textproduct: La Crosse

KEY MESSAGES

- Scattered showers and storms linger for much of the overnight hours, with overall minimal impacts other than localized pockets of heavier rainfall.

- Increasing heat is expected Monday and Tuesday when heat indices climb into the mid 90s. Daily storm chances also exist, some of which may be strong, though confidence in if they develop is low (10-20%).

- The potential for showers, storms, and warmth continues into the late week.

DISCUSSION

Issued at 550 PM CDT Sun Aug 30 2026

Tonight: Scattered Showers and Storms Linger

Anvil shading from convection across Iowa kept the lower troposphere from mixing as efficiently as initially progged earlier today. This resulted in not only cooler temperatures, but a larger pool of MUCAPE for convection to realize this afternoon and overnight as the LLJ increases after 03-06Z. We saw widespread convection develop during the mid-afternoon over northeastern Iowa that the short-term guidance essentially fell flat on their face in predicting. Only the RAP, with its shallower BL depths, has caught onto current trends and was used heavily as guidance into the overnight. It develops scattered to even widespread convection along the nose of the LLJ along and east of the Mississippi River early tonight that festers for much of the night as the jet shifts eastward.

With the line parallel steering flow, repeated rounds of storms could impact the same area. It is not out of the realm of possibilities that we see localized pockets of 1-2" of rain (<10% chance at a given location), but the drier sub-cloud depths may help limit the efficiency of the rainfall rates. These showers and storms should exit the local area by sunrise as the LLJ weakens and pivots to the east.

Monday - Tuesday: Storm Chances and Increasing Heat

The warm front begins to lift northward Monday morning as a closed upper low across southern Canada moves into the region bringing a surface trough into the Upper Midwest. Additional shower/storm potential exists along this boundary for Monday afternoon though confidence is pretty low regarding timing/location given the sporadic coverage depicted in the CAM solutions, likely influenced by the fair degree of capping across the region. The environment does appear to be conducive to storm development, some possibly strong to severe, given MUCAPE upwards of 2000-3000 J/kg and effective shear of 35-40kts depicted in model soundings although this potential is highly dependent on if storms can get past the cap. Probabilities for storms developing sit around 10-20%, highest along/south of Highway 18 where forcing along the surface trough will be strongest, though recent CAM solutions are beginning to indicate development may occur north of I-90 where capping is slightly weaker. If storms do develop, gusty winds and hail will be the main concerns.

Apart from shower and storm potential, Monday looks to be a hot day as the persistent warm advection and southerly winds result in surface temperatures in the mid to upper 80s and dewpoints in the upper 60s to low 70s. While cloudiness associated with the various waves may act to moderate temperatures slightly, ensemble guidance is in fairly good agreement with the warm temperatures with the 30.12z HREF suggesting an 60-90% chance for highs to exceed 85 degrees area wide. Given the warm temperatures and high dewpoints, heat indices Monday afternoon are expected to climb into the mid 90s, potentially near 100 degrees within the river valleys.

The wave train continues Tuesday as a weak shortwave trough at 500hPa lifts northward out of the central Great Plains atop a developing stationary boundary at the surface. This set-up may lead to some additional showers and storms late Tuesday (20-30%) though greater chances come early Wednesday morning. Heat also continues Tuesday with highs in the upper 80s to around 90 degrees again, resulting in heat indices in the mid to upper 90s, warmest south of I-90.

Wednesday - Friday: Potential for Showers, Storms, and Warmth Continues

Shower and storm potential continues through the mid to late week as the active upper level pattern remains in place. As previously noted, Wednesday morning has the highest agreement regarding precipitation amongst the 30.00z LREF members as another 500hPa shortwave traverses the surface warm front. Current probabilities of precipitation sit around 40-70% in the LREF grand ensemble and 40-60% in the 30.07z NBM, highest along and north of I-90 at this point. A greater spread amongst the ensemble membership exists past Wednesday, though daily chances for showers and storms exist ranging from 20-50% per the 30.07z NBM.

Apart from precipitation chances, continued bouts of warm advection associated with predominantly southerly low level winds should keep high temperatures around the mid 80s, which is depicted in the 30.06z ENS/GEFS/GEPS interquartile range. Depending on how the potential continuous barrage of rain plays out, the forecast may ultimately trend towards a warmer or cooler solution as the aforementioned divergence of the ensemble membership does have outliers ranging from the low 90s to upper 70s. Would tend towards the cooler side given the potential cloudiness and rainfall, but there's still plenty of time for the trend to make itself known.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 550 PM CDT Sun Aug 30 2026

Mainly VFR conditions are forecast for the TAF period with localized MVFR/IFR restrictions owing to scattered showers and thunderstorms (20-40% chance) overnight. These storms push to the east of the region by sunrise with mainly dry conditions for the day on Monday--though there is a low risk (10-20% chance) for additional afternoon storms. Winds generally remain from the south to southeast overnight at 5-15 kts--strongest at higher elevations. Where winds decouple at lower elevations, pockets of LLWS will develop between 04-12Z. The surface winds begin to veer to the northwest behind a front Monday afternoon north of a CCY to ISW line before 00Z.

ARX WATCHES/WARNINGS/ADVISORIES

WI...None. MN...None. IA...None.