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FXUS63 KFSD 270333 AFDFSD

Area Forecast Discussion National Weather Service Sioux Falls SD 1033 PM CDT Fri Apr 26 2024

KEY MESSAGES

- Showers and storms continue to expand in coverage early this afternoon. Strong to severe storms possible into this evening (greatest risk remains through (9PM). Rain chances decrease late tonight.

- Through Sunday night, an additional widespread beneficial rainfall of an inch or more is expected (>70% chance), with higher chances across northwestern IA. Low chances (40% or less) for an additional 2" in northwestern IA.

- Chances for showers and storms return late Saturday night through Sunday, with rainfall accumulations forecast to range between half an inch to an inch, though locally higher amounts may still occur. A few storms may become strong to severe.

- 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.

UPDATE
Issued at 634 PM CDT Fri Apr 26 2024

With the loss of daytime heating and as they move north into more stable air, have begun to see a weakening trend over the last 30 minutes or so. Given the trends, expect the severe to continue to diminish quickly. Still cannot rule out an isolated elevated hail threat in northwestern IA through 8 PM.

DISCUSSION
Issued at 304 PM CDT Fri Apr 26 2024

REST OF TODAY AND TONIGHT: Convection has begun this afternoon, with elevated hailers earlier this afternoon in northwestern IA. Elevated instability is evident on satellite by the convective CU developing over the stratus, even into south central SD. Further to the southeast, convection is developing across eastern NE, along the dryline and clearing. Instability, including surface based CAPE, is beginning to increase in this area. Dewpoints are increasing across eastern NE as well. Closer to our area, clearing in the stratus has been noted in the more stable areas of the James River Valley, although we are starting to see some thinning via satellite of the lower clouds in western IA.
Continuing to monitor the clearing of the stratus in northwestern IA this afternoon, as clearing soon rather than later will help us destabilize, leading to an increasing conditional severe weather threat.

Showers and storms will continue to develop across NE and move to the northeast, entering our southern area by 21z/4PM based on the latest timing of development and hi-res guidance. Large hail, generally to golf ball size, remains the main threat with steep mid level lapse rates over 7 deg C/km. Shear values within the 0-6 km range remain high as well, 25 knots or more, and increasing as you move to the east (with values from most of the deterministic guidance and mesoanalysis over 40 knots in northwestern IA). Expect supercells, or at minimum some rotation in the storms, which will aid in the keeping the hail threat in play into the evening hours across the area. Uncertain how far north storms will maintain their strength, but can't rule out a stronger storm or two approaching/crossing the I-90 corridor.

Beginning to see that surface destabilization mentioned in the previous paragraph via the SPC Mesoanalysis page, with values increasing 400+ J/kg over the last couple of hours across northeastern and eastern NE. LCL heights in this area are low (less than 1500 ft), and SRH values are over 200 m^2/s^2. If we can continue to see this trend over the next hour or so, concerned with conditional tornado threat increasing across the southern MO Valley into the Hwy 20 corridor in northwestern IA.

Strong wind gusts to 60 mph are possible with storms mixing down the stronger winds in the low level jet. However, this is the lesser threat, and looks more likely with linear mode of storms.

In summary, large hail to golf ball size is the main threat with any strong to severe storm this afternoon and into the evening. If a storm can stay discrete, 2" hail may be possible. If we can continue to clear out stratus before 4PM or so, a few tornadoes may be possible, mainly across the Hwy 20 corridor into northwestern IA.
Damaging wind gusts to 60 mph may occur as well - particularly if storms are able to become more linear. Locally heavy rainfall is expected, but flash flooding concerns remain low. Localized rises and ponding are possible.

Severe threat diminishes after 8-9PM this evening, with some lingering showers and isolated storms into the overnight hours.
Rainfall amounts this afternoon through tonight generally an inch to 1.5", although could see higher amounts within stronger thunderstorms. Lows in the upper 40s/mid 50s. Breezy winds gust around 20 to 25 mph tonight, and begin shifting from southeast to northwest.

SATURDAY: Some lingering showers are possible early in the day and especially along the Hwy 14 corridor as the low pressure pivots off to the northeast, but otherwise a mostly dry day is expected. Cloud cover remains in place, with highs in the 50s and 60s. Warmest conditions will be along and southeast of IA/MN Hwy 60, where flow looks to take the longest to switch to the northwest. Winds shift to the northwest then northeast as the low moves away. We'll be a bit breezy, with gusts 20 to 25 mph at times.

SATURDAY NIGHT THROUGH SUNDAY: Rain chances return to the forecast Saturday evening as a pronounced upper level low near the Rockies edges closer to the region. Despite the model variability that still exist, most guidance generally suggest that showers/storms will begin along the MO River Valley prior to midnight, then continue to lift northeastward throughout the day Sunday as the aforementioned low moves over western SD/NE. Do think it's worth noting however, that the sfc low low will be a bit more displaced from the upper low, so not expecting the system to become as stacked at Friday's - at least for the time being.

As alluded to in the previous discussion, the severe threat will largely depend on track of this complex in addition to where any sfc boundaries set up. Timing could also be a limiting factor, as this system is progged to reach our area during the late morning/early afternoon, limiting the opportunity to tap into our diurnal heating. Nonetheless, sounding profiles shows ample shear (40+ kts), instability (~1000J/kg), and steep mid level lapse rates (7-8 degC/km) ahead of the wave. Thus, can't completely rule out the possibility of seeing a few severe storms, which would be capable of producing large hail, damaging winds, and isolated tornadoes. SPC's latest Day 3 Outlook does include a small portion of our area in a Marginal Risk (Level 1 of 5) for severe weather, mainly across NW Iowa - so will continue to monitor trends. In any event, we strongly encourage you to have multiple ways to receive warnings. Lastly, in regard to rainfall totals, expect widespread amounts between half an inch to an inch across our area, with isolated pockets near 1.5 inches also possible.

MONDAY: Upper level flow turns quasi-zonal Monday as the low lifts northeastward towards the International border. As a result, should see dry conditions prevail across the region, with highs ranging from the mid 50s across SW Minnesota to mid 60s along the MO River Valley. Winds during this time look to remain primarily out of the west, with gusts between 20 to 25 MPH possible in areas east of I-29 through the afternoon.

TUESDAY THROUGH THURSDAY: Scattered rain chances return for the remainder of the period as multiple shortwaves push eastward across the Northern Plains. Still a bit of model variability exist concerning the timing of these waves, though most generally agree that showers will be possible throughout the daylight hours Tuesday, with a relative lull Wednesday, followed by another round of showers Thursday. Again, too much uncertainty still exist to put confidence behind any one solution just yet. Otherwise, look for highs to rise into the 60s to lower 70s, with lows generally in the 40s.

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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