textproduct: Sioux Falls

KEY MESSAGES

- Hotter conditions remain on track for Sunday and likely Monday where heat indices could climb into the low 100s. A Heat Advisory has been issued for locations along and west of the James River on Sunday from 1PM to 9PM.

- Temporary reductions in visibility will be possible through Sunday especially across southwestern MN due to wildfire smoke. Any sensitive groups should continue monitoring their local air quality.

- Chances for showers and storms return to the area early tomorrow morning and persist through Monday. Coverage is not expected to be widespread. Isolated strong to severe storms are possible Sunday night and on Monday.

UPDATE

Issued at 943 PM CDT Sat Jul 18 2026

Quiet conditions are expected through most of the overnight hours as temperatures fall mainly into the 60s for lows Sunday morning. Widely scattered elevated showers look to develop late tonight into early tomorrow morning over north-central South Dakota, but perhaps as far southeast as where the Missouri River Valley and Highway 20 intersect. This activity will gradually push eastward through Sunday morning. Instability may increase just enough before these showers arrive that a couple of thunderstorms cannot be ruled out mainly near and west of the James River Sunday morning, but severe weather is not expected. This activity will gradually weaken with eastward extent as it runs ahead of any appreciable instability. These showers will help limit us from reaching full heating potential Sunday, so did lower the high temperature forecast by a couple of degrees; however, this cooling will be offset by higher dew points and thus the current heat advisory remains in good shape.

After these morning showers fade and exit the area, instability will increase but the cap looks to be firmly in place. Thus, currently not expecting to see new thunderstorm development Sunday afternoon and early evening. The main concern for stronger thunderstorms will come overnight as storms develop to our north and west and move towards parts of our area, more on that threat in the discussion below.

DISCUSSION

Issued at 201 PM CDT Sat Jul 18 2026

Quiet conditions continue this afternoon with highs warming up to the 80s. Winds remain light out of the east/northeast, making for a pleasant Summer day. Light concentrations of smoke remain across the area. Sensitive individuals could have some issues with the smoke across the area. With the light winds in place, the light smoke looks to be here to stay. Dry conditions will persist through the rest of today but things will change for the overnight hours. A cold front that has passed through the forecast area this morning will begin to return northwards as winds turn to out of the south. This will mainly affect the elevated front between 850 to 700 mb as the warm air advection (WAA) associated with this southerly flow looks to develop elevated convection along this boundary. Mean flow will be parallel to the boundary, resulting in any storms that form to travel down this boundary. Instability looks to be on the weaker side with CAPE values only up to around 500 to 700 J/kg. Effective shear values are also weak on the order of 30 knots or less. Thus, these storms are not expected to be strong to severe, but drier air in the low levels could result in a few brief wind gusts and perhaps small hail. The storms look to develop across central South Dakota tonight before sliding southeastwards along the boundary into the forecast area. Instability values will be waning as they enter the forecast area, so they are expected to be weakening with southeastward extent.

Sunday will be the warmest day this weekend as the thermal ridge continues to push eastwards. High temperatures look to warm to the mid 80s to about 100F with the warmest temperatures occurring along and west of the James River Valley. With a return to southerly flow in place, slightly higher dew points will be advected into the area. Dew points look to moisten to the 60s to potentially reaching 70F in a few locations. This will push heat indices up to the 90s to low 100s across the bulk of the area. The highest heat indices will occur with the hottest temperatures along and west of the James River Valley. As such, a Heat Advisory has been issued for this area from 1PM to 9PM. Aside from the heat, a few showers could persist into the morning hours before dissipating by the afternoon timeframe. Otherwise, the rest of the afternoon hours look dry. Light concentrations of smoke look to persist across the area despite the southerly flow. The dry conditions will not last long though as another chance for showers and storms look to make a return to the area Sunday night. A shortwave trough will push out of North Dakota and dive southeastwards. This looks to being shower and storm chances to mainly southwest Minnesota at this time. The previously mentioned boundary will continue to return northwards, serving as the focal point for this new round of convection to develop on. Currently, it seems like the storms will develop along the 925mb front and track down it through the night. Vertical shear will be stronger but instability values look to remain on the weak side of things with a magnitude between 500 to 1000 J/kg. Thus, isolated severe storms are possible across mainly southwest Minnesota Sunday night with large hail and damaging winds as the primary hazards.

The core of the thermal ridge will finally make its way over the forecast area on Monday. This will push high temperatures all the way up to the mid 90s to low 100s. With sufficient moisture remaining in the area, heat indices will reach up to the 90s to low to potentially reaching up to about 104 to 105F. The hottest temperatures and heat indices will occur along and south of I-90. Some uncertainty remains regarding these hot temperatures as a second round of thunderstorms is possible Monday afternoon. A cold front will be pushing through the area after the first round of showers and storms passes through the area Monday morning. This front looks to result in a second round of showers and storms Monday afternoon. As of now, it looks like storms could develop east of I- 29 at this time. If they do form, they will again have the potential to be strong to severe. There is uncertainty regarding where storms develop as well as if these storms may affect afternoon highs enough to limit diurnal heating potential. Heat headlines may be needed but that decision will be made once confidence increases in storm coverage, location, and temperatures.

Much drier air is in store behind the previously mentioned cold front. High temperatures will return to near seasonable for the rest of the work week with highs in the 80s. A few chances (15-35%) for rain are possible throughout the week but details are uncertain at this time. Temperatures may trend back to above average by next weekend as zonal flow returns aloft.

AVIATION /06Z TAFS THROUGH 06Z MONDAY/

Issued at 1256 AM CDT Sun Jul 19 2026

Widely scattered light showers and very isolated thunderstorms will move through areas mainly near and west of the James River Sunday morning. Added a PROB30 group for -SHRA to account for this at KHON, but it does look like the better thunder potential will at least be south of the site. This activity pushes east and fades through the early afternoon, but is unlikely to cause much if any impacts to KFSD or KSUX. Another round of showers and thunderstorms will move in Sunday night, but not enough confidence to include mention of this in the TAF sites at this time as the better chances currently reside over southwest Minnesota.

Winds will turn more south-southeasterly heading into the day on Sunday, remaining fairly light through the period with the highest gusts up to only 20 kts near and west of the James River during the late morning through middle afternoon. Any stronger storms that move through Sunday night may be capable of isolated wind gusts to 50 kts or slightly stronger.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Heat Advisory from 1 PM this afternoon to 9 PM CDT this evening for SDZ038-050-052-053-057>060-063>065-068-069. MN...None. IA...None. NE...None.