Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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289 FXUS63 KFSD 231811 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 111 PM CDT Thu May 23 2024 .KEY MESSAGES... - Strong to severe storms move across the northern Plains tonight. Timing of initiation and stronger storms has been delayed 1-3 hours from previous forecast. Main hazards will be large hail/isolated damaging wind with initial storms west of the James River early evening. This transitions to increasing threat of damaging winds through portions of the Missouri River to Hwy 20 corridor late evening to overnight, with isolated embedded tornadoes also possible. - Breezy conditions with south winds gusting as high as 30-35 mph this afternoon. Gusty winds continue for Friday as winds become northwesterly gusting to 30-40 mph. - Periodic showers and thunderstorms for Memorial Day weekend. While we are not currently expecting strong storms, lightning is still possible. When thunder roars, go indoors! && .UPDATE... Issued at 111 PM CDT Thu May 23 2024 Isolated showers/storms have been developing in portions of the James River Valley this midday, along the eastern edge of the EML characterized by steeper 8-9C/km mid-level lapse rates. Expect this activity to weaken/lift north through mid-afternoon. Attention then turns to surface low currently over northwest SD, with attendant dryline/cold front extending south into the Nebraska panhandle, as well as associated mid-upper level trough swinging through the northern Rockies. Seeing good consensus among various models in farther westward storm initiation as the surface boundaries move into central SD late this afternoon, which delays timing of convection reaching our far western CWA until closer to 7-8pm CDT. This initial activity should be more discrete cells in central SD, but given the delayed timing, storms may be somewhat weaker as they approach our western areas. Still looking at a threat of large hail up to golf ball size, along with isolated damaging winds up to 70 mph with this activity. As storms progress east through the evening and deeper forcing associated with the upper level trough arrives, expect storms to evolve into a linear system, complete with bowing segments that will translate east along and south of the Missouri River corridor and portions of northwest Iowa into early Friday morning. Still some uncertainty regarding how far north the better instability will extend, but seeing modest consensus in the CAMs that support a threat of damaging winds up to 70+ mph moving east through at least our southern tiers of counties, roughly along/south of a line from Yankton to Le Mars to Storm Lake. 0-3km shear vectors are forecast to be WSW around 30kt at the leading edge of the line, which could support circulations and perhaps isolated embedded tornadoes in portions of the QLCS oriented NW to SE, which appears to more likely be the portion of the line near and north of Hwy 20. The main question regarding the tornado threat would be nocturnal timing of the line, as it would be fighting a somewhat more stable layer below about 2kft AGL. Storms look to exit our northwest Iowa counties by around 3-4am, though some trailing showers/scattered storms may linger through shortly after daybreak. && .DISCUSSION... Issued at 308 AM CDT Thu May 23 2024 THURSDAY: Southerly winds will increase throughout the morning as a low pressure system approaches from the west, tightening the SPG. Look for sustained winds to peak around 15-20 mph with gusts 25-30 mph by mid-afternoon, highest through the James River Valley. Plentiful WAA will warm our highs into the upper 70s and 80s for today. Our southerly winds will also transport modest moisture northward, helping dewpoints to climb into the mid-upper 50s west of the James River Valley and low 50s east. These will support some moderate instability of around 1500-2000 J/kg MUCAPE over central South Dakota. Model soundings continue to indicate a capping inversion today, that will erode quickly once we reach peak afternoon heating. At the same time the cold front will be approaching from the west, forcing convection initiation. Strong vorticity from the top of the boundary layer through the mid-levels, coupled with 35-45 kts deep layer shear, is enough to support a few strong updrafts. This set of model runs also indicates similar steep mid-level lapse rates mentioned in the previous discussion which will support large hail up to 1.5-2 inches in diameter. Dry mid- levels at convection start and high DCAPE values around 1000 J/kg support strong down drafts, and so damaging wind gusts up to 70 mph are also possible. As the storms progress eastward, 700 mb steering winds are nearly parallel with the forcing, indicating quick upscale growth into a line. From here, the better instability is along and south of the Missouri River along the South Dakota and Nebraska border. This should focus the stronger storms along our southern counties for the evening/overnight hours. As the storms move into northwestern Iowa around 05-06Z, they look to become more elevated and hail will become the more dominant threat. However, at the same time the low level jet kicks into gear, which increases the chance that a few strong to severe gusts could make it to the ground. Storms should move east of the region by early Friday morning. FRIDAY AND BEYOND: Things dry out for Friday with breezy northwesterly winds gusting up to 30-35 mph. Strong CAA limits highs to the 60s. Saturday warms a little more into the upper 60s and low 70s. Meanwhile, another upper trough is digging south along the Pacific Northwest. Saturday night into Sunday there is another chance for showers and thunderstorms. Currently, we do not expect these storms to be strong, as the better dynamics are over southern Nebraska and Kansas. Light scattered showers continue for most of Memorial Day and highs remain on the cool side of normal, in the low 70s. An upper ridge begins to build in for Tuesday, finally allowing us to dry out and warm up a bit through the end of the week. Look for highs to warm from the mid 70s Tuesday into the upper 70s and mid 80s by Thursday. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 111 PM CDT Thu May 23 2024 Southerly winds gusting 25-30kt this afternoon, but main focus will be increasing thunderstorm chances moving east across the forecast area 24/00Z-12Z tonight. Initial storms west of the James River this evening (00Z-03Z) may produce large hail or isolated gusts in excess of 40-50kt. Greater threat for damaging winds, perhaps in excess of 60kt, will come as storms evolve into a line which is projected to move through the lower Missouri River Valley and portions of northwest Iowa, including KYKN-KSUX-KSLB overnight (04Z-09Z). In addition to the potential for strong winds, MVFR-IFR ceilings and visibility may accompany the thunderstorms. Storms are driven by a cold front which will be followed by increasing west-northwest winds on Friday, with gusts as high as 30-35kt. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ UPDATE...JH DISCUSSION...AJP AVIATION...JH