946 FXUS63 KFSD 230805 AFDFSD AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 305 AM CDT Sun Aug 23 2020 .SHORT TERM...(Today and Tonight) Issued at 305 AM CDT Sun Aug 23 2020 The primary forecast issue in the short term is temperatures and fog. Per observations and the GOES-16 nighttime microphysics imagery, some patching fog has been forming in the Missouri River valley near Sioux City and also just east of Spirit Lake and Spencer in northwestern Iowa. Observations also show light fog around Cherokee. Some denser fog may form in eastern Dickinson and Clay counties through sunrise. Similarly, could see more fog development in the Little Sioux and Floyd River valley. Not expecting this fog to become widespread with only localized visibilities less than one mile. Any fog that forms is expected to dissipate quickly after sunrise. Otherwise, the well advertised heat will continue. One area of concern is the impact of smoke on high temperatures. Satellite showed smoke moving into southeastern South Dakota at sunset and the experimental HRRR smoke forecast shows the smoke in the upper troposphere spreading across all of southeastern South Dakota and southwestern Minnesota by mid morning and then into northwestern Iowa and northeastern Nebraska this afternoon. Discussions with WFO Rapid City indicate smoke may have kept temperatures 2-4 degrees cooler than expected in this air mass. Given smoke will be thickening during the day, have lowered highs about 1 or 3 degrees than full mix out would give with the largest differences in southeastern South Dakota. This still puts highs in the lower to mid 90s today. With light south winds overnight, lows will again fall into the mid to upper 60s. .LONG TERM...(Monday through Saturday) Issued at 305 AM CDT Sun Aug 23 2020 The heat will be the main story through Thursday across the area. Except for the exact timing of the frontal passage on Thursday, there are few model differences with all models showing anomalously warm conditions. The NAEFS ensemble mean has 850 and 700 mb temperatures near or above 99th percentile from Monday through Wednesday. This gives high confidence the hot temperatures will continue through Wednesday or Thursday. The only uncertainty will continue to be the impact of smoke which is expected to remain over the area through midweek before an approaching upper trough may switch the source of air from the US west coast to southwestern Canada. Without the smoke, many areas would reach the upper 90s and could even crack 100 degrees Monday through Wednesday. With confidence higher that thicker smoke will be over the area on Monday, lowered highs into the lower to mid 90s along and north of I90. In the Missouri Valley and northwestern Iowa, kept highs in the mid to upper 90s. For Tuesday through Thursday, did not adjust temperatures lower yet given there is still some uncertainty on how thick smoke will be on those days and what its exact impact will be on temperatures. But if the smoke remains fairly thick through mid week, it is likely highs will be 2-5 degrees cooler than currently forecast on those days. Fortunately, dew points will remain in the 60s so that heat indices should remain in the upper 90s to around 100 rather than feeling like 100-110 as it did earlier this year. In terms of precipitation, very little is expected through Thursday. The aforementioned near record 700 mb temperatures will serve to cap any surface-based convection until Thursday. One item of interest is an upper wave currently off the Mexican coast that is expected to move across the area on Monday night. This wave will bring some mid-level moisture above the capping inversion where lapse rates of 8-9 C/km will be in place. Both the NAM and GFS show isolated high-based thunderstorms developing Monday evening. At this time did not include storms in the forecast given the uncertainty on how much midlevel moisture will accompany the wave and the very limited coverage if storms do occur. Otherwise, a cold front will approach the area on Wednesday night and move through on Thursday. Some dying convection from western and northern South Dakota may be able to move into south central and east central SD Wednesday night but any rainfall would be light and isolated. As the front moves through Thursday and Thursday night, there could be post-frontal convection as 700 mb temperatures begin to cool. But the probability remains low as 700 mb temperatures remain above +10C until Friday morning which could keep the atmosphere capped. For Friday and into the weekend, temperatures will return to normal. With the westerlies also moving south, there will be the potential for some upper level waves to moving through which may bring some rain next weekend although timing differences result in low confidence as to when and where any rain would occur. && .AVIATION...(For the 06Z TAFS through 06Z Sunday night) Issued at 1040 PM CDT Sat Aug 22 2020 Still some potential for fog later tonight into early Sunday morning but confidence on how low the visibility will go still a little low. Currently leaning towards mainly MVFR visibility with the better chances in northwest IA where a weak boundary and moisture pooling exist. Outside of the fog potential VFR conditions are expected. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$ SHORT TERM...Schumacher LONG TERM...Schumacher AVIATION...08