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FXUS63 KFSD 270905 AFDFSD

Area Forecast Discussion National Weather Service Sioux Falls SD 405 AM CDT Sat Apr 27 2024

KEY MESSAGES

- Mostly dry through the day today, with perhaps spotty sprinkles or drizzle. Showers and a few storms return tonight and Sunday, ending by early Monday. A few strong to severe storms possible in portions of northwest Iowa, though confidence in timing/location is low.

- Additional rainfall of 0.50-1.00" for many areas through Sunday night, with locally higher amounts possible. Ponding of water in low-lying or poor drainage areas is possible, and will have to monitor isolated areas which received heavier rain on Friday.

- Unsettled pattern continues through next week, with periodic rain chances, though precipitation chances and temperatures are both low confidence forecast due to widely varying model solutions.

DISCUSSION
Issued at 400 AM CDT Sat Apr 27 2024

TODAY: Widespread low stratus still blankets much of the forecast area this morning, and with weak cold advection in the wake of our departing storm system, do not expect to see much in the way of clearing today. Despite the dreary skies, breezy northwest winds, and below normal temperatures for most areas, today should be the nicer day of the weekend. Low-level thermal boundary lingers east of IA/MN Highway 60 this morning, and is expected to slowly drift east through this afternoon. Locations southeast of the boundary will have the best chance to see periods of sun that could help push temps into the lower to perhaps mid 60s, while the stagnant clouds west of the boundary hold readings in the 50s.

TONIGHT-SUNDAY NIGHT: Our next mid-upper level wave begins to slide northeast toward the CWA tonight, becoming negatively-tilted and deepening as it moves across the Dakotas and Minnesota through Sunday night. Expect rain to expand in coverage through tonight as a lead mid-level wave lifts north, and the 850mb boundary rotates back to the west/northwest as an inverted trough, allowing for broad warm advection ahead of the upper trough. Surface to 925mb boundary looks to remain near the extreme southeast corner of our CWA, if not just outside. As such, instability remains extremely limited across our forecast area through daybreak Sunday, perhaps 100-200 J/kg of MUCAPE in our far southeast late tonight with mid-level lapse rates near- below 7C/km. Thus think the threat of strong-severe storms tonight is low, but could see pockets of heavier rain.

Still some uncertainty for Sunday with respect to severe potential, especially regarding the track of surface low and timing/location of the attendant warm front by mid afternoon. Greater consensus appears to have the low lifting northeast near to just east of the Highway 60 corridor during the late afternoon to early evening. Latest RAP projections show potential for a ribbon of deep layer shear >40kt in northwest Iowa Sunday afternoon, as CAPE values near/south of the aforementioned warm front climb to around 1000J/kg. This, of course, will be dependent on ability for these areas to recover from morning rains, with stratus clearing enough to allow for sufficient warming.
However, cannot disagree with the potential for isolated strong to severe storms in parts of northwest Iowa Sunday afternoon as seen in the SPC Day 2 Marginal (Level 1 of 5) Risk if this destabilization is realized.

The trough and associated surface low continue to lift northeast of the forecast area, which should gradually bring an end to the rain by late Sunday night. While additional rainfall amounts with this second system will again be more variable due to convective influences, looking at the potential for a broad 0.50-1.00" of rain across the area. While we did see pockets of rainfall in excess of 2.00" over the past 24-36 hours, broader rain totals averaged 0.50-1.50". The additional rain, which is not expected to fall at excessive rates given the limited instability in most areas.

MONDAY-FRIDAY: The region remains in a quasi-zonal to broad troughing pattern through much of the week ahead. Temperatures slowly recover to near-above normal by Tuesday-Wednesday, but a stronger trough seen in some models dropping into the northern Plains by Thursday could bring cooling back for the end of the week. Latest runs of the deterministic GFS/ECMWF/Canadian show modest agreement in some sub-zero 850mb temperatures plunging south across the Dakotas/Minnesota by Friday. However, their respective ensembles show a much broader spread, with the NBM 10th-90th spread still exceeding 20F across the area. Latest NBM high temp for Friday in the mid 50s-lower 60s is on the cooler side the spectrum, near to slightly below its 25th percentile, but this does not look unreasonable given the cooling seen in the deterministic model solutions. Will not alter highs given the high degree of uncertainty, but would expect to see further flucuations in late week temperatures over the coming days.

As far as precipitation chances, there are multiple shortwaves sliding through the fast flow aloft, though timing/strength varies among the solutions. First wave looks to be focused on the daytime hours Tuesday, but not enough confidence at this range to make adjustments to the generally low NBM pops beyond that period.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/
Issued at 1020 PM CDT Fri Apr 26 2024

A mix of VFR to LIFR tonight as showers and thunderstorms continue to move off to the northeast. These storms are not expected to be severe, but visibility could be reduced to 4 miles under heavier rainfall. Clouds will be persistent through the period, reducing ceilings to around 500 ft AGL, gradually increasing through Saturday afternoon to 1500 to 2500 ft AGL, with the highest ceilings south of I-90. Toward the end of the period ceilings deteriorate again as a second low pressure system begins to move into the area.

The current low pressure is situated roughly over eastern South Dakota. Winds are highly dependent on your location in relation to the low. KHON has winds out of the northwest, KFSD out of the west, and KSUX out of the southwest. Winds will be veering through the period, eventually settling to the northeast by the end of the period. Winds will be slightly breezy, 10-15 kts sustained with gusts around 20 kts decreasing through Saturday afternoon. Winds increase again and shower and thunderstorm chances return late Saturday night into Sunday.

FSD WATCHES/WARNINGS/ADVISORIES
SD...None.
MN...None.
IA...None.
NE...None.

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