603 FXUS63 KUNR 210448 AFDUNR Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 1048 PM MDT Fri Aug 20 2021 .DISCUSSION...(This Evening Through Friday) Issued at 152 PM MDT Fri Aug 20 2021 The latest water vapor images showed the upper low spinning across far northwestern SD as a trailing short-wave trough was moving through western SD. This was producing scattered showers and a few thunderstorms over parts of northeastern WY, the BLKHLS, and northwestern/west-central SD. The low and wave will continue to move northeast this afternoon and evening, allowing for the rain to come to an end. Otherwise, a tightening pressure gradient around a surface low in southeastern ND--along with low-level cold advection-- will foster gusty west to northwest winds across much of the CWA for the remainder of the afternoon into the first half of this evening. On Saturday a weak short-wave ridge will maintain mostly dry conditions, but an approaching short-wave trough will produce return southerly flow across northeastern WY. Combined with steepening midlevel lapse rates and modest low-level moisture return, MLCAPE of 500 J/kg will develop in the afternoon and may lead to a few thunderstorms over Campbell County. There will be a better chance of showers and thunderstorms Saturday night as the short-wave trough crosses the CWA and a stout LLJ (45-kt 850-mb winds) helps maintain elevated thunderstorms. A weak Pacific cold front will cross the CWA Sunday in association with the upper trough, leading to gusty westerly winds, especially across southwestern SD. There also will be a very narrow window for a few severe storms over south-central SD before the front passes that area; MLCAPE should reach near 1500 J/kg by midday, along with sufficient shear for organized storms. The rest of Sunday through most of Tuesday will be dry and warmer before the next upper-level trough approaches. Highs will reach well into the 90s for most of the area on Monday, but still remaining below record levels. Fire weather conditions also will be near critical for parts of the area Sunday and Monday. The next cold front moves through the CWA on Tuesday, with potentially unsettled weather late Tuesday through Friday. Although ensembles show a range of timings and placements for short-wave trough passages, they generally are consistent with MLCAPE of 1000-1500 J/kg Wednesday through Friday afternoons. Temperatures also should be close to average for Tuesday through Friday. && .AVIATION...(For the 06Z TAFS Through 06Z Saturday Night) Issued At 1045 PM MDT Fri Aug 20 2021 VFR conditions are expected across most the area through Saturday afternoon. Isolated showers and storms are possible across northeast WY by late Saturday afternoon into the evening. && .FIRE WEATHER... Issued At 152 PM MDT Fri Aug 20 2021 A front will move through the area late Saturday night and Sunday morning, which will produce gusty westerly winds on Sunday, especially across southwestern SD where winds could gust from 30 to 35 mph. With drier air moving into the area, the relative humidity will drop to 15 to 20 percent, producing near critical fire weather conditions. It is possible that a fire weather watch may be issued for this area during later shifts. Monday will be the hottest day of the period ahead of the next cold front. Highs well be in the upper 80s to mid/upper 90s for most places, and relatively humidities will drop into the lower teens for parts of northeastern WY and southwestern SD. Deep mixing will promote gusty southwest winds of around 25 mph, again producing near critical fire weather conditions. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...None. WY...None. && $$ DISCUSSION...Bunkers AVIATION...15 FIRE WEATHER...Bunkers