Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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298 FXUS63 KFSD 181121 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 621 AM CDT Tue Jun 18 2024 .KEY MESSAGES... - A passing front will bring new risks for strong to severe convection to areas east of I-29 by mid-late afternoon. Primary risks will be 1.5" hail and brief 60 mph wind gusts. - Near advisory level southwest winds may impact areas east of I-29 this morning. - Additional rainfall expected through the Little Sioux River basis later this afternoon and evening may push current flooding higher. - A brief dry day on Wednesday will give way to yet more rain into Thursday. Non-severe thunderstorms will again bring widespread rain. Probabilities of >0.50" totals are at 80% with 1" probabilities around 40%. Several high resolution models do suggest some potential for greater than 2" totals. - Thunderstorms risks continue into Friday, with potential for heavy rainfall Thursday night, and then lower 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 218 AM CDT Tue Jun 18 2024 THIS MORNING: Widespread convection continues over western and north central South Dakota this morning. With the responsible wave moving northeast into North Dakota, and mean steering flow also pushing northeast, this strong activity should avoid the CWA outside of a bit of an outflow/gravity wave sent eastward by daybreak. Further southeast, gusty southeast winds up to 40 mph continue through the higher elevation areas. The increased mixing is holding temperatures in the 70s in most locations, providing a warm start to Tuesday. TODAY: Rising surface pressure over the Western High Plains will begin to push a cold front southeast into the Tri-State area today. This front will begin to approach the I-29 corridor early this afternoon, with some indications of high based elevated showers possible ahead of the front. Soundings also show a fairly stout warm later around 750:700 mb ahead of this boundary. We may need a few of these elevated showers to help cool the warm nose enough to allow for stronger surface based convection to form east of I-29 by 3-4PM. Most of the surface based convection will reside within a corridor of 1800-2500 J/KG MLCAPE, but also on the edge of stronger deep layer shear AOA 25-30 knots. So while the predominate nature of activity will be multi-cellular, an isolated embedded supercell could be possible. Primary hazards will be hail up to 1.5", and given low-lvl flow running mostly parallel to the convection, some modest bowing segments could be possible. A narrow corridor of heavy rain is also possible once deeper convection develops over northwest Iowa. HREF PMM data suggests totals between 0.5-2", focused mostly through the Little Sioux River basin which may force currently flooding higher. TONIGHT: High pressure moving southeast through the Dakotas may force the front southeast overnight, but some continued overrunning of the low-lvl baroclinic zone could lead to additional shower activity along the Missouri River and into northwest and western Iowa overnight. Further north, temperatures will fall into the upper 50s to lower 60s with lighter winds. WEDNESDAY-THURSDAY: Mid-lvl ridging over the eastern seaboard will retrograde back westward on Wednesday and Thursday, leading to a northward retreat of both the surface and elevated front late Wednesday into Thursday. Most of Wednesday is likely to be cooler and dry, but shower and isolated thunderstorm risks spread northward quickly Wednesday night into Thursday. Broad warm advection in combination with a weak mid-lvl perturbation moving northeast through the Dakotas into Thursday will bring high rain chances. GEFS/ECE both bring up to an 80% probability of 0.50" of rain with nearly a 40% probability of greater than 1" of rain by Thursday afternoon. The surface warm front lifts northward into Friday morning, but with an increasing LLJ approaching 40-50 knots through the Plains, the concern of deepening moisture return and renewed convection north of the warm front could pose another heavy rain risk into Friday. FRIDAY: Most medium range guidance pushes the warm front north of the area, planting the CWA into the warm sector. Both shear and instability look marginal into Friday night, but slow storm motion in any storm that develops may create locally heavy rainfall into Saturday morning. SATURDAY-MONDAY: A deeper upper trough will swing eastward along the Canadian border and into the Great lakes over the weekend. This will bring a period of cooler more pleasant conditions to the region this weekend. This cooler period will end quickly as heights rise, warm advection returns, and both temperatures and rain risks increase late Monday into Tuesday. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 616 AM CDT Tue Jun 18 2024 Scattered MVFR ceilings will continue to develop this morning, perhaps becoming broken at times. Strong southwest winds ahead of an approaching cold front will gust up to 40 mph at times, especially east of I-29. The aforementioned front will reach I-29 towards early afternoon with some scattered showers developing ahead of the boundary. Eventually enough cooling in the low-lvls may lead to these storms becoming surface based across northwest Iowa and southern Minnesota. Periods of MVFR ceilings and visibility may be possible at times. Behind the front, winds will turn northwesterly through the overnight hours, as VFR conditions return. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...Dux AVIATION...Dux