textproduct: La Crosse

KEY MESSAGES

- Storms develop this evening west of the Mississippi River and push east overnight. Storms may become strong to severe with large hail being the primary threat initially before transitioning to a damaging wind threat later into the evening.

- Additional storms are likely both Saturday evening/night and Sunday evening. While strong to severe storm potential appears more limited on Saturday night, severe potential increases more on Sunday with much of the area in a slight risk (level 2 out of 5) for severe weather.

- Monday currently has the greatest risk for severe weather over the next four days across the entire local area. Large hail, damaging wind gusts and tornadoes are on the table in this setup as storms move through the region on Monday.

- With multiple rounds of storms expected this evening through Monday, rises on area rivers into flood stage will be possible where storms frequent the same locations.

DISCUSSION

Issued at 135 PM CDT Fri May 15 2026

This Evening & Tonight: Storms Likely, Several Strong to Severe Possible

A fairly notable increase in convective coverage has been observed across the CAMs over the past couple of forecast cycles resulting in increasing confidence for strong to severe storms later this evening and into the overnight hours. Currently, dewpoints have noticeably increased this morning, reaching into the 50s as of noon for many locations. While dewpoints are increasing, soundings at this point remain well mixed with a fairly dry boundary layer. As we head over the next few hours, the conceptual model in this case will essentially be a waiting game for the boundary layer to become saturated sufficiently such that you can lift saturated parcels into the deepening convective layer. Once this occurs, expecting storms to develop quickly (around 6-8pm as shown in the CAMs) along a stationary boundary over portions of northern IA through southeast MN and into western WI. At convective initiation, the environment seems primed for supercells at first, capable of large to even very large hail with mid-level lapse rates to around 7 C/km and very elongated hodographs. This coupled with deep inverted-v soundings would suggest damaging wind threat with any RFD in developing supercells.

However, as we head later into the evening and storms move eastward, deep-layer shear begins to weaken which will result in storms clustering and trending more linear with time as they approach the Mississippi River as hinted at in a number of the CAMs. As a result, expecting the threat for hail to decrease with a maintaining damaging wind threat. Some tornado potential cannot be ruled out during this period either as low-level hodograph curvature increases as the low-level jet ramps up in addition to greater saturation in the boundary layer after dark. However, this is contingent on if we can get saturation closer to the surface as LCL heights at convective initiation would not be conducive for tornadoes. Eventually, storms begin to push eastward into western WI and eventually exit the area after midnight. Not anticipating any flooding concerns with this first round of rainfall with amounts generally ranging in the 0.25" to 1" range with storms being fairly progressive in their movement.

This Weekend: Additional Rounds of Storms

As we head into tomorrow, skies clear out for the morning and afternoon hours on Sunday with zonal flow persisting aloft. However, as we head into the evening and overnight hours in particular on Saturday, a weak axis of 850mb moisture transport transverses a stationary boundary situated across portions of northeast IA and southwest WI. As this occurs, some of the CAMs initiate convection across portions of eastern IA that slowly pushes northward through the overnight and into Sunday morning. Overall, the environment is not overly favorable during this period as instability is relatively limited, generally under 1000 J/kg. Consequently, this coupled with weak deep layer shear should keep this to more of a stratiform precipitation band with embedded convective cells with limited severe potential.

The more significant implication of this round of convection on Sunday morning will be how much we recover going into the afternoon and evening on Sunday. Currently, the synoptic factors in play to advect moisture into the region are quite favorable as there is a profound 850mb moisture transport axis that pushes north into the local area on Sunday evening. As a result, instability values (2000- 3000 J/kg of MLCAPE) increase rather quickly during the late afternoon and into the evening with the moisture transport axis aiding as a focus for convective development. Shear profiles are somewhat displaced from the instability with the main synoptic trough situated over the rockies still with a stronger upper-level jet streak to the north. As a result, storm updrafts may struggle to maintain, however given the aforementioned ample instability and mid- level lapse rates of 8-9 C/km, this would certainly still favor a severe hail risk. Damaging wind gusts cannot be fully ruled out with any descending hail cores, however given a noticeable capping inversion in the NAM/GFS soundings during this period, storms will likely be elevated in nature which would also minimize the tornado threat as well.

Monday: Potential for Severe Storms Areawide Monday

Monday currently appears to have the greatest risk for severe storms areawide over the next 4 days as the previously advertised upper- level trough pushes through the Great Plains and into the Upper Midwest and becomes neutral to perhaps negatively tilted on Monday evening. As this occurs, cyclogenesis occurs at the exit region of this feature with a pronounced cold front in eastern MN. This coupled with a fairly profound 850mb moisture transport axis into the region will enable ample moisture to push northward through the area with the grand ensemble (GEFS/EC/Canadian ensemble) having very high probabilities (80-100%) for dewpoints over 65 degrees. As a result, instability profiles are fairly impressive across deterministic and ensemble guidance with modest probabilities (50- 70%) for MUCAPE exceeding 1500 J/kg on Monday evening in the EC ensemble. The one question that remains is exactly how shear profiles will manifest as the 250mb jet streak is somewhat displaced from the main 500mb shortwave. As a result, while hodographs exhibit some low-level and mid-level elongation, the upper-levels are more disorganized. Consequently, this may result in a QLCS storm mode assuming upper-level support continues to remain absent which would favor a damaging wind and embedded tornado threat. That being said, ample instability and mid-level lapse rates of 7-8 C/km in the recent GFS would suggest at least some severe hail is possible as well, particularly if we can get any supercellular structures going.

The environment for heavy rainfall in this particular setup seems rather favorable as well with precipitable waters pushing over 1.5" which are about the climatological maximum based on KDVN sounding climatology. The key question from a heavy rain/flooding perspective will be: 1. How the rainfall from the previous rounds of storms manifests and 2. How progressive the storm evolution is on Monday evening and if we have multiple rounds of discrete and linear modes throughout this timeframe. Overall, the inter-quartile range in the grand ensemble (GEFS/EC/Canadian) for rain amounts with just this Monday round ranges from 0.25" to 1" with 95th percentile values up to 2". So certainly could contribute further to any flooding potential where storms occurred from previous rounds.

In all, Monday appears to be a fairly active day from a severe weather perspective with all hazards on the table depending on how all the various severe weather ingredients shake out. Now is the time to be thinking about your severe weather safety plan and have multiple ways of receiving warnings if you have not already done so, especially considering each day from this evening through Monday contains at least some degree of risk for severe weather.

Tuesday - Thursday: Cooling Trend

As we head past Monday and head later into the week, cold air advection behind the front will allow for cooler temperatures to move in for the middle of the upcoming week. As a result, the 10th to 90th spread for highs in the NBM falls to the upper 50s to upper 60s for much of the local area on Wednesday, likely our coolest day for the upcoming week.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1221 PM CDT Fri May 15 2026

Main taf concerns are thunderstorm chances this evening at both RST/LSE taf sites. Convection is expected to develop late this afternoon/evening and move across the area this evening. Timing of convection at both taf looks to be 23z today to 04z Saturday. There could be a brief period of MVFR/IFR conditions under the heavier showers/storms. Have introduced a tempo for this brief period potential. By 05z Saturday convection will move east of the taf sites and VFR conditions are expected to prevail after 05z Saturday.

ARX WATCHES/WARNINGS/ADVISORIES

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