Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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310 FXUS63 KFSD 240922 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 422 AM CDT Mon Jun 24 2024 .KEY MESSAGES... - River crests have already occurred or will occur within the next 24-48 hours. While no significant amounts of rainfall will occur, areal flooding caused by the prior excessive rainfall continues across portions of the area. - Hot and humid today, with heat indices above 100 degrees. The previously issued Heat Advisory has been expanded to include the entire forecast area. - Highly conditional threat for severe storms this afternoon and evening. While we could see some showers and weak storms, if storms are able to tap into more of the instability, they may quickly become severe with 2 inch hail and wind gusts of 70 mph. - The pattern through mid week does not favor any long duration rain, with another chance for isolated storms Tuesday afternoon. Conditional risk of severe weather exists on Tuesday afternoon and into the evening. Additional scattered rain chances return later this week into next weekend. && .DISCUSSION... Issued at 418 AM CDT Mon Jun 24 2024 CURRENTS: A warm and muggy morning is on-going, with 3 AM CDT observations showing most with temperatures in the upper 60s to mid 70s. Dew points remain in the 60s and lower 70s, so with increasing WAA, not expecting temperatures to fall more than 5 degrees or so from where they currently sit. Have started to see isolated showers and storms develop in the last hour over northwestern IA thanks to the aforementioned WAA and the increasing LLJ. Stout dry layer should help limit precip reaching the ground, but can`t rule out amounts around a tenth or quarter of an inch of rainfall (especially if showers/storms develop over the same area). TODAY/TONIGHT: Robust WAA continues today as mid level ridge and wave move east. We`ll mix into this anomalously warm (both 850 and 700mb in the top 1% of climatology) air aloft through the day, leading to highs in the 90s. Increased temperatures a degree or two in the east with increasing probability of exceeding 90 degrees. Paired with the incredibly high dew points in the lower to mid 70s, heat indices climb into the 100-110 degree range. Have expanded the Heat Advisory to include the rest of the forecast area (adding southwestern MN and Brookings county). Pondered issuing an Excessive Heat Warning for the Hwy 20 corridor; however, given the uncertainty with mid cloud development and subsequent impacts to highs, left the Advisory in place. Regardless, the heat and humidity will lead to unpleasant to dangerous conditions to those working outdoors today or without air conditioning. If you are spending time outdoors, please take frequent breaks in the shade or air conditioned areas, stay hydrated, and know the signs of heat illness. Breezy southerly winds will not provide much in the way of relief, even with gusts at times 25 to 30 mph. Additional showers and thunderstorms are possible this afternoon and evening with the approach of a weak mid level wave and surface cold front. With the robust layer of warm air aloft, atmosphere is strongly capped. Although we`ll be mixing into the cap, not sure we`ll mix deep enough to erode it entirely to allow convection to develop. Timing of this relatively weak wave and uncertainty in the location of the cold front relative to this leads to some uncertainty in if we`ll even see much more than elevated ACCAS type showers and isolated storms. However, if we can break the cap, showers and storms could quickly become strong to severe. Bulk shear values are the strongest off to the northeast in central MN, but despite this, we`ll still have over 3500 J/kg of CAPE (instability) to work with if we can break the cap. Steep mid level lapse rates over 8 deg C/km and sounding profiles (as well as downdraft CAPE near/above 1000 J/kg) are supportive of severe weather. Threats if we can break the cap are large hail of 2+ inches in diameter and wind gusts to 70 mph. Given the expected elevated nature of storms, tornadoes are not anticipated. Although widespread heavy rain is not expected, any thunderstorm could produce a quick inch to 1.5" of rain. Isolated showers and storms are possible near/west of the James River into the Hwy 14 corridor overnight as the front stalls out and the next weak wave/WAA push swings through. Lows tonight remain in the upper 60s to lower 70s. TUESDAY: With the cold front washed/stalled out over the area (somewhere near I-90), another day of very conditional severe weather threat exists Tuesday. Similar atmospheric conditions, although temperatures in the upper 80s/lower 90s and highest dew points will generally be confined to areas near Hwy 20. Same severe weather hazards are in play, with uncertainty to due forcing mechanisms, although the cap looks a little weaker during the afternoon and evening Tuesday. Again, widespread heavy rain is not expected but any thunderstorm could produce a quick inch or more of rainfall. Highs Tuesday in the 80s to lower 90s and with lower dew points, heat indices will be a bit more bearable (into the mid 90s across northwestern IA). Lows in the upper 50s to lower 60s, with rain chances ending late evening/early overnight. WEDNESDAY: Dry conditions prevail on Wednesday as surface high pressure moves over the region and mid level ridging develops to the west. Slightly cooler (upper 70s/lower 80s) with northeast to east flow. WEDNESDAY NIGHT ONWARD: Next wave moves in late Wednesday night, with rain chances largely focused west of I-29 overnight, moving east through Thursday. More active/unsettled pattern continues into the weekend with a few mid level waves and at least one surface low swinging across the northern/central Plains. Models do vary on the timing and strength of each wave, but generally have a stronger one Thursday night. Thursday into Friday would be the period of concern for more widespread heavy rainfall potential (totals of an inch or more). Guidance shows increased moisture advection, as well as PWATs near 2" (in the top 10% of climatology). Temperatures should be within 5 to 10 degrees of normal. && .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1036 PM CDT Sun Jun 23 2024 VFR conditions are expected for the TAF period. Light southeasterly winds will persist through the overnight hours. The low level jet (LLJ) is on track to strengthen tonight which will bring some low level wind shear (LLWS) to KSUX. Some showers and thunderstorms look to develop on the nose of the LLJ but latest guidance keeps any chance for rain confined to parts of northwest Iowa, east of KSUX. As such, have kept out any mention of rain in at KSUX but trends will be monitored. Winds will turn southerly during the morning hours while gusts strengthen up to around 20-25 knots. A cold front will sweep through the area during the afternoon and evening hours tomorrow, turning the winds to out of the northwest. The northwest winds will finish out the TAF period. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Heat Advisory from 1 PM this afternoon to 8 PM CDT this evening for SDZ038>040-050-052>071. MN...Heat Advisory from 1 PM this afternoon to 8 PM CDT this evening for MNZ071-072-080-081-089-090-097-098. IA...Heat Advisory from 1 PM this afternoon to 8 PM CDT this evening for IAZ001>003-012>014-020>022-031-032. NE...Heat Advisory from 1 PM this afternoon to 8 PM CDT this evening for NEZ013-014. && $$ DISCUSSION...SG AVIATION...Meyers