Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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509 FXUS63 KFSD 150354 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1054 PM CDT Fri Jun 14 2024 .KEY MESSAGES... - While a few showers and isolated thunderstorms will be possible near and west of the James this evening, a high chance for showers and storms comes overnight and through Saturday morning. There is a low chance for an isolated severe storm or two in south central SD. - A few strong to severe storms are possible near and east of a Marshall to Sioux Falls to Yankton line Saturday evening. - Sunday night into Monday brings a moderate to high (50-80%) chance for more organized rain and thunderstorms. Scattered severe storms and locally heavy downpours are possible overnight. The highest risk for heavy rain and/or severe weather is for locations near and north of a Yankton SD to Spencer IA line. - Summer heat brings highs in the 80s and 90s through Monday with a high chance of heat index readings of 90 to 100 degrees. The pattern remains active with periodic rain chances through the remainder of next week. && .DISCUSSION... Issued at 233 PM CDT Fri Jun 14 2024 Mid to upper level moisture will increase through the afternoon and into the evening ahead of an upper level wave moving out of the Rockies. Initially, most of the area will see only the increase in clouds and the fairly deep 10-12 kft dry layer below this moisture provides no instability and makes and weak precipitation development aloft difficult to reach the ground. After about 03z moisture from the south begins to deepen as the wave moves onto the Plains. The instability remains fairly weak and elevated thus limiting any severe potential, but the strength of the wave and the availability of increasing mid level moisture should allow showers and thunderstorms to overspread the area. GEFS and EC ensemble support a very good (40-70% chance) for a half an inch or more of rain, with higher chances near and south of I-90. The wave that will bring the overnight activity slowly moves through the area Saturday, which could leave a small corridor of instability across mainly northwest IA and southwest MN late Saturday afternoon and evening. Not a lot of model agreement on dissipating clouds and allowing heating in this area, so instability values are all over the place. A worst case scenario looks like maybe 1500-2000 J/kg CAPE and 20-30 knots of shear. So would be enough to support a hail and wind threat but likely remain isolated to scattered. With the wave in place Saturday eastern SD and northeast NE will likely see some scattered shower and isolated thunderstorm activity but it should not be severe. Saturday night into Sunday weak mid and upper level ridging spreads across the area which should keep mainly quiet conditions in place. Low pressure aloft deepens to the north with a front expected to slowly sag southward through the day. The environment ahead of this front becomes very unstable, but with a prominent cap in place not expecting any development. But with the strong cap in place and nearby boundary we may see some areas with dew points exceed 70 degrees, so starting to feel pretty humid. Better chances for this oppressive humidity will be northwest IA, southwest MN and far southeast SD. Sunday night into Monday becomes very interesting as a boundary should be in place near the Missouri River in the low levels and a little farther north and west in the mid levels. As a right entrance region of the jet races into ND, a mid level wave is expected to eject into western NE and central SD. This should force some stronger southerly flow in the low levels and bring some strong forcing across the low and mid level front. The question then becomes just where along this front will activity try to develop. Barring breaking the cap, the timing of the mid level wave suggests that development will be more likely after 3z, and if capping can hold from about 900 mb to the surface, capping looks much weaker in the 800-850mb layer. Even lifting from this level yields about 2000- 3000 J/kg CAPE with about 30 knots of shear so severe weather would become fairly likely in this environment. Likely not tornadoes but hail and wind. However if storms get going around 925 mb then a bit more shear and instability could support a more organized linear system. One other concern will be heavy rain. Mid and upper level flow will likely run normal to the low level boundary which would support training and with such deep instability and a warm cloud depth around 14000 feet some high rainfall rates will be likely. Still a lot of unknowns right now, but later Sunday night into Monday definitely shows potential. Tuesday through Friday remains active with southwest flow aloft and occasional mid level waves to trigger some activity. A lot to happen between now and then but for now Tuesday night into Wednesday and Friday appear to be the best chances. && .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...08 AVIATION...BP