textproduct: La Crosse

KEY MESSAGES

- A storm complex moves into the region tonight along and north of I-90 (40-70%). Storms could be strong to severe with damaging winds the main threat.

- Additional storm chances exist Wednesday afternoon (10-20%) and Thursday morning (30-60%).

- Daily shower/storm chances continue through the weekend with warm temperatures though these may trend cooler by Sunday.

DISCUSSION

Issued at 113 PM CDT Tue Sep 1 2026

Tonight - Wednesday Morning: Strong to Severe Storms Late Tonight

A shortwave trough lifting northward out of Nebraska this afternoon will spark showers and storms across the northern Great Plains before growing upscale into an MCS that moves eastward towards the Upper Mississippi River Valley. MUCAPE and effective shear are expected to be supportive of strong to severe convection with this MCS generally 1500-2500 J/kg and 35-45kts, respectively though the majority of the effective shear occurs within the 0-3km layer. Thinking the MCS will be strongest across our Minnesota counties though some stronger updrafts may continue into Wisconsin especially with support from the nocturnal low level jet. Storms should largely be elevated given the stable near surface layer, mitigating the tornado threat, and skinny CAPE profiles should limit hail growth such that the main threat is damaging winds as DCAPE values exceed 750 J/kg.

Placement and timing of the MCS is a bit challenging with many different CAM solutions and quite a bit of run to run variability. As of right now, arrival of the MCS to the I-35 corridor looks to be between 12-4 am, moving through our area during the early morning hours. CAMs suggest it moves along and just north of I-90 where the MUCAPE gradient is maximized. Some solutions do suggest a further northward placement may occur, though these are outliers and confidence is highest in the near I-90 solution, especially with a southward trend noted in the 01.12z HREF as compared to 12-24 hours ago.

Another aspect to watch is rainfall amounts due to PWATs of 1.5-1.6 inches, above the 90th percentile of climatology, and the skinny CAPE profiles. Probabilities for rainfall amounts exceeding 1 inch are 20-40% in the 01.12z HREF though locally higher amounts of 2+ inches are possible per the 01.12z HREF LPMM. Fortunately the system will be progressive and areas along and north of I-90 have also been fairly dry meaning the soil will be able to handle even the higher end rainfall amounts.

Wednesday Afternoon - Thursday Morning: Afternoon and Overnight Storms

After the morning's MCS, another warm and humid day is in store for Wednesday while a quasi-stationary frontal boundary remains situated across our southern counties through the afternoon. Given the warm and humid conditions, instability, albeit capped, builds across the region in the vicinity of the boundary with MUCAPE values of 2000-3000 J/kg. While forcing is weak, somewhat nebulous, and the low levels remain capped some convective development appears possible during the afternoon given warm air advection and moisture transport into the boundary. This potential is low however, generally 10-20%. AI/ML guidance does suggest some potential for a stronger storm exists due to the instability present though this is a very low chance (<5%) with the more favorable environment existing to our east.

Another MCS is possible Wednesday night into Thursday morning as the 850hPa nocturnal low level jet intensifies again. Can't rule out some stronger storms, mainly north of I-90 as the surface boundary begins to lift northward, with MUCAPE values of 1500-2500 J/kg remaining across the region although shear is less favorable for strong to severe convection as an upper level jet streak propagates eastward. Could see another round of rainfall to around 1/2 inch with probabilities for greater than 1/2 inch in the 01.00z LREF around 20-30%.

Thursday Afternoon - Sunday: Daily Storm Chances and Warm

Additional shortwaves traverse the northern extent of a building upper ridge through the remainder of the week into this weekend. Daily chances for showers and storms exist with these waves, ranging from 20-60% in the 01.13z NBM, but overall details are not clear. Warm temperatures also continue through Saturday with the NBM mean suggesting highs should be in the mid to upper 80s though current trends suggest a bit of a cool down is in store for Sunday with highs in the upper 70s to low 80s.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1233 PM CDT Tue Sep 1 2026

VFR conditions currently present across the region with generally light and variable winds. This will continue through the evening hours and into the early portions of the overnight hours. A cluster of showers and storms is expected to move east into the region later overnight, mainly after the 06Z timeframe. Probabilities for rain at the terminals are high in the 60-80% range but thunderstorm chances are lower between 30-40% as guidance has the complex of storms gradually decaying as it moves through the region so have left the thunderstorm mentions in the PROB30 groups. This will be a progressive system so rain chances should be done by 12Z with VFR conditions prevailing through the end of the period.

ARX WATCHES/WARNINGS/ADVISORIES

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