textproduct: La Crosse

KEY MESSAGES

- Widespread rainfall is expected Friday into Saturday, heavy at times, with totals around 1-4 inches expected, amounts of 5+ inches possible (5-15%, mainly in NE IA), and amounts up to around 7" cannot be ruled out. Localized flooding and river rises are expected where the heaviest rainfall occurs. While the location of the heaviest rainfall remains uncertain, confidence in occurrence was high enough (50%) to issue a Flood Watch including areas along and south of I-90 and north of US Highway 18.

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

DISCUSSION

Issued at 1223 AM CDT Fri Sep 18 2026

Widespread rain Friday into Saturday

WV satellite shows and upper high over the ArkLaTex region, a low moving over northern CA, and a longwave trough advancing over eastern Canada. Multiple shortwaves are seen within southwesterly flow aloft from the Four Corners to the northern Plains. At the surface, lee pressure falls are occurring in NE CO with southerly return flow in the southern Plains to the east of this feature, a high is building in western Ontario in the wake of the Canadian upper trough, and a quasi-stationary front is present roughly along Interstate 70 where the northeasterly winds associated with the Ontario high meet the northern extent of the return flow.

Friday into Saturday, aforementioned front will tighten as it is forced northward by the strong return flow across the central CONUS. Broad lift across the area looks to occur by mid-morning as a disturbance ejects over MN so could see a few lighter showers early. As time goes on, strong return flow will lead to PWATs approaching 2", near the top of climatology. This return flow will also serve to both force the front northward and tighten the front. As 850/700mb frontogenesis increases over the CWA in the afternoon, additional showers will occur. Over time, the front should stall out over the forecast area and, as a LLJ ramps up during the evening, rounds of moderate to heavy rain will occur with a few rumbles of thunder as elevated instability increases. Overall, 17.19z NBM QPF values suggest most areas have a 50% chance for final rain totals to be within the 1-4" range and a small (5-20%) chance for more than 5", mainly in northeast Iowa north of US Highway 18. Taking a look at the 17.12z HREF and 17.18z REFS to get a sense of high end possibilities, probability-matched mean values of 5-7+" are seen between I-90 and a latitude defined by the IA/MN border. Needless to say, this would lead to flooding issues for affected basins and affected paved areas, as discussed further in the Hydrology section below.

Cooler with additional shots at rain next week

The front that fills the focus of all our rain problems Friday into Saturday looks to be shunted back southward as the Ontario surface high continues to build. This will lead to cooler temperatures through next week. Quasi-zonal flow aloft favors periodic potential for rain as disturbances translate downstream. With the aforementioned front favored to be to our south, best chance for additional rain during each of these periods looks to be in northeast Iowa and southwest Wisconsin. No periods stand out as having a higher amount of hazard but, depending on where heaviest rain falls Friday/Saturday, any additional rain in NE IA/SW WI would be unwelcome and delay falls in rivers.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 543 AM CDT Fri Sep 18 2026

Low clouds will remain in place, leading to MVFR or lower flight categories. Rain showers will move in over the region for much of this 24 hour period, resulting in lower visibility with the heavier rainfall. Isolated thunderstorms are possible, but the potential and confidence in a storm being near a terminal are too low to mention in the TAF at this time. IFR to LIFR ceilings are likely towards the later half of this TAF cycle. The wind is forecast to be out of the east with the potential for gusts to reach 20-25 kts this afternoon into this evening.

HYDROLOGY

Issued at 1223 AM CDT Fri Sep 18 2026

With highly moist profiles expected, efficient rain rates could lead to flooding issues, particularly in paved areas with known runoff problems, such as Austin MN. As for river flooding, just a few miles will make all the difference with regards to which basins are most affected but ensemble river forecasts suggest minor to moderate flooding would occur in basins where the higher end amounts discussed above occur with a small (5% or less) chance for major if high end amounts affect multiple areas within one of the quicker responding basins (i.e. Trempealeau, Black, or Kickapoo).

Given the slowing movement of the front and the potential for repeat generation of showers and thunderstorms along it, flooding concerns in urban areas and within any basins are high enough to warrant a Flood Watch for Friday evening into early Saturday afternoon. Have endeavored to include counties with the best signal for greater than 5" of rainfall per 17.12z HREF and 17.18z REFS PMM QPF output, mainly along and south of I-90 and north US Highway 18. This may need to be adjusted in space and time as more shorter term guidance arrives with exact locations remaining uncertain.

ARX WATCHES/WARNINGS/ADVISORIES

WI...Flood Watch from this evening through Saturday afternoon for WIZ041-042-053>055-061. MN...Flood Watch from this evening through Saturday afternoon for MNZ086>088-094>096. IA...Flood Watch from this evening through Saturday afternoon for IAZ008>011.