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FXUS63 KFSD 260944 AFDFSD

Area Forecast Discussion National Weather Service Sioux Falls SD 444 AM CDT Fri Apr 26 2024

KEY MESSAGES

- Widespread rain/embedded thunderstorms early this morning diminish in coverage after daybreak. Strong to severe storms develop this afternoon into this evening (greatest risk 3pm-9pm).

- Rain chances decrease late tonight, with a mostly dry Saturday. However, showers/storms return Saturday night and Sunday. A few strong to severe storms possible in portions of northwest Iowa.

- Through Sunday night, widespread beneficial rainfall of an inch or more is expected (>80% chance). Moderate chances (40-60%) for more than 2 inches, focused across northwest Iowa and adjacent areas.

- 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 444 AM CDT Fri Apr 26 2024

THIS MORNING: As anticipated, we're seeing widespread rain in the broad warm advection regime ahead of an approaching mid-upper level trough early this morning. Instability is on the weaker side except perhaps across our far south, so thunder is largely limited to the southern portions of the forecast area. Expect this area of rain to diminish from the south/southwest after daybreak as the lead wave lifting north ahead of the trough slides north of the forecast area, and mid-level drying spreads across the CWA Low level moisture remains firmly in place, though, and with weak lift continuing, could see spotty light rain/drizzle persist even after the more widespread moderate rain lifts north.

THIS AFTERNOON/TONIGHT: Our attention then turns to the main low pressure system which will slowly lift northeast through central Nebraska this afternoon, then track across our forecast area tonight. The track appears to have shifted slightly westward from projections at this time yesterday, but timing is similar. Still some questions on ability to see sufficient clearing work into our far southern CWA by mid-late afternoon to allow for stronger destabilization. However, the southerly mid-level jet ahead of the negatively-tilted trough results in very strong deep layer shear (better than 45-50kt in several models), which could overcome the weaker instability (high shear/low CAPE environment) and still support at least elevated storms (possibly supercells) with a threat of hail up to golf ball size across the southeast quadrant of our forecast area (generally east of Hwy 81 and near/south of I-90)
where mid-level lapse rates top 7.5-8C/km. This corresponds well with the SPC Slight (Level 2 of 5) severe weather risk for today.

Back to the uncertainty, still some high-res models which show the low clouds breaking up/clearing through portions of the Missouri River Valley by mid-late afternoon as the warm front lifts north, while others stand firm on the warm front remaining farther south and the low clouds persisting into this evening. Those that show the potential clearing would warm enough to break the surface-based capping inversion, with 0-1km shear of 30-40kt and LCL heights below 2kft supporting a risk of tornadic supercells. Again, this would largely be focused near the surface boundary/clearing line, with storms likely becoming elevated with a reduced tornadic threat as the move north of the boundary.

Potential severe window looks to be relatively short, perhaps beginning as early as 3-4pm in the Missouri River Valley, and exiting our eastern CWA by 8-9pm this evening as the wave slides east. Thereafter, expect coverage of showers/storms to gradually wane through the overnight hours. Given the expected quick-moving storms, think any threat of overland flash flooding is low. However, storms may contain rainfall rates in excess of an inch per hour so localized ponding in low-lying/poor drainage areas or urban areas prone to flooding may see brief periods of standing water.

SATURDAY: This could end up being a relatively nice day, certainly the nicer day of the weekend, as brief mid-upper level ridging keeps much of the CWA dry through the day. Low-level thermal boundary will likely be lingering southwest-northeast somewhere in the CWA though, with consensus pointing to near to southeast of the IA/MN Highway 60 corridor. This will separate widespread cloudiness & cooler temps to the northwest from at least partial sunshine and near normal temps southeast.

SATURDAY NIGHT/SUNDAY: A second potent mid-upper level trough swings northeast across the region during this period, though a little less stacked than the system today. Greatest uncertainty in this period is where the low level boundary will be located, along with the exact track of the mid-upper level trough which still has some variance among the models. Higher confidence that much of the forecast area will again see rain at times later Saturday night-Sunday, with much lower confidence in potential for any strong-severe storms which may occur near/east of the aforementioned surface boundary. Timing also differs from today in that the system reaches our area earlier in the day, so not working with max diurnal heating. Nevertheless, still see a period of strong shear ahead of the wave from late Saturday night into Sunday afternoon, so cannot rule out isolated strong to severe storms across our eastern counties.

Combining both of these systems together, we have high (>80%)
probabilities of seeing widespread rainfall amounts exceeding an inch by Monday morning. The greater potential for 2 inches or more remains focused across northwest Iowa and nearby areas, where periods of deeper convection are more likely with each of the systems.

MONDAY-THURSDAY: Mid-upper level flow pattern turns a bit more zonal after the Sunday wave exits, but indications of one or more stronger waves sliding across the northern Plains through early-mid week which could bring at least scattered rain chances at times. Temperatures return to near or slightly above normal for the transition to May with highs back into the 60s to at least lower 70s. May have to monitor temps toward the end of this week, though, as latest GFS/ECMWF bring a shot of colder air southward behind one of the stronger waves in the latter half of the week. Wide variance in solutions on this, though, with better than a 20F spread between the NBM 10th and 90th percentiles for high temperatures on Thursday.

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/
Issued at 1108 PM CDT Thu Apr 25 2024

VFR ceilings have begun to deteriorate as stratus moves in, and will continue to deteriorate to MVFR with scattered IFR for the remainder of the period. Scattered showers and thunderstorms have begun across the region. This first round is expected to remain sub-severe, however dime to nickle size hail and stronger wind gusts up to 50 mph under thunderstorms are possible.

Winds are out of the southeast at 15-20 kts sustained with gusts 30- 35 kts. The highest gusts will shift from west to east during the overnight period and will be predominate over northwestern Iowa by Friday morning. Winds will gradually decrease through the afternoon to less than 20 kts. There exists some concern for LLWS at KSUX near the end of the period. Directional and speed shear may be present between 1500 and 2000 feet AGL at 160 degrees and 45 kts.

A second, stronger round of showers and thunderstorms will develop over northern Nebraska in the next few hours. Some of these storms could be strong to severe with hail up to an inch and damaging wind gusts. Several additional waves of showers and thunderstorms are expected through Sunday as a low pressure system moves through the area. Some of these storms could again be strong to severe.

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

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