210 FXUS63 KFSD 241456 AFDFSD AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 956 AM CDT Tue Aug 24 2021 .UPDATE... Issued at 946 AM CDT Tue Aug 24 2021 A couple of updates this morning. Storms have continued to develop across southwestern MN into northwestern IA, with backbuilding this morning. A few areas of focus for this development: 850mb WAA coupled with a lingering low level jet and cold pool from the earlier MCS across portions of MN. These storms have been severe for nearly the last two hours, producing 60 mph wind gusts and quarter-sized hail (per spotters in the area). Additionally, storm motion has been northeast, allowing storms to train over the same areas. Although no concerns at present for flash flooding, will continue to monitor this potential given the training nature - storms have begun just a bit more of an eastern shift. Model guidance this morning (and through the previous overnight) has not been handling this convection well in any manner. Thus, it remains to be seen at present how this could potentially impact our severe chances for this afternoon and evening as the cold front over central SD begins to move to the east. && .SHORT TERM...(Today and Tonight) Issued at 433 AM CDT Tue Aug 24 2021 Large complex of storms north of the area this morning will put out an outflow which will leak to just south of the Highway 14 corridor this morning, with just a temporary influx of recycled cooling and drying, along with winds shifting briefly to the north/northeast. Elsewhere, the feeding low-level jet inducing a smattering of elevated showers/thunderstorms across southwest MN, which will lift away during the early morning hours. Another storm has been wandering through northern NE, set off a heat burst at KVTN, and now will likely follow the predecessors and weaken heading out of the better elevated instability and into the drier airmass with the EML poised to the east. Today will be another warm and very humid day, as a wave moves out of the Canadian Rockies and along the International Border. Low- level jet will weaken during the morning and veer as the complex passes by mostly to the northeast of the area, likely clipping areas around KMML/KTKC with a very early day risk of thunder. While could see the southern end of an area of gusty outflow winds, the severe threat with this feature looks to remain north of the area. The other area today to watch will be those areas near NE with presence of the elevated mixed layer and some mid-level moisture. There is always the risk in this pattern of a random accas shower/storm popping up, but usually like to see the mid-level lapse rate closer to 9C/km instead of 8-8.5C/km. Nevertheless, even with the rising heights into the afternoon, should at least see a scattered mid- level deck spread in across far southeast SD, northeast NE and into northwest IA, and will have to monitor to see if any precip with a very small positive area can actually make it through the warm/dry layer as precipitation during the early to mid afternoon. Impact of this could be to gnaw away at what appears to be a formidable cap to surface-based convection, from 200-300 J/kg depending on what one believes, through a fairly deep 200 hPa layer. It will be this environment of a strong cap which the approaching frontal boundary moves into far southeast SD by late afternoon into very early evening. At this point, may be a stretch to expect development along the boundary, but cannot fully rule out if cap is weakened by mid-level showers, or if temps further south along the boundary in NE reach much warmer and actually heat out to start development along the boundary. The area remains in a marginal risk of severe storms today, and it cannot be stated enough that this is a conditional risk. Should storms develop, the mean MLCAPE is a fairly thick 2000-2500 J/kg, with appreciable DCAPE, but shear on the southern edge of the westerlies only 30-35 kts. Convective mode may be a bit messy, but a somewhat more normal direction suggests at least odds to keep more isolated. Potential for upscale growth into bowing segments in play as well, and the 0-3km shear is even more normal especially from the lower Mo River into northwest Iowa, so mesovortices also a possibility. Best stated as a remote chance for hail up to golf ball size, winds to 75 mph and a brief tornado from very late afternoon into the evening hours, and primarily areas south of I-90 in far southeast SD into northwest IA. Otherwise, through the day, will start to see heating out and moisture levels underestimated in most models beneath the warm layer aloft, so have gone closer to NBM 90th percentile, with the strong exception of areas where the wind