textproduct: Sioux Falls

KEY MESSAGES

- Very low relative humidity and wind gusts around 25 mph along US Highway 14 lead to critical fire danger today. Red Flag Warning in effect today 9 AM to 6 PM CDT. Elevated to near critical fire conditions elsewhere due to humidity at or below 25%.

- Shower and thunderstorm chances return this afternoon and evening, mainly across northwestern Iowa. A few storms may be strong to severe between 4 and 9 PM. Main threats hail to 2 inches and wind gusts to 75 mph. Have a way to receive warnings!

- Significantly higher thunderstorm risks develop Saturday night through Monday. Strong to severe storms possible overnight Saturday into early Sunday. Potential for widespread severe storms remains focused Sunday afternoon and early evening. Monitor the forecast and review severe weather plans now!

UPDATE

Issued at 318 AM CDT Fri May 15 2026

No changes to the forecast into this morning. Forecast for today's fire concerns remain on track as well, with Red Flag Warning in effect today for areas along and north of US Hwy 14 - more details in the Fire Weather Section of this discussion. Changes were made to tonight with increasing severe risk; details below.

Increased pops in northwestern IA for this afternoon and evening to account for convective initiation. Some of the 15.00z CAMs (NAM Nest, ARW) break the cap with the front as far west as the SD/IA border and as early as 3 PM. However, other guidance like the 15.06z RAP and 15.00z Fv3 are as late as 6-7 PM with initiation and keep it more localized to the front in our far southeastern counties. HRRR trends are also maintaining a later and more easterly initiation through the 15.06z run. Based on this and the trends in the HREF, think that the greatest severe threat for our area will be from roughly 4 to 9 PM this evening, as nearly all guidance has storms exiting our area by 9-10 PM. Duration of severe threat is highly dependent on where and when convection initiates.

Main threats are large hail to 2 inches in diameter and wind gusts up to 75 mph. With increasing shear and steep mid level lapse rates, expect hail to be the main threat with any of the more discrete and isolated cells. As the front moves east across northwestern IA this afternoon and evening, 0-6km shear vectors become more parallel to this boundary, supporting upscale growth and increasing wind threat with gusts to 75 mph possible. Biggest uncertainty with today's severe threat boils down to where and when the cap breaks. Secondary concern is our moisture return, impacting the instability. Trends in observations will need to be closely monitored today.

Concerns for strong to severe storms continue through the early part of next week. More details below.

DISCUSSION

Issued at 158 PM CDT Thu May 14 2026 18 THIS AFTERNOON: Synoptically driven strong southwesterly wind gusts of 30 to 45 mph continue to develop ahead of a surface front moving eastward gradually. We've had quite a few reports of elevated dust as well as blowing dust impacting west to east roadways this afternoon. While widespread visibility remains above 5 miles, very highly localized visibility may drop below 1 mile in higher gusts. While temperatures are slower to respond today with the cloud cover, we're still headed towards critical fire danger. See separate discussion for more details. Otherwise, a few lingering high based showers will remain over NW Iowa into the late afternoon, with isolated shower/tsra possible along the aforementioned front moving east through sunset.

TONIGHT: The frontal boundary will slide into the eastern CWA after dark tonight with some persistence of mid-lvl clouds along it. Elsewhere, winds do turn light and variable as temperatures cool into the upper 40s and 50s.

FRIDAY-FRIDAY NIGHT: We'll begin to see gradual shifts in the mid- lvl pattern on Friday as zonal flow develops. Two concerns on Friday, one being marginal RFW conditions along the Highway 14 corridor in the afternoon, and the second being potential convection forming along surface convergence zone over eastern Nebraska and western/northwestern Iowa late in the afternoon. Soundings along the Highway 20 corridor show some inhibition lingering at mid- afternoon, but dissipating as convective temps in the low 90s are met. With mid-lvl flow increasing late in the day, just enough forcing on the boundary could force isolated to scattered storms along and south of Highway 20 in the evening. MLCAPE around 1000- 1500 J/KG could support marginally severe hail for a few hours as storms will track southeast with the mean flow away from the CWA. Further north, weaker low-lvl flow should prevent widespread convection from forming into Saturday morning, but it's also a non-zero chance.

