textproduct: La Crosse

KEY MESSAGES

- Line of showers, possibly a rumble of thunder, late tonight. Overall impacts look to be minimal.

- Risk remains for heavy rain and possibly severe weather for late Monday through Tuesday, but the devil is in the details for how this event will unfold.

- Trending cooler for the latter part of next week with additional storms possible.

DISCUSSION

Issued at 144 PM CDT Fri Sep 11 2026

Tonight: Line of Showers Moves Through

A narrow line of elevated showers and possibly a few thunderstorms are progged to develop ahead of an approaching cold front within a pocket of narrow mid-level theta-e advection/30-40-kt LLJ above a stout lower tropospheric warm nose. The window of opportunity for storms at a given location is short (1-2 hours) and is predicated on the moistening of that critical 800-700-mb layer in time. MUCAPE values of 500-1000 J/kg would certainly support general thunderstorms, though the wind fields would limit any organization as indicated by the complex hodographs. Most of the 11.12Z HREF members have this convection waning by sunrise as it moves towards central Wisconsin.

Monday - Tuesday: Heavy Rain/Severe Weather Risk

The main period of interest in the forecast remains from Monday into Tuesday as an early fall extratropical cyclone ejects off the northern High Plains and tracks across southern Manitoba and Ontario. The attendant cold front slides southeastward with return flow pushing warm sector PWATs close to 2 inches and warm cloud depths of 12+ kft as Pacific moisture overruns lower level Gulf moisture. While the thermodynamic environment will support efficient rainfall processes, how and where this rain falls will be dictated by the position of the front and convective evolution ahead of the front, which is a source of model variability between the medium range solutions. Recent runs of the EPS/GEFS have started trending towards a more progressive frontal passage, but there still exist a few members that have the front more parallel to the mean upper level flow, which would be more favorable for repeated rounds of heavy rain. There are some solutions that fails to push the front south of the area on Tuesday and linger the heavy rain potential into Wednesday.

The risk for severe weather also hinges on how the warm sector evolves during the day on Monday and the influences of the morning convection, variables we will not have sorted out until later this weekend.

Late Week: Possibly More Storms

Looking beyond early next week, the local weather pattern will hinge on the evolution of Pacific Northwest wave Tuesday into Wednesday. Roughly 20-30% of the current guidance have this wave propagating eastward across the region and deepening, resulting in the surface baroclinic zone lifting northward into our region and being the focus for renewed rounds of storms and heavy rain later in the week. These solutions look to be falling more into the minority and some of the more recent deterministic runs also are favoring this shortwave energy cutting off over California and the northern stream energy lifting northeastward and not affecting the region. The blended NBM forecast understandably has low (20-30%) PoPs for much of next week, but if the cutoff solution unfolds, we may stay dry and cool for much of the remainder of the week--or until the cutoff low finally ejects eastward.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

Issued at 618 PM CDT Fri Sep 11 2026

The main aviation concern over the next 12 hours will be the potential for thunderstorms, mainly at KLSE. Confidence continues to be high enough to keep PROB30 mention for storms at that site. KRST still has a potential for storms in the vicinity, but confidence is too low (around 10-20 percent) to mention in the TAF. Low clouds may also develop for a few hours behind a frontal passage, which may result in brief MVFR ceilings at both sites. KLSE has the higher potential in this occurring so have dropped ceilings to MVFR with this TAF cycle. The wind will shift from out of the south to out of the northwest behind the front. Wind gusts could be as high as 20-25 kts during the day tomorrow.

ARX WATCHES/WARNINGS/ADVISORIES

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