textproduct: La Crosse

KEY MESSAGES

- Isolated light showers are possible through this afternoon (10-15%), mainly south of I-90.

- Widespread rainfall is expected Friday into Saturday, heavy at times, with totals of 1-3 inches expected (30-60%) and amounts of 4+ inches possible. Localized flooding and river rises are expected where the heaviest rainfall occurs.

- Additional rainfall chances continue into the new week (20-50%) with cooler than normal temperatures expected.

DISCUSSION

Issued at 143 PM CDT Thu Sep 17 2026

Today - Tonight: Mostly dry, Isolated Showers South of I-90

The wave and weak moisture transport responsible for this morning's rainfall have largely shifted eastward this afternoon and along with them, more widespread rainfall. However, can't rule out some lingering isolated showers south of I-90 through the afternoon as lingering low level moisture across the region along with some weak isentropic ascent on the 305K surface should be enough for said development. These forcings move east overnight with cloudy conditions remaining as we head into Friday morning.

Friday - Saturday: Widespread Rainfall and Flooding Potential

A 500hPa shortwave trough is expected to eject off of the closed low currently situated across the Pacific Northwest this afternoon, moving eastward tonight while an associated surface low moves into the Upper Midwest. The warm front associated with the surface low moves into the Upper Mississippi River Valley Friday morning and stalls across the region which will be the catalyst for a widespread rainfall event across the Upper Mississippi River Valley Friday into Saturday.

South of this boundary, a very moist airmass is expected to be in place, characterized by PWATs around 1.7-1.9 inches. Moisture transport associated with a strengthening 850hPa low level jet will also nudge into the boundary, ushering the saturated airmass northward which combined with strong 850hPa warm air advection and 800-700 frontogenesis should provide ample forcing for rainfall development. This rainfall could be fairly efficient at times Friday given the saturated atmosphere though an overall lack of instability throughout the day should hinder convectively enhanced rainfall. Heading into the overnight hours Friday, the aforementioned moisture transport strengthens further per the 17.15z RAP while MUCAPE increases to around 250-500 J/kg at which time some convectively enhanced rainfall and rain rates of 1+ inches per hour would become more favorable. Storm motion vectors also parallel the boundary during this time which leads to the potential for training showers/storms as we head into Saturday morning. Given the favorable set-up for widespread rainfall which could be heavy at times, confidence is fairly high that amounts should be around 1-3 inches (30-60%) though locally higher amounts of 4+ inches are possible, especially where convectively enhanced precipitation occurs.

While confidence in efficient rainfall and a few inches of rainfall is fairly high, the location of said heaviest rainfall is less certain with a couple scenarios in place. The first and higher confidence scenario (70-80%) is that the axis of heaviest rainfall develops generally along and south of I-90 where the various CAMs, and by association the 17.12z HREF, are currently in the best agreement as to where convectively enhanced rainfall is most likely to develop overnight Friday into Saturday. The second scenario brings the heaviest rainfall a bit further north and a bit further south with those along I-90 largely being spared the heaviest rainfall (20-30%). While confidence is higher in the first scenario, some of the CAM guidance does suggest higher rainfall rates are favored across northeast Iowa which may result in this axis tending towards the second scenario. Overall, it appears to be more of a question of where will the axis of highest rainfall amounts occur rather than a question of if it will occur.

From an impacts standpoint, areal flooding and river rises are likely for areas that receive the greatest rainfall amounts and for those areas that received more abundant rainfall amounts earlier this week. Flash flooding is possible, although this threat appears to be greatest overnight Friday into Saturday when convectively enhanced rainfall and training storms are favored. For additional information, please see the Hydrology section.

Sunday - Early Next Week: Additional Rainfall and Cooler

The upper wave responsible for the widespread rainfall moves east of the region Sunday while the aforementioned upper low begins to shift eastward across the northern United States. The quasi-stationary boundary responsible for the rainfall Friday- Saturday is currently expected to remain stalled across northeast Iowa into southwest Wisconsin which should result in some additional showers and storms along the boundary as the upper low continues eastward. The upper level pattern remains fairly active through the beginning of the new week while the aforementioned boundary remains situated across northeast Iowa and southwest Wisconsin, resulting in additional chances for showers and storms (20-50%).

Apart from the rainfall potential, cooler temperatures are on the horizon with highs topping out in the upper 50s to low 60s for the end of the weekend and into next week per the 17.13z NBM, generally 5 to 10 degrees below normal for this time of year.

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1241 PM CDT Thu Sep 17 2026

MVFR to IFR cigs this afternoon will improve to VFR cigs by evening with continued VFR conditions overnight. As we head into the morning hours on Friday, rain will overspread the region with ensuing MVFR cigs and vsbys. Based on the HREF and the potential for some heavier rainfall rates, cannot rule out (10-30% chance) for some IFR vsbys reductions towards 18z as well. However, the bulk of IFR reductions likely will not occur until after 18z on Friday. Winds will remain light from the northeast for much of the TAF period before shifting east and increasing to around 10-15 kts towards 18z on Friday.

HYDROLOGY

Issued at 143 PM CDT Thu Sep 17 2026

A prolonged period of efficient rainfall is anticipated Friday into Saturday, beginning Friday morning. While the exact axis of where the heaviest rain will ultimately situate itself is uncertain, confidence is medium to high that a widespread 1-3 inches of rain will occur across the Upper Mississippi River Valley (30-60%) though areas of 4+ inches are possible where storms frequent which may lead to some instances of areal flooding.

Rises on area rivers are expected given the most likely amounts of 1-3 inches, some of which may approach action to minor flood stage. River flooding would become much more likely where showers/storms frequent and higher totals are realized.

Flash flooding is somewhat less of a concern as rain rates should be on the lower side, especially early in the event. However, any convectively enhanced areas of heavy rain will certainly pose a flash flood risk, primarily overnight Friday into Saturday when rainfall rates of 1+ inch per hour are favored. If the current trends continue, a Flood Watch will likely be needed.

ARX WATCHES/WARNINGS/ADVISORIES

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