textproduct: Sioux Falls

KEY MESSAGES

- Near to critical fire weather continues this afternoon along Highway 14.

- Thunderstorm risks grow after 5pm for a narrow portion of NW Iowa. Hail to 2 inches and 70 mph winds possible. Storms move southeast quickly with the risk ended by 9pm.

- Thunderstorms increase in coverage Saturday night into Sunday. Severe weather risks including 1.5" hail and 60 mph wind gusts will be possible.

- A significant severe weather episode will be possible late Sunday afternoon into Sunday night. Very large hail, straight line winds, and tornadoes will be possible. This risk may extend into Monday.

- If you have outdoor plans Sunday afternoon through Monday, the time is now to prepare and have alternative plans in place.

DISCUSSION

Issued at 129 PM CDT Fri May 15 2026

THIS AFTERNOON: Poorly defined "cool" front continues to slowly sink southeast this afternoon now entering southern Minnesota and far northwestern Iowa as of 1pm. Southerly winds are slowly advecting 50 dewpoints into the area ahead of the boundary, but generally models have been too quick to pull higher dew point air northward. By late this afternoon a weak impulse in the 700:500 mb layer will arrive into the Tri-State. This increased lift combined with slight pressure rises to the northwest of the front should create a bit more convergence sufficient for convection development in a portion of NW Iowa (along and southeast of a line from Sioux City to Spencer). Soundings indicate MLCAPE values may approach 1000- 1500 J/KG, with an increase in effective bulk shear towards 40+ knots. In any storm that develops, large hail (potentially 2" or greater) will be possible as mid-lvl lapse rates break the 8C/KM mark. Inverted V soundings along the periphery of the CWA boundary in NW Iowa would suggest downburst potential increases as well. With the mean wind increasing from the west northwest, storms should begin to congeal into one or more linear clusters as they head into central Iowa. The peak timing for development and impact falls within the 6pm to 9pm timeframe.

TONIGHT: Storms move southeast of the forecast area very early in the overnight period, leaving quiet and dry conditions overhead. Soundings show a pronounced low-mid lvl dry layer advecting southward through the night.

SATURDAY: The aforementioned frontal boundary will be pushed by convection closer to I-80 by Saturday morning, leaving dry and quiet conditions in the local area for most of Saturday. A persistent easterly 850 mb wind fetch is not expected to bring meaningful moisture northward through the day with the surface boundary tracking northward towards Highway 20 by mid-evening. Temperatures under filtered sunshine peak in the lower 80s, and with persistent low RH and breezy easterly surface winds fire danger may be elevated.

SATURDAY NIGHT: Today's guidance has slowed the northward advection of rich theta-e air northward which will likely keep the area convection free deep into the evening. We will be watching areas of central and southern Nebraska tied to surface low pressure and the existing warm front that remains in place. Convection is expected to develop as a stronger wave ejects into the Plains. One or more clusters of strong to severe storms will attempt to track north or northeast along the advancing elevated moisture axis and towards the MO River valley after midnight. MUCAPE remains somewhat marginal with the initial surge of moisture, and mid-lvl lapse rates are only in the 6.5-7 C/KM range throughout the hail growth zone. Nevertheless, effective shear may just be strong enough to produce a few elevated supercells capable of larger hail into Sunday morning. Tend to feel the straight line wind risk may be lower, but should the surface front lift northward or boundary layer become only loosely stabilized, then a few stronger gusts could transport down through the MO River Valley into NW Iowa into daybreak.

SUNDAY-SUNDAY NIGHT: Significant severe weather potential is possible Sunday afternoon into Sunday night. After morning convection lifts northeast of the CWA early in the morning, guidance is becoming more confident in a period of shortwave subsidence moving overhead through a large part of the daytime hours of Sunday. While we'll enter a quiet period, moderate to extreme instability will continue to advect northward into the Tri-State area with some models hinting at potential for AOA 3000 J/KG by mid-afternoon. Soundings across the area suggest an EML will hold in place into at least the mid afternoon hours as we wait for a deeper trough to lift into the Plains. This trough should work to push the effective warm front further north while reinforcing a pre-existing cold front across central NE/SD. The arrival of deeper synoptic lift after 21Z could lead to explosive convective development by early evening. With the large reservoir of instability, bulk shear over 50 knots, and strong low-lvl buoyancy all modes of severe weather will be likely. Numerous sounding analogs suggest 2-4" hail in any discrete or initial updraft that forms. Storms staying discrete is somewhat in question, with guidance hinting that the potential for quick upscale growth into a linear line with embedded supercells progressing east northeast instead. The increase in the low-lvl flow after dark and and slight backing of surface winds could lead to a much greater potential of QLCS activity into the late evening and overnight hours as storms track into Iowa and Minnesota.

As we move past midnight, models suggest the highest severe weather threat will migrate eastward with the line. However, there remains a high degree of uncertainty in the evolution after midnight. ECMWF/NAM/CMC are holding back the upper trough to the west, and actually allow deeper instability to lift back northward into Monday morning. The persistent 40-50 knot LLJ could lead to scattered hail producing supercells continuing into Monday morning. Still very uncertain.

MONDAY: A continued severe weather risks seems plausible on Monday depending on what happens overnight. Should the GFS based models hold true, then both surface based and elevated instability slides east of the area. However ECMWF/NAM solutions would pull the surface front back westward while leaving a large degree of elevated instability westward. There's just too much uncertainty to get into specifics, but Monday could also feature potential for higher end severe weather especially in areas along or east of I-90.

TUESDAY-THURSDAY: We'll see a distinct cooldown in temperatures as we head towards the middle of the week. Highs on Tuesday with a cooler northwest wind only rise into the the upper 50s. A slow recovery towards the lower to middle 60s is possible by Wednesday. The pattern through the rest of the week remains quasi-zonal, suggesting one more more weak disturbances moving through the flow creating a few rain chances by the second half of the week.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1203 PM CDT Fri May 15 2026

VFR conditions will continue into the evening and overnight hours. Scattered CU will develop along and ahead of a slowly advancing frontal boundary in NW Iowa, Minnesota, and Nebraska this afternoon. Isolated to scattered strong storms may form along and east of a line from Sioux City to Spencer for a few hours.

Elsewhere winds turn light and variable into Saturday, with upper clouds and then mid-lvl clouds arriving towards daybreak.

FIRE WEATHER

Issued at 129 PM CDT Fri May 15 2026

We continue to see very low relative humidity values this afternoon throughout the region. However, wind gusts have been slow to develop with only a few areas along and north of Highway 14 near red flag warning criteria. Winds will settle down by early evening ending the risks.

Easterly winds on Saturday will bring breezy afternoon gusts to the area with another day of lower relative humidity values at or around 15 to 20 percent. That said, gusts are anticipated to stay low enough to prevent red flag conditions. Elevated fire danger will be possible along the Missouri River valley however.

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.