SATURDAY: The vast majority of the daytime hours of Saturday is anticipated to be dry as brief mid-lvl ridging slides through the area. The surface front that passes through the area today will settle near or north of I-80 but may make a slow retreat northward in the afternoon. By early evening deeper synoptic lift associated with an upper trough ejecting out of the Central Rockies will focus low-lvl convergence and strong theta-e advection south of I-90 or towards Highway 20. This is where some uncertainty begins to grow in the forecast. Models have some variability on the northward extent of surface based activity but strongly support elevated storm development across northern Nebraska eastward into north central Iowa. There may be some potential given 40 knots effective shear, for a few supercells with very large hail to form initially. Later in the evening a potential MCS may try to form over northern Nebraska and lift northeast through the nearly 1500 J/KG MUCAPE bringing both a risk for very large hail but some modest strong wind risks into the Tri-State area into daybreak. Pockets of heavy rain will be possible in this thunderstorm activity.

SUNDAY: While some considerable uncertainties remain on Sunday, there continues to be increased potential for impactful and potentially widespread severe weather in the area. As alluded to in Wednesday's discussion, the greatest uncertainty continues to revolve around the extend of the northward advection of the warm sector into the Plains during the day. Overnight and morning convection may play a large role in this process, but ensemble data has trended towards instability shifting further north over the past 24 hours, even showing MLCAPE exceeding 3000 J/KG over portions of NW Iowa by late afternoon. The other limitation we'll need to monitor is the EML that remains in place for much of the daytime hours once morning convection passes. A stubborn warm layer continues to present itself in soundings near 700 mb deep into the afternoon, but thoughts are the amplitude of the slightly negatively tilted trough may be enough to overcome this limitation. Given the magnitude of bulk shear, supportive mid- lvl lapse rates, low-lvl helicity and buoyancy there is certainly potential for a higher end severe weather day well into the evening/overnight hours which includes all modes. This thought is backed up by AI-learning outlook products and CIPS analogs that support a significant severe weather potential. Still quite a few issues to work though and plenty of time for things to change, but it's time to really start monitoring the forecast if plans exist on Sunday or Sunday night.

MONDAY: Uncertainty grows further on Monday as models diverge on the positioning of a cold front and how far east it may track. Ensembles of these models are also split with the GFS shoving the effective boundary east. The ECMWF/CMC both stall the front over the eastern CWA and allow it to retreat westward as a secondary trough lift out of the Central Plains. These drastically different scenarios mean a big difference in severe weather risks especially east of I-29 Monday afternoon and evening.

QPF TOTALS: A wide array of potential QPF totals remain with this system with NBM continuing to spread low probabilities of higher QPF totals, but expressing a wide array of 10/90th percentile potential between 0.25-2". Convection will ultimately drive the highest totals, and would not be surprised to see pockets of 2"+ by Monday evening.

TUESDAY-THURSDAY: Behind this trough, cooler weather will arrive through the middle of the week with minimal hazards expected.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 622 AM CDT Fri May 15 2026

VFR conditions for most of the area through the period. Guidance this morning continues to show showers/storms developing near or east of the SD/IA state line into northwestern IA later this afternoon and into the evening hours. With low confidence in how far west this activity develops, have removed the PROB30 TSRA from KSUX for this cycle. Any showers/storms that develop could lead to less than VFR conditions in heavier rain. Additionally, strong to severe storms may produce large hail and wind gusts around 60 knots.

Wind gusts today around or less than 20 knots, strongest for areas along and north of US Hwy 14. Direction will shift through the period as a couple of boundaries move across the region.

FIRE WEATHER

Issued at 334 AM CDT Fri May 15 2026

Very dry conditions continue today with relative humidity values around and below 25% area wide. Winds remain light over much of the area, although gusts to 25 mph are expected through the day along and north of US Highway 14. A few gusts to 25 mph are possible for parts of south central SD, but these are expected to be brief late this afternoon. This combination leads to near critical and critical fire conditions. Red Flag Warning will be in effect for the US Hwy 14 corridor and northward where confidence in near critical to critical conditions is highest from 9 AM to 6 PM CDT.

Rain and storm chances return to portions of the area tonight. Low humidity Saturday (around and less than 25%) through the mid to late afternoon with breezy conditions (gusts to 25 mph) may lead to elevated to near critical conditions for a brief window.

Widespread rain and storm chances return to the region Saturday night through Monday, tempering widespread concerns.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Red Flag Warning until 6 PM CDT this evening for SDZ038>040. MN...Red Flag Warning until 6 PM CDT this evening for MNZ071-072. IA...None. NE...None.