Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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995 FXUS63 KFSD 172003 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 303 PM CDT Mon Jun 17 2024 .KEY MESSAGES... - Continued uncertainty for severe weather risks today, given cloud cover, lingering showers/storms, and the surface warm front displaced well south of where previous models had projected. May see renewed potential for elevated storms late this afternoon and evening, mainly north of I-90. - A progressive front moves through on Tuesday, bringing yet another risk for strong storms and locally heavy rainfall. Brief bursts of 60 mph winds and marginally severe hail possible. - An unsettled week is ahead, with numerous thunderstorm risks into next weekend. The risk for locally heavy rainfall will be possible each day, with slightly more muted severe weather risks with lower shear. && .DISCUSSION... Issued at 303 PM CDT Mon Jun 17 2024 Surface effective warm front was pushed well south of earlier projections by our morning convection, and has returned to near a Norfolk NE-Storm Lake IA line as of 1930Z. Additional storms have developed along/north of the suspected 850mb warm front, now near and north of US Highway 18 in northwest Iowa with the storms extending north into southwest Minnesota. This activity continues to progress to the northeast ahead of a triggering mid-level wave. Southeast portions of the CWA have seen the greatest clearing this early afternoon. We are also seeing a decrease in coverage of the lower stratus across southeast SD per satellite and surface obs, though still plenty of mid-level cloud cover around to reinforce the cooler air currently north of the warm front. TONIGHT: Confidence in high-res models is on the low side given poor handling of ongoing convective line now in IA/MN. However, they seem to show fair agreement in developing scattered to numerous showers/storms ahead of the elevated warm front as it lifts north of I-90 during the later afternoon and evening. Forecast soundings toward the Highway 14 corridor show steep mid-level lapse rates that could support isolated larger hail, but effective shear for the elevated parcels is much lower (near 30kt) than the full 0-6km bulk shear of over 40-45kt, and thus an unsure whether elevated storms will become organized enough for more than a pulsy hail threat through this evening. By ~04Z, the elevated warm front is focused well north of our forecast area, and should take much of this convective threat north along with it. Will have to watch for potential afternoon development to our southwest in western Nebraska, as some CAMs indicate these storms evolving into a linear cluster lifting northeast across Nebraska through the evening. This activity would be closer to the surface warm front, which could present a slightly greater threat of damaging wind gusts. However lack of agreement in this scenario among the models (some have a notable lack of activity in this area) leads to lower confidence in seeing additional rain moving into the Missouri River Valley overnight. TUESDAY-TUESDAY NIGHT: After an expected quiet late night into Tuesday morning, will see a renewed threat of strong to severe storms near and more likely east of I-29 as a cold front and associated upper trough slide east across the region. Strong influx of low level moisture ahead of the front will lead to MUCAPE values 1500-2000 J/kg, though soundings remain strongly capped to surface-based convection until the front arrives, so may again be looking at potential for elevated storms for which shear is on the weaker side. Similar to tonight, expect large hail and pockets of heavy rain to be the greatest threats. The cold front is expected to exit our eastern counties by mid Tuesday evening, but the southern end of the front may hang up to the southwest, keeping a lingering chance for additional showers/storms across at least the southeast half of the area into later Tuesday night. WEDNESDAY-SATURDAY: Unsettled pattern continues through much of the extended forecast as blocking high pressure remains stagnate across the eastern US - keeping us in a southwesterly flow regime. Outside of a cooler day on Wednesday, temperatures largely near normal (+/- 5 degrees or so). Periodic breezy conditions prevail. Showers and storms continue Wednesday, mainly focused during the day across our southeastern counties with the focus of a weak mid level wave, lower level WAA, an elevated boundary, and the right entrance region of the upper jet all line up. Showers and storms continue into Wednesday night, likely becoming more widespread as the next weak mid level short wave tracks out of western NE. This wave continues to move northeast through Thursday, keeping more widespread storm chances in place. Severe weather looks unlikely from Wednesday into at least the first half of Thursday as instability is lacking. PWATs during the day on Wednesday look to be around 1.0-1.5", highest across northwestern IA. Higher values (up to 2") lift northward into the day Thursday as moisture advection increases and lifts north. Next slightly stronger wave moves through during the latter half of the day Thursday into Thursday night, with instability increasing to around 1500 J/kg. However, best bulk shear is offset from the instability to the north and west, so severe threat appears low at this time. Again, heavy rainfall remains a concern with PWATs still around 1.5-2" and becoming more widespread over the area. Mid level waves train overhead in quick succession Friday into Saturday, with a shift to more westerly flow into the weekend thanks to a more defined mid/upper trough diving southeast from southern AB/SK and northern MT. Rain and storm chances continue, although timing and location are a bit more uncertain due to timing issues with the short waves during this period. Surface low in tandem with the more defined trough will lift off to the northeast through the latter half of the weekend. SUNDAY-MONDAY: May see a reprieve from rain Sunday into Monday as some ridging begins to develop in the western portions of the northern/central Plains; however, confidence is low given variance in model data. Additionally, nearly all deterministic guidance brings another short wave in on the heels of this ridge, so not sure how long of a break from the unsettled pattern we might have. Continued near normal (+/- 5 to 10 degrees from average) temperatures. && .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 144 PM CDT Mon Jun 17 2024 Low confidence forecast with regard to persistence of MVFR stratus early in the period, as well as timing/location of thunderstorm chances through the next 12-18 hours. Most likely scenario lifts current line of thunderstorms with relatively brief but very heavy rain northeast through southwest MN and the Iowa Great Lakes. Additional scattered storms will be possible through the later afternoon and evening, mainly north of the I-90 corridor. Broader clearing is expected through the overnight hours, but could see MVFR stratus again develop after sunrise Tuesday with a strong push of low level moisture from the south ahead of an approaching cold front. The front will trigger additional storms as it moves near and east of I-29 after this TAF period. A strong southerly low level jet is expected to develop this evening and persist through just after sunrise Tuesday. This will result in low level wind shear, with speeds as high as 45-50+ kt within the low level jet. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...JH/SG AVIATION...JH