textproduct: Sioux Falls

KEY MESSAGES

- Showers and thunderstorms develop after 3am Monday morning. A low chance for a marginally severe storm with hail could be possible through 9am as activity moves northeast.

- A very conditional risk for severe storms forms late Monday afternoon along Highway 20. All hazards could be possible, though cloud cover may play a significant role in potential.

- We'll remain near to slightly below normal for temperatures during the upcoming week.

- Scattered non-severe rain chances will focus in three timeframes, with the highest probabilities of rainfall focused Wednesday and again late Thursday night and Friday.

DISCUSSION

Issued at 1240 PM CDT Sun Sep 13 2026

THIS AFTERNOON: Another cool, but pleasant early Fall day in the region as continued mid-lvl clouds cross the CWA. With a light east southeast wind, we'll continue to see temperatures rise towards the upper 60s.

TONIGHT: Two thirds of the overnight hours are expected to remain quiet. However as we move past midnight, a steady increase in a broad LLJ through the Plains, along with the approach of mid-lvl vorticity will begin to develop showers and thunderstorms. Two areas of focus for convection will develop through the morning. One area will be across central and north central South Dakota, with a second band of elevated convection forming from south central South Dakota into eastern Nebraska where strong theta-e advection begins to move northeast. While widespread severe weather isn't anticipated, soundings show elevated CAPE around 1500 J/KG through the CWA. Mid- lvl lapse rates are marginal, but shear could be strong enough for a few storms briefly capable of quarter size hail.

MONDAY: Ongoing convection at daybreak Monday will continue to move northeast during the morning hours. HREF/NBM mean QPF suggests fairly widespread 0.25-0.50" totals by mid-day, but the ceiling is also quite high with outside potential for an inch of rain in very isolated locations. While the initial band of moderate rain will move away from the CWA by early afternoon, model consensus keeps the orientation of the LLJ favorable to allow persistent showers or even isolated thunderstorms well into the afternoon. Depending on cloud cover, a very narrow window for surface based convection will still be possible by mid/late afternoon as a surface warm front attempts to lift northward. While most guidance has backed off on the northward extent of destabilization and kept this south of Highway 20, a few models remain steadfast that upwards of 2000+ J/KG MLCAPE could develop from Sioux City to Storm Lake by 23Z. A quick upscale growth situation would then develop as lift is maximized with an approaching mid-lvl vort. IF storms could develop, all hazards would be possible initially, with activity rapidly shifting SE early in the evening.

The persistent cloud cover and development of widespread low stratus will leave cooler temperatures over the CWA. I have again attempted to lower highs towards the cooler side of NBM guidance. Barring a late afternoon jump if clouds scatter, most locations especially east of I-29 may struggle to reach 70 degrees and those in Southwest Minnesota could still be too warm with their forecast highs.

One final item of note for Monday will be winds. A sharpening of the SPG through the morning will promote sustained winds as high as 20 mph at times. Gusts, especially behind departing rain, may rise into the 30 to 40 mph range into early afternoon.

TUESDAY-WEDNESDAY: High pressure arrives for Tuesday, signaling dry conditions in most locations with continued slightly cooler than normal high temperatures. A slight shift of mid-lvl flow back to the southwest by Wednesday could bring enough moisture return for scattered showers in most areas during the day Wednesday. The focus of the strongest moisture return, and resultant risk for heavier precipitation continue to stay focused south of Highway 20.

THURSDAY-SUNDAY: Shortwave ridging moves back through the Plains for Thursday and early Friday, spelling a return of dry conditions and some potential for temperatures to return to normal levels to finish the week. As alluded to in yesterday's discussion, signals remain favorable for rain by Friday as a stronger wave ejects out of the Southern Rockies and into the Plains. Still lots of details to work out with this system, but as expected, the 24 hr probabilities for >0.25" ending Saturday morning have jumped over 50% in all three major ensembles. Quite a bit of uncertainty for next weekend in regards to the eventual shift of mid-lvl flow. Ensembles showing low percentage PoPs over a large area suggesting a significant spread in guidance. While severe weather risks are expected to remain low, temperatures stay near seasonal normals with an 8 degree spread in the 25/75th percentile data split either side of normal.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 647 PM CDT Sun Sep 13 2026

Scattered to numerous showers and storms will develop late tonight and continue into Monday morning. Behind the storms will be MVFR to locally IFR stratus that looks to linger through much of the day, potentially scattering out by the mid to late afternoon hours Monday. There are low chances of additional thunderstorms over northwest Iowa during the afternoon Monday, but left out of mention at KSUX at this time as development will heavily depend on when and how much the aforementioned stratus scatters out.

Breezy conditions are expected through much of the period, with southeasterly winds picking up in south-central South Dakota this evening. These stronger winds will spread eastward through the overnight hours and into Monday morning before slowly decreasing area-wide toward the end of the period. Strongest wind gusts will be around 30-35 kts. Lastly, a cold front will move into central South Dakota to end the period, turning winds to the west and then northwest.

FSD WATCHES/WARNINGS/ADVISORIES

SD...None. MN...None. IA...None. NE...None.