textproduct: La Crosse

KEY MESSAGES

- Severe storm threat returns through the area today with primary concern for damaging winds and secondary concerns of large hail and isolated tornadoes. Higher confidence for severe storms and storm initiation lies southwest of the forecast area into central and southwest Iowa with storms weakening as they lift northeast through the evening hours.

- Colder with showers for Tuesday with colder temperatures sticking around into the weekend. Low chance for some local areas to see Frost Tuesday and Wednesday nights.

DISCUSSION

Issued at 316 AM CDT Mon May 18 2026

Morning Observations & Synoptic Setup Today:

An upper level longwave trough over the Rocky Mountain West on GOES water vapor imagery loops, evidently amplifying in enhancement of meridional GOES-derived upper level jet streaks lee of the Rocky Mountains, causes cyclogenesis over the Central Plains and subsequent reinvigoration of low level moisture transport, resulting in a similar severe weather set up as Sunday for the Upper Mississippi River Valley through this evening into tonight.

Low Level Capping Limiting Local, Widespread Confidence Today:

While it remains quite moist in the low levels early this morning, GOES upper level water vapor imagery loops and derived upper level winds depict a meagerly zonal perturbation eradicating moisture from the local forecast area to the east, tied to the long bowing non-severe radar returns from southern Iowa through northern Wisconsin. High resolution model analyses (18.06/18.00Z) suggest 850mb/925mb drying persisting through the early afternoon before cyclogenesis along the lingering low level baroclinic boundary spanning from the Mid Missouri River Valley to the northeast, kinked by anticyclonic rotation over Minnesota, initiates low level moisture transport northeastward towards the forecast area. The local drier, warmer low level air will cause another capping inversion, inherently increasing dependence on northern extent of moisture return (i.e., warm front) through the forecast area today. Current, further limiting factors include diurnal timing close to sunset and deamplification of the upper level wave as it nears the Upper Mississippi River Valley through the afternoon, both decreasing confidence in northerly frontal transport.

Summarily, the current temporal and resultant spatial window for convection to become surface based or even achieve sufficient forcing to access enhanced MUCAPE (2000+ J/kg) from steeper mid level lapse rates limits local confidence. Furthermore, high resolution soundings and spatial analysis of available shear magnitudes are messy, at best. Splotches of severe storm supportive shear values across the forecast area remain tied closer to the actual frontal boundary, from southwest to northeast Minnesota through the afternoon and evening, eventually coinciding into the nighttime hours when a local capping inversion is more likely.

The two synoptic factors or scenarios that would increase local severe threat through today would be a stronger wave lifting north out of the Southern Plains this morning or limited eradication of low level moisture this morning.

Severe Storm Potential Timing, Location, & Impacts Today:

Current confidence for damaging winds as the main hazard primarily along southwestern peripheral counties in northeast Iowa potentially into bordering counties of southwest Wisconsin and southeast Minnesota with secondary hazards of large hail and isolated tornado(es) this evening into tonight. While all hinge on northern extent of return flow and warm front location, damaging wind concerns are more widespread into western and central Wisconsin given the rapid ejection of the trough causing quick storm motions of potentially 50+ kts.

Colder with Patchy Showers Tuesday:

Eventually the upper level trough axis traverses east through the forecast overnight into Tuesday morning, advecting light rain showers into early Tuesday morning with additional cumulus popcorn shower potential into the daytime from steepening low level lapse rates within a low level CAA regime. Quite chilly showers to boot with daytime highs in the mid 50s to 60s and confidence for 925mb/850mb temperatures dropping 15+ C from 19.00Z to 20.00Z.

Colder & Mostly Dry Into The Weekend:

The colder than normal temperatures expected to stick around the Upper Mississippi River Valley until the weekend due to lingering low level anticyclonic flow locking in advection from a Canadian Continental quasi-polar airmass situated over the northern Great Lakes. Current confidence in lowest temperatures Tuesday night may graze Frost headlines along western peripheral counties in southeast Minnesota as well as northern peripheral counties in central Wisconsin Tuesday and WEdnesday nights.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 549 AM CDT Mon May 18 2026

VFR expected through much of the morning hours as MVFR ceilings remain well upstream into central Minnesota at 18.12Z TAF issuance, eventually attempting to flirt closer to the forecast area through today. Haven't accounted for potential but may be an impact through the late morning to early afternoon should low level saturation come to fruition.

Subsequent storm chances increase from southwest to northeast this evening into tonight. Storms quickly progress from with damaging winds as main severe hazard and large hail/isolated tornadoes as secondary threats. Storms exit the area with frontal passage overnight into Tuesday morning.

ARX WATCHES/WARNINGS/ADVISORIES

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