Area Forecast Discussion
Issued by NWS Rapid City, SD
Issued by NWS Rapid City, SD
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120 FXUS63 KUNR 300437 AFDUNR Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 1037 PM MDT Wed May 29 2024 .KEY MESSAGES... - Thunderstorms this afternoon and evening; Severe storms are expected with damaging wind and large hail the primary hazards. - Active pattern continues through the weekend with daily chances of showers and storms. - Trending warmer by Saturday, with seasonable to above average temperatures expected next week. && .DISCUSSION...(This Evening Through Wednesday) Issued at 134 PM MDT Wed May 29 2024 Upper level analysis shows upper ridge sliding eastward into the Upper Midwest with upper trough axis centered through central Idaho. Water vapor imagery shows shortwave pushing into eastern Montana with ascent notable on IR/WV beginning to overspread Montana and Wyoming. At the surface, diffuse low pressure/surface trough extends from northeastern Wyoming southward into eastern Colorado with a cold front extending from central Montana into western Wyoming. A band of convection has formed east of the Bighorns and has pushed into Campbell County. Additional thunderstorms are expected to form across northeastern Wyoming and western South Dakota over the next 1-3 hours, spreading eastward through the afternoon and evening, some of which are expected to be strong to severe. Ample instability is in place (MUCAPE ~1000-2000 J/kg) with steep lapse rates (~8-9C/km) atop a well-mixed boundary layer. Given the dry boundary layer, MLCAPE is muted (~500-1250 J/kg) and MLCIN has been slow to erode across western SD. Expect the remain MLCIN to weaken over the next 1-2 hours with continued heating and increasing upper ascent. Additionally, the dry/well-mixed BL has resulted in DCAPE ~1200 J/kg (per SPC mesoanalysis), which will persist through the evening hours. Forecast soundings show rather meager bulk shear (EBWD ~25-30 knots); hodographs show ample curvature and shear within the lowest 3 km, although modest winds aloft will limit the overall shear available. Storm mode is expected to be multi-cellular with transient supercellular characteristics. Given these ingredients coupled with anticipated storm mode, expect mainly a damaging wind threat through the evening. Regarding hail potential - modest shear will limit storm organization, although sufficient instability exists for large hail. Additionally, forecast soundings show relative lack of CAPE below the freezing layer, which will result in increased residence time for any parcel within the hail growth zone - this will help mitigate the lack of shear throughout the cloud bearing layer. Ultimately, expect the best hail threat will be limited to any stronger, longer lasting updraft cores. Deterministic and ensemble guidance both depict fairly active, zonal flow through at least mid-week next week. This would result in near daily chances of showers and thunderstorms the rest of this week into next week. Cooler temperatures will prevail tomorrow and Friday behind the surface cold front before more zonal/southwesterly flow resumes for next week and temperatures warm. Temperatures across the higher elevations of the Black Hills are likely to fall into the mid 30s tonight and Thursday night; some snow may mix in with rain across the higher elevations of the Black Hills Thursday night/Friday morning, although no accumulation is expected. Ensembles and cluster analysis highlight broad ridging building across the western half of the country by mid-week next week. && .AVIATION...(For the 06Z TAFS Through 06Z Thursday Night) Issued At 1037 PM MDT Wed May 29 2024 Showers and thunderstorms will continue to affect northeast Wyoming and western South Dakota late tonight. MVFR/IFR conditions with gusty, erratic winds are possible in and around any storms. A cold front will bring breezy northwest winds and drier air to the region and end the precip early this morning. Some MVFR & IFR CIGS will be possible as the front pushes through, but VFR conditions will soon return areawide by mid morning Thursday. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...None. WY...None. && $$ DISCUSSION/KEY MESSAGES...SE AVIATION...13