textproduct: La Crosse

KEY MESSAGES

- Low potential for thunderstorms exists this afternoon (10-30%). The overall potential for storms to become strong has decreased with higher potential now over northeast Wisconsin.

- Shower and storm potential exists south of I-90 Tuesday afternoon (10-30%) while dry conditions are expected to the north.

- Daily shower and storm chances along with warm temperatures continue through the remainder of the week.

DISCUSSION

Issued at 106 PM CDT Mon Aug 31 2026

This afternoon - Tonight: Low Thunderstorm Potential

A weak 500hPa shortwave trough has been moving northeastward throughout the morning and has made its way into the Upper Mississippi River Valley this afternoon. An unstable atmosphere exists across the region given the warm conditions today with MUCAPE values of 2000-3000 J/kg analyzed. Effective bulk shear of 35-40 kts is also depicted, strongest north of I-90 where 500hPa wind speeds associated with a closed low moving eastward across the United States/Canada border are maximized. However, fairly strong capping exists across much of the area, especially south of I-90 where boundary layer dry air depicted in model soundings has largely kept convection at bay through the morning hours.

This capping decreases with northward extent given increasing low level moisture ahead of an approaching surface front. Forcing along the front combined with the northeastward moving wave should be able to develop some showers and storms across the region late this afternoon into this evening (10-30%) given the favorable instability and shear profiles. Areas east of the Mississippi River and north of I-90 have the greatest potential to see a shower or storm though thinking the bulk of any convective activity, and with it the potential of a stronger storm, will exist northeast of our area where low level capping is minimized.

Given the high humidity across the region, some valley fog development seems reasonable overnight, possibly spilling out into surrounding areas. The 31.12z HREF suggests 30-50% probabilities for visibilities less than 3SM across the region although partly cloudy skies could hinder more widespread development. Regardless, the typically foggy river valleys should see some visibility restrictions heading into Tuesday morning.

Tuesday: Shower/Storms South of I-90, Dry to the North

A bit of a bimodal weather scenario is expected Tuesday afternoon. The previously mentioned surface front stalls out generally just south of Minnesota/Iowa border where pooling surface moisture along the boundary will allow dewpoints to remain in the upper 60s to low 70s, building MUCAPE values upwards of 3000 J/kg once again. 500hPa winds remain maximized over the Upper Midwest throughout the day, resulting in effective bulk shear values of 35-40kts remaining across the region and leading to another environment favorable for convective development. However, there remains the issue of low level capping and whether or not parcels can ascend through said cap. There appears to be an overall lack of a sufficient forcing mechanism, but with the front in the vicinity, some isolated storms appear possible across our far southern counties (10-30%). Some of these storms could become strong to potentially severe with gusty winds and hail being the main concerns.

North of the boundary, drier conditions are expected as lower dewpoints are ushered into the region. Tuesday could be a fairly efficient mixing day for these areas as RAP/HRRR soundings depict a pretty stout dry layer up to 500hPa. Cloud cover will be the main limiting factor to deeper mixing through Tuesday afternoon with these same soundings suggesting mixing tops out around 800hPa. However, if skies are clearer or turbulent mixing atop the boundary layer is more efficient than currently expected, deeper mixing is possible resulting in minimum RHs bottoming out in the 15-20 percent range per the 31.12z HREF.

Wednesday - Friday: Daily Storm Chances and Warm

Several additional chances for storms exists across the region through the remainder of the week as several shortwave impulses traverse the top of a ridge axis situated from the Southeast United States into the Upper Midwest. Wednesday morning is the next most favorable time for a wetting rain across the region (40-70%) although recent runs of the GEFS/EPS/GEPS has begun to suggest the higher probabilities exist across central Minnesota into northwest Wisconsin. If this trend continues, the bulk of any precipitation may miss our area. However, there is signal within these ensembles for rainfall amounts upwards of 1/2 inch nudging into southeast Minnesota and western Wisconsin (10-30%), mainly north of I-90.

Early thursday morning brings another round of rainfall, this time further south. Probabilities for rainfall in the 31.13z NBM are 40-70% across the entire region though highest along I-90. Friday morning looks to be fairly rinse and repeat of Wednesday and Thursday with current probabilities for rain sitting around 40-60%.

Can't rule out some rumbles of thunder with the rainfall any of these mornings as at least a few hundred J/kg of MUCAPE look to be present. Some of the AI/ML severe guidance even suggest the parameter space could support some stronger storms, though these details are murky at best.

Warm temperatures also continue through the late week with daily high temperatures currently sitting around the mid to upper 70s per the 31.13z NBM mean. However, the various rounds of showers/storms and any residual cloudiness can and likely will influence these temperatures.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 1232 PM CDT Mon Aug 31 2026

VFR conditions currently present across the region with light winds out of the west/southwest at or below 10 kts. Valley fog formation looks likely tonight with a sufficiently deep light wind layer overnight (greater that 3 kft) and plenty of low level moisture. Have introduced BCFG and a scattered low ceiling to KLSE for the late overnight period with further degradations possible in future package updates. Areal fog is also possible but confidence in this occurring is too low to include in the KRST taf at this time. Light winds will prevail through tomorrow morning.

ARX WATCHES/WARNINGS/ADVISORIES

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