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Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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892 FXUS63 KFSD 182005 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 305 PM CDT Tue Jun 18 2024 .KEY MESSAGES... - A passing cold front will bring a risk for strong to severe convection across the eastern half of the forecast area into the early evening. Primary risks will be 1.5" hail and brief 60 mph wind gusts, though isolated tornadoes and flash flooding are possible. - Additional rainfall expected through the Little Sioux River basis into this evening may push current flooding higher. - A mostly dry day Wednesday will give way to yet more rain into Thursday-Thursday night. Non-severe thunderstorms will again bring widespread rain with higher amounts hopefully remaining near/north of the I-90 corridor (Little Sioux River basin). Probabilities of additional >0.50" totals are at 80% north of I-90, with 1" probabilities above 50%. Several high resolution models do suggest some potential for greater than 2" totals. - Thunderstorms risks continue into Friday, with a low potential for strong to severe storms Friday afternoon and evening. - Cooler temperatures and likely dry conditions this weekend into early next week, though rain chances return by Tuesday. && .DISCUSSION... Issued at 305 PM CDT Tue Jun 18 2024 As of 19Z, well-advertised and well-defined cold front is located near the MN-SD state line, then arcs slightly to the southwest into northeast Nebraska. As anticipated, have begun to see a line of showers and scattered thunderstorms develop along the front over the past hour or two, and these storms should intensify over the next hour or two as the front pushes east into a warm and very moist air mass characterized by surface dew points in the lower-mid 70s, fairly impressive for this early in the season. This is contributing to SBCAPE values near to above 2000J/kg ahead of the front, with pockets of modest deep layer shear of 25-35kt (stronger shear is displaced west of the front). With low level shear largely oriented parallel to the boundary, expect pulsy multicell activity to dominate most of our area, though could see more of an isolated tornado threat north of I-90 where the the line arcs slightly more to the northwest and becomes closer to normal with the low level shear vectors. Precipitable water values remain near/above 1.75+ inches ahead of the front into this evening, or above the 95th percentile of climatology per the NAEFS and EC ensembles. This should allow for efficient rainfall producing storms bringing a potential for heavy downpours and isolated flash flooding as they move across areas with already saturated soil conditions and high river levels. The front and associated stronger storms are on track to push east of our forecast area by 00Z-01Z this evening, though could see some showers/isolated thunderstorms along the trailing elevated boundary linger a bit longer into the evening. With focus on the current severe threat, did not spend too much time evaluating the outer periods of the forecast. Surface high pressure slides east across the Dakotas/Minnesota on Wednesday which should keep the area mostly dry, though could see some light elevated post-frontal showers or isolated storms as a wave slides northeast along the mid-level boundary. Return flow and the 850mb warm front will lift back to the north later Wednesday night, bringing renewed chances for rain and storms. Most significant change noted from previous forecast is a shift to the north with the potential for heavier rains from late Wednesday night into Thursday night. The higher probabilities for amounts > 1.00" are now focused mostly north of I-90, which if it pans out, would bring some relief to the water-logged Little Sioux River basin and other areas across northwest Iowa. Although finer details may vary, we are still looking at a modest shortwave sliding northeast across the northern Plains later Friday-Friday night. Could see temperatures push well into the 80s or even lower 90s ahead of this system, and with dew points again approaching 70F, will be looking at heat indices at least into the lower to perhaps mid 90s. Temperatures should ease a little lower through the weekend with mostly dry weather expected. && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1255 PM CDT Tue Jun 18 2024 Surface cold front nearing I-29 as of 1730Z will push east across southwest MN and northwest IA through early this evening. As the front moves into a more unstable airmass, scattered to numerous showers and thunderstorms are expected to develop. Better chance for thunder looks to be east of KFSD but will likely impact KSUX at times through 18/23Z-19/00Z. Variable gusty winds, IFR or lower visibility in pockets of heavy rain, and possibly hail may accompany the strongest storms. Threat for these stronger storms moves east of our forecast area by 19/01Z with showers and perhaps isolated thunderstorms lingering near and south of US Highway 18 into Wednesday morning. Away from any thunderstorms, MVFR ceilings have become more widespread west of the cold front, but improving conditions are expected through this evening as deeper dry air builds in behind the front. Could see additional MVFR stratus sag southward into areas mainly north of I-90 late in the period, but confidence is low in areal coverage/timing, so kept KHON at SCT below 3kft AGL for now. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...JH AVIATION...JH