Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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431 FXUS63 KFSD 231933 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 233 PM CDT Thu May 23 2024 .KEY MESSAGES... - Strong to severe storms move across the region tonight. Main hazards will be large hail/isolated damaging wind with initial storms west of the James River Valley 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! && .DISCUSSION... Issued at 232 PM CDT Thu May 23 2024 TONIGHT: Focus for tonight will be on potential for strong to severe thunderstorms moving east across the region. 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 7-9 pm 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 60-70 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. FRIDAY-SATURDAY: Upper low is a little slower to eject northeast on Friday, but with strong push of cold advection in the morning, could see a few gusts to around 40 mph near/west of the James River Valley through midday. As the day progresses, approaching surface ridge from the southwest should allow winds to gradually decrease, before becoming light/variable Friday night. As for precip chances, could see some lingering light showers or an isolated storm across parts of southwest MN/northwest IA through mid-morning before the upper wave exits, but otherwise looking at a brief stretch of dry but cooler weather as we head into the weekend. Coldest temps aloft swing through during the day Friday, which should keep highs in the 60s at best, perhaps limited to the mid-upper 50s in some areas. Slight warming builds in for Saturday, though temperatures look to remain on the cool side of normal with highs in the upper 60s-lower 70s for the start of the Memorial Day weekend. SATURDAY NIGHT-MEMORIAL DAY: Cooler than normal temperatures persist across the area as we head through the latter half of the holiday weekend. Another deepening trough slides across the region Saturday night-Sunday with another wave diving southeast later Sunday night into Monday. These systems could produce scattered showers and perhaps a few storms, but instability and shear are both on the weaker side, so not looking at a severe risk at this point. TUESDAY-THURSDAY: Mid-upper level ridging builds in behind the departing weekend system, bringing dry weather and warming temperatures through the middle of the week. The ridge slides east/flattens with an approaching trough by Thursday, which could bring scattered precip back to the region by the end of this forecast period. && .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. && $$ DISCUSSION...JH AVIATION...JH