939 FXUS63 KUNR 060502 AFDUNR Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 1002 PM MST Fri Feb 5 2021 .UPDATE... Issued at 645 PM MST Fri Feb 5 2021 Current surface map show arctic front backing through central Montana into eastern Wyoming. Snow is increasing in coverage across northeast Wyoming/southwest South Dakota. With expected snowfall amounts in the 2-4" range, have issued Winter Weather Advisory for tonight/early Saturday with passage of shortwave. Early morning wind chills will drop to -20F to -25F across northwest South Dakota, have also issued Wind Chill Advisory for that area. && .DISCUSSION...(This Evening Through Friday) Issued at 141 PM MST Fri Feb 5 2021 Current surface analysis places a stationary front oriented from NW to SE across the forecast area. Colder and drier air is situated to our east across central SD into ND with a slightly warmer and more moist air mass southwest of the boundary. Upper level analysis and water vapor imagery depict a fast moving shortwave and associated mid-level jet streak currently dropping southward out of southern Alberta and Saskatchewan into the Northern Plains. As large scale ascent overspreads the region, enhanced frontogenesis along the low-to-mid-level baroclinic zone will support snow overnight tonight, beginning by mid-evening. The bulk of the accumulating snow will fall across the southwestern 2/3rds of the forecast area, 2 to 4 inches possible across portions of southwest SD and the Black Hills, where some upslope enhancement is likely. The baroclinic zone will surge southward in the wake of this system with much colder air spilling into the forecast area for the weekend. Unsettled flow will result in quick moving waves pushing through the area through the weekend. The upper level ascent coupled with easterly, upslope flow will support continued snow late Saturday into Monday with additional accumulations expected. Expect to see high snow ratios with the weekend snow as the colder air mass settles into the region, which should allow for quick accumulations. Highs Saturday and Sunday will be in the low teens to single digits with overnight lows 0 to 10 below zero. While some areas in NW SD along the ND border will likely see wind chills fall to 25 below or colder tonight and Saturday night, have opted to hold off on a Wind Chill Advisory. Should we see more clearing than is currently forecast, the potential is there for large portions of Perkins, Harding, and northern Ziebach counties to hit advisory criteria. Longwave troughing will continue into next week as a deep, upper low slowly drops southward out of central Canada. Very cold air will spill into the Northern Plains for next week. Highs will struggle to push out of the single digits most days and lows will fall to 5 to 15 below. Will likely need a long duration Wind Chill Advisory for Monday night through at least the end of the week for dangerously low wind chills. Confidence is high that much of the area will see overnight/early morning wind chills bottoming out between 20 and 35 below zero. Looking beyond the end of next week - confidence is low in the overall temperature trends. While the ensemble means show a gradual warming trend to near normal temperatures, the surface temperature spreads are in excess of 60 degrees by Day 9-10+...precluding any confidence in predictability. && .AVIATION...(For the 06Z TAFS Through 06Z Saturday Night) Issued At 1001 PM MST Fri Feb 5 2021 Snow will continue across northeast Wyoming, the Black Hills, and southwest South Dakota tonight...with widespread MVFR/IFR conditions. Visibilities will improve Saturday morning as the snow ends, with MVFR/Local IFR ceilings lingering into Saturday afternoon. More snow is expected late Saturday afternoon into Saturday evening, with a return of IFR visibilities. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...Winter Weather Advisory until 8 AM MST /9 AM CST/ Saturday for SDZ012-024>031-041>044-047-072>074. Wind Chill Advisory until 11 AM MST Saturday for SDZ001-002-013- 014. WY...Winter Weather Advisory until 8 AM MST Saturday for WYZ054- 056>058-071. && $$ Update...7 DISCUSSION...Eagan AVIATION...7