shift will move in to the west, staring later this morning. Mean mixed layer dew points here will drop into the upper 40s and 50s during the day as wind swings around northwest to north. Fire danger looks to be at least high, but is close to reaching very high if temps warm a bit more than expected with humidity 20-25 percent. Further east, the lid on high dewpoints along with expected warming will drive the heat index into the 90s to just over 100 degrees this afternoon. Have gone ahead of issued a small heat advisory for portions of far southeast SD, northeast NE and the Sioux City area of northwest Iowa with several hours of the heat/high dew points leading to heat indices above 100 degrees. Tonight, after convection winds down, will see much drier air slowly spread in northwest to southeast behind the front. Winds in blends look a bit anemic, and boosted closer to 90th percentile to account for advection. Some mugginess holding on toward U.S. 20 in IA, otherwise a more comfortable night shared by most as temps fall to upper 50s to mid 60s. .LONG TERM...(Wednesday through Monday) Issued at 433 AM CDT Tue Aug 24 2021 Likely one of the only truly quiet periods in what looks like several days of unsettled weather will be on Wednesday. Cooler and stabilizing low-level airmass settles southward as the frontal boundary is likely driven toward a more southern end of model distributions into KS/MO. Several members show an increase in mid- level frontogenesis in entrance of exiting jet streak and exit of approaching jet stretching out along U.S. 14 in the afternoon, which given the very dry low-levels, would expect to be only a cloud signal. Some solutions are trying a few showers, and that would not be impossible with a 8-9C/km mid-level lapse rate poised just south of this area. Mostly 80s for highs, with some upper 70s east central SD into southwest MN, a bit on the higher side of the distribution. A wave will begin to push out into the western Plains on Wednesday night, increasing the low-level jet. Very unstable air aloft to the south above the frontal surface, leading to high potential instability. Storms also likely to initiate west during the evening and translate eastward. Much latitudinal variation in where convection is expected to blossom especially later Wednesday night, likely a result of the broad and fairly gradual frontal slope from NE to northern SD. As flow impinges on the east-west boundary, should see expanding coverage of elevated storms. With steep lapse rates and decent directional shear, elevated supercells are possible with potential for hail up to 2 inches in diameter. Marginal risk for severe seems in order, as location and coverage remains a question at this point. Unsettled conditions continue into the weekend, with stronger wave moving out Thursday night, and another around later Saturday. Where boundaries end up with set up the potential for severe as well as areas of heavy rainfall. Much will be determined as the earlier events shape the resting places of important features. However, with the persistence of the southwest flow aloft and the available moisture and instability, it is important to keep up to date on the day-to-day changes. A marginal risk is already in play on Thursday. In terms of rainfall, distributions have some fairly burly amounts, but a tremendous variance in amounts as well. Certainly potential for heavy rainfall, starting as early as Wed night/Thursday. At least after stronger trough passage, should quiet down and dry out on Sunday. && .AVIATION...(For the 12Z TAFS through 12Z Wednesday morning) Issued at 645 AM CDT Tue Aug 24 2021 Tail end of a complex of thunderstorms with surface wind gusts to around 35-40 kts will push through the remainder of southwest MN by 13z, leaving behind a fairly stout and stable outflow pool down to around KEST to KFSD before synoptic set up slows and stalls. Could see a little MVFR to just north of KFSD through 14z, but a stronger secondary push toward KHON will spread in a couple hours of broken ceilings, with heights likely in MVFR range. In terms of later day convection, confidence in development is lacking to place thunder in KFSD/KSUX forecasts. If any storms manage to develop along the frontal boundary after around 22-23z, there would be a threat for severe. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Heat Advisory from 1 PM this afternoon to 6 PM CDT this evening for SDZ065-066-068>071. MN...NONE. IA...Heat Advisory from 1 PM this afternoon to 6 PM CDT this evening for IAZ020-031. NE...Heat Advisory from 1 PM this afternoon to 6 PM CDT this evening for NEZ013-014. && $$ UPDATE...SG SHORT TERM...Chapman LONG TERM...Chapman AVIATION...Chapman