textproduct: La Crosse

KEY MESSAGES

- Isolated to scattered showers and storms are possible this afternoon, mainly along and south of Highway 18 (10-20%). - Showers and storms develop Thursday morning (40-70%) before a potential MCS moves through the area Thursday night.

- Periodic showers and storms continue through the holiday weekend.

DISCUSSION

Issued at 1158 AM CDT Wed Sep 2 2026

This Afternoon - Tonight: Isolated to Scattered Showers/Storms

The quasi-stationary boundary that has plagued the Upper Mississippi River Valley the last several days remains situated along and south of I-90 through the rest of today. With the boundary in place and weak 850hPa warm air advection and moisture transport into it, there is some low potential for the showers and storms ongoing across north-central Iowa to continue eastward into northeast Iowa and southwest Wisconsin (10-20%). Given little in the way of airmass change, fairly impressive instability is expected to build across the region again through the afternoon hours with RAP/HRRR soundings suggesting MUCAPE should be upwards of 1500-2500 J/kg. However, more favorable instability is expected south and east of our area and the low level capping also remains so confidence in how the ongoing storms evolve is fairly low as the low level jet weakens and pivots into eastern Iowa.

If storms are able to continue eastward this afternoon, there is a low possibility they will be able to produce some higher rainfall totals given favorable storm motion vectors parallel to the boundary which would lead to training storms. Training has already been observed with them across north-central Iowa, so this will be something to monitor through the afternoon if storm coverage begins to increase. Also can't rule out a stronger storm given the energy available, but diminishing 500hPa winds will result in less than favorable effective shear values and should largely inhibit any strong/severe threat.

Thursday - Friday: Strong to Severe Storm Potential

Early Thursday morning, warm advection and moisture transport increase across the region as the nocturnal low level jet increases. When combined with a weak shortwave at 700-500hPa moving through the region, convective development should be able to occur across much of the Upper Mississippi River Valley, continuing through the remainder of the morning hours. There is quite a bit of model inconsistency regarding the timing and evolution of this convection and overall confidence in precisely what will happen is low, but confidence is medium to high that convection will develop (40-70%). Similar to the last few days, sufficient instability for stronger storms remains across the region, though wind shear will be the be the primary limiting factor for severe weather. South of I-94, effective shear is fairly weak, on the order of 20-30kts whereas north of I-94 this increases towards 35-45kts with straight hodographs. If storms develop north of I-94, such as the 02.12z HRRR suggests, some more discrete to supercellular structures are possible. The 02.09z RAP suggests mid-level lapse rates steepen to 7- 7.5 C/km in this already favorable environment which would support hail development along with damaging winds given DCAPE around 1000 J/kg. South of I-94, it currently looks as though convective development would be on the weaker side given the less favorable shear profiles.

Thursday evening and overnight, there's the potential for an MCS to roll through the area as storms that develop near the Minnesota/North Dakota/South Dakota border. Confidence in when, where, or even if this system develops is pretty low as its evolution will largely hinge on how the convection earlier in the day plays out. As of right now, would expect damaging winds to be the primary threat though a tornado or two can't be ruled out within favorably oriented bowing segments given 0-3km shear of 30-35kts, assuming the system become surface based.

Another bout of showers and storms are expected Friday afternoon along the boundary that remains situated across the region. A rinse and repeat atmospheric set-up regarding low level moisture and warmth will lead to instability favorable for some stronger updrafts, but weak 500hPa winds and rising heights may largely limit any sort of strong/severe potential. The SPC currently has part of our area outlined in a Marginal Risk (Level 1 out of 5) and will be another period to monitor.

Holiday Weekend: Persisting Showers and Storms

The Upper Midwest remains on the northern edge of a mid to upper level ridge through the weekend. Several shortwave troughs move along the northern extent of this ridge, directly over the Upper Mississippi River Valley, which will support periodic shower and storm development through the weekend. Daily probabilities for rainfall sit around 20-40% in the 02.13z NBM. While instability will be present for some rumbles of thunder, effective shear is expected to be fairly weak, limiting the potential for severe weather through the weekend. This is supported by AI/ML guidance which have little to no footprint for strong/severe storms across the Upper Mississippi River Valley.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 1225 PM CDT Wed Sep 2 2026

Largely VFR conditions present across the area outside of portions of NE Iowa seeing some MVFR visibilities from a small line of showers and thunderstorms early this afternoon. This line of showers and storms is expected to slowly sag south into Central and Eastern Iowa by later in the afternoon with VFR conditions expected to remain everywhere else. Light winds generally out of the south and/or east will continue through the period at or below 12 kts. An area of showers and storms is expected to develop late overnight and into tomorrow morning with confidence in POPs increasing after 12Z for both terminals. Have added prevailing showers with PROB30 thunder at this time. How this activity will evolve tomorrow morning is still pretty uncertain so have continued the PROB30 through the end of the period.

ARX WATCHES/WARNINGS/ADVISORIES

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