Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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712 FXUS63 KFSD 150808 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 308 AM CDT Sat Jun 15 2024 .KEY MESSAGES... - Rain will continue to move northeast through the morning hours of Saturday. Persistent showers into the afternoon may limit temperatures. - Very conditional risk for a few stronger storms over Northwest Iowa and southern Minnesota later this afternoon and evening. The greatest risk will be from hail. - Heat advisory criteria may be approached over Nebraska and Iowa Sunday and Monday afternoon. - Potential for a higher end severe weather risk Sunday night into Monday across the CWA. Very large hail will be the primary risk, but some wind risk could develop. - Additional concerns for heavy rain and flash flooding late Sunday night into Monday. Though some uncertainty exists on where this rain axis may develop. - Very unsettled pattern through most of next week. Ensemble guidance through the week suggests nearly a 50% probability for as much as 4" of rain by next weekend. && .DISCUSSION... Issued at 304 AM CDT Sat Jun 15 2024 THIS MORNING: Large MCS continues to work eastward across eastern Nebraska this morning. Further north, rainfall has generally been very light due to the presence of high bases and sub-cloud dry air. CAMs all indicate rainfall will increase throughout the Tri-State area after 3am, with the associated lobe of vorticity moving into southwest Minnesota by mid-morning. Rainfall rates have been less than expected, and overall QPF has lowered into the morning. TODAY-TONIGHT: Most CAMS suggest this initial lobe of vorticity moves north east of the CWA by late morning. However have some concerns that the persistent southeast low-lvl warm advection persisting into the afternoon ahead of a slightly positively tilted mid-lvl trough moving into eastern SD/NE may keep scattered showers going well into the afternoon. Regardless the increase of cloud cover should cause temperatures to stay much cooler in the 70s, with some potential for a late day climb towards 80. Uncertainty remains fairly high for severe weather potential later this afternoon/evening. In the wake of the morning MCS, we may see some sort of differential heating boundary develop south of the CWA by the afternoon which will try to lift northward into early evening. CAMS are all over the place as to how far this can travel this afternoon, but there remains some potential for upwards of 2000 J/KG MLCAPE across portions of southeast South Dakota, northeastern Nebraska, and northwest Iowa by 21Z. With lift arriving from secondary trough sliding eastward, convection may initiate quickly by 21Z and then track into northwest Iowa. While deep layer shear is marginal, given steepening mid-lvl lapse rates, large hail up to 1.5" seems to be the most likely threat, with a secondary wind threat. Depending on how the surface evolves, cannot discount the tornado threat as long as storms can stay rooted into the boundary layer. 0-1km helicity values push 100-150 m2/s2, 0-3KM CAPE is pushing 250 J/KG, and LCLs are quite low at 1800 ft AGL across most of northwest Iowa. The positive is that this threat will not go long into the overnight with most of the risk east of the CWA by 9pm. In the wake of the rain light winds may allow for some fog development. Further west, a few models hint at decaying convection reaching central South Dakota after midnight. SUNDAY: A surface front is expected to sink southward into northeast Nebraska and stretch into southern Minnesota by mid-day Sunday. 70 degree dewpoints will likely pool along and south of this front by Sunday afternoon and with temperature surging into the 90s over eastern Nebraska and portions of northwest Iowa, heat index values may push 100 degrees. Further northwest, temperatures will be cooler in the lower 80s. SUNDAY NIGHT: Increasing concerns for both severe weather and heavy rain risks Sunday night. Aforementioned frontal boundary will serve as a focal point (especially north of the boundary) for convection to develop after dark. An intensifying LLJ, sharpening frontogenesis, and broad dPVA within strong southwesterly mid-lvl flow will provide all the ingredients needed for convection to develop. Soundings suggest the greatest focal point for lift will be within the 850mb zone, which would provide upwards of 2500-3000 J/KG MUCAPE. Deep layer shear 40-50 knots, and mid-lvl lapse rates within the prime hail growth zone approaching 7-7.5 C/KM would support the potential for very large hail AOA 2". If storms stay rooted closer to the surface front, then a damaging wind risk could also develop. The strong moisture convergence within the 925:850 mb layer also push PWAT values over 175% of normal by late Sunday night. Given steering flow nearly parallel to the surface boundary, very high warm cloud layers surpassing 14K ft AGL, and large volume of MUCAPE, would anticipate some very high rainfall rates. The biggest uncertainty is where the surface front will become established and where the zone of greatest lift will form. All-in-all, agree with WPC slight risk for excessive rainfall, with some CAMs suggesting 2-3" of rain. MONDAY-MONDAY NIGHT: Convection north of the boundary may continue well into Monday, which will keep a high temperature gradient across the front into the early afternoon. Eventually there are some thoughts that as heights deepen over the Central Rockies, we`ll begin to shift the frontal boundary northward and westward late in the day, placing the greatest risk for afternoon and evening redevelopment west of the James River and north of Highway 14. Temperatures will have a late day surge south and east of the boundary, with most areas reaching into the upper 80s to lower 90s, though where the front ends up is still highly in doubt. PoPs remain fairly high into the overnight hours of Monday, through this is likely due to the high degree of uncertainty in ensemble guidance and impacts on the NBM. In reality the chance for rain and associated QPF may be much lower. Another corridor of very heavy rain is likely Monday night wherever the sfc warm front become established. This should be north of the CWA into central Minnesota, but we`ll have to watch closely. TUESDAY-SATURDAY: It`s difficult to go into too much detail for the rest of next week as there are so many mesoscale details that will have to play out first. The overall pattern continues to be one that is going to be very active into next weekend. Eventually the frontal boundary moves back south on Tuesday bringing more rain to the region. However the mid-lvl flow flattens through the Plains for the rest of the week, keeping the surface front and low-lvl baroclinic zone very near the CWA into next weekend. This may end up being a classic setup for repeated rounds of convection along the boundary, and given the high PWAT values in the region, significant rainfall potential exists. In fact, the probability of >4" of rain by next weekend is around 50% in the ECE, 40% in the GEFS, and 70% in the CMC. Even more concerning are the spotty 10% probabilities for as much as 8" of rain in the Tri- State area. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1052 PM CDT Fri Jun 14 2024 VFR conditions may deteriorate to MVFR overnight as an area of showers and scattered thunderstorms spread in from central/eastern Nebraska. Southeast winds remain fairly light, near or below 15 kts with occasional gusts, but may see gusts in the 20s mix down overnight with rain. MVFR and occasionally IFR ceilings are expected with scattered showers and storms continuing through Saturday morning. Southeast winds will weaken Saturday afternoon as ceilings lift and rain clears out. May see some storms redevelop and/or move back in during the afternoon and evening, especially east of I-29, but confidence was too low to include in the TAF at this time. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...Dux AVIATION...BP