350 FXUS63 KFSD 130436 AFDFSD AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 1036 PM CST Sun Dec 12 2021 .SHORT TERM...(This Evening through Monday) Issued at 302 PM CST Sun Dec 12 2021 Unseasonably warm air below 850 mb has spread across most of the CWA this afternoon and will remain into Wednesday. The forecast challenge will be how much of that warm air reaches the surface, and how much melting snow may result in fog later tonight. A weakness in the surface pressure gradient will move across the area after sunset and winds will gradually diminish to light and variable later this evening. Except for patchy cirrus, skies will be mostly clear so places should rapidly fall into the 20s with areas in sheltered valleys with snow cover falling into the teens. With dew points in the lower to mid 20s, some concern that as temperatures approach 20, fog will begin to form in those sheltered valleys. Similar to previous forecast, we continued with patchy fog developing after midnight. With winds remaining light and the very low sun angle, the fog may be slow to burn off on Monday - especially if it is dense. Winds will slowly increase out of the southeast Monday afternoon west of I-29. Once again 925 mb temperatures will be from 5-10 C. Most of the area remains snow covered but bare ground is now appearing on satellite in the Missouri Valley east of Yankton and also along Highway 20 in northwestern Iowa. Snow has also begun to disappear north of Hwy 14 in Kingsbury and Beadle County. In both areas have raised highs into the lower to middle 40s. Coolest temperatures will again be in the thicker snow cover from Sioux Falls into southwestern MN in the mid to upper 30s. .LONG TERM...(Monday Night through Sunday) Issued at 302 PM CST Sun Dec 12 2021 The big concern in the long term will be the potential for dense fog Tuesday night and then strong winds, and maybe even a thunderstorm, Wednesday and Wednesday night. Monday night will again be mostly clear and winds will be under 10 mph in most areas. That will again allow temperatures to fall into the lower 20s. With melting again on Monday, there is some concern fog could reform on Monday night but enough wind may keep fog limited to a few river valleys east of I-29. Tuesday will see increasing clouds as the upper wave moves into the Rockies and cirrus streams northeastward into the plains. 925 mb temps will again be from 5-10C - this time with low level winds of 10 to 15 mph. With snow cover continuing to be eroded, temperatures are more uncertain both south of Hwy 18 in northwestern Iowa and north of Hwy 34 in SD. Should snow cover be completely melted these areas could warm well into the 40s with lower 50s possible around Sioux City and Storm Lake. Huron could also rise into the mid to upper 40s. In the deeper snow cover, climatology suggests that highs in the upper 30s to lower 40s is more likely - depending on the depth of the snow. For now kept highs somewhat conservative in the upper 30s where snow cover is expected and mid to upper 40s where snow cover will be melted off. On Tuesday night, ample moisture will be transported northward on a 40-50 kt LLJ. Dew points at 925 mb are expected to be above freezing. As this air mass overrides the cold ground and even colder snowpack, stratus will develop and there is a good chance (40-50%) that fog will develop overnight and could persist into Wednesday. If surface dew points do rise above freezing, areas with snow cover could see a prolonged period of dense fog Tuesday night and Wednesday. There is still a lot of uncertainty on both the extent of the snow cover and and how high dew points will be late Tuesday night and Wednesday. At this time, did not put fog into the forecast since it is still 2.5 to 3 days in the future. Regardless, as this moisture advects northward, evening lows are expected Tuesday night with temperatures will slowly rising much of night. Evening lows will be in the 30s but temperatures will likely be in the 40s in portions of northwest Iowa by sunrise. Wednesday will be a windy and very warm day - especially east of the James River. There does exist a lot of uncertainty of the exact storm track. A farther west and slower solution would bring very warm air all the west to the James River or even Chamberlain. A faster and more east track would likely limit the warmest air east of I-29 and bring cold air and falling temperatures into areas west of I-29 by the afternoon. Add to the fact that there is also uncertainty on how widespread snow cover will be and the result is a large range in temperatures on Wednesday. At this time the potential for record temperatures, perhaps even all-time December records exists in northwestern Iowa where highs are likely to reach 60 degrees and could even reach 70 if there is a slower progression of the cyclone. As one heads northwest, the probability of record temperatures decrease - both because of snow cover and the lower probability the warm air is able to transport that far. At this time, kept areas where snow cover is expected in the 40s in Minnesota and along I-29 in South Dakota. But if the snow cover is melted, then these locations could warm well into the 50s. In addition to the unseasonably warm air, several models are also suggesting unstable air - with MUCAPES > 100 J/kg developing across portions of northwest Iowa and southwestern Minnesota. 700 mb temperatures are very warm and that may cap the atmosphere until the cold front moves east of the area. For this reason, there remains uncertainty how much precipitation will for along the cold front. Any precipitation that forms is expected to exit the area Wednesday evening. The other impact of this system will be winds. Southerly winds ahead of the system Tuesday night and Wednesday will be 20-30 mph with gusts to 45 mph. Wind will be even stronger behind the front. Sustained winds for a period Wednesday night could be over 30 mph. Winds will be at or above 50 kts around 1km so gusts over 50 mph will be possible. Fortunately, winds should slowly decrease late Wednesday night and into Thursday. Beyond Thursday, ensemble model solution begin to diverge. There are essentially two cluster of solutions - one in which a ridge begins to rebuild over the central US such that temperatures will begin to moderate by next weekend. A second cluster of solutions has a second trough move across the Plains next weekend with the result that a second shot of much colder air would move into the area during the weekend. No solution is favored at this time so we will continue to monitor to see where temperatures may be headed the week before Christmas. && .AVIATION...(For the 06Z TAFS through 06Z Monday night) Issued at 1033 PM CST Sun Dec 12 2021 Recent snowmelt and light winds would still suggest at least some potential for fog overnight and Monday morning although guidance continues to be rather unenthusiastic about this prospect. Have continued some minor mention at KHON and KFSD where forecast soundings are somewhat more convincing. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$ SHORT TERM...Schumacher LONG TERM...Schumacher AVIATION...Kalin