exceeding 100 F climb above 70%. Heat advisories will likely be needed Friday and Saturday, particularly across our southeastern zones as highs and heat indices climb above 100 degrees and approach 105 degrees; however, duration criteria for extreme heat products will probably not be met. Therefore, have held off on issuing an Extreme Heat Watch at this time. Sunday through early next week, the low-level thermal ridge breaks down as a trof sweeps across the region. The midlevel trof may not quite accompany the low-level wave, with ensembles favoring a continuation of deep southwesterly flow over the region. The result will be cooler (though, really, more seasonable) temperatures and potentially more unsettled conditions over the first half of next week. && .AVIATION...(For the 06Z TAFS Through 06Z Thursday Night) Issued At 940 PM MDT Wed Jun 18 2025 VFR conditions expected to prevail through the period. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...None. WY...None. && $$ DISCUSSION/KEY MESSAGES...Sherburn AVIATION...Wong