textproduct: La Crosse

KEY MESSAGES

- Widespread rainfall potential increases this afternoon, continuing through Thursday. The heaviest rainfall is favored across northeast Iowa into southwest Wisconsin where the potential for 2+" of rainfall is 40-60%. Localized amounts of 3-4+" are possible.

- High temperatures will remain seasonal in the 60s to 70s.

DISCUSSION

Issued at 1230 AM CDT Tue Sep 29 2026

Today - Thursday: Widespread Rainfall

An upper level low is expected to develop across the Desert Southwest this morning after becoming cutoff from a longwave trough across the western United States. After this cutoff occurs, the longwave trough continues eastward across the northern United States and knock's on the Midwest's door this afternoon. Ahead of these features, southerly flow advects tropospheric deep moisture across the Central Plains into the Midwest, characterized by PWATs of 1.5-1.75 inches. The deep moisture and forcing associated with the upper level features will spark widespread moderate to heavy rainfall beginning this afternoon which largely continues through Thursday. The main caveat is that a break in the rainfall looks to occur Wednesday morning behind the trough and ahead of the cutoff low which begins to lift northward towards the Midwest on Wednesday, bringing another round of heavy rainfall Wednesday afternoon into Thursday.

Still some uncertainty with regard to where the axis of heaviest rainfall sets-up through midweek. The ECMWF suite (ECMWF/EPS/AIFS) have been very consistent with more southward solutions, putting northeast Iowa into southwest Wisconsin most at risk for higher rainfall amounts. In comparison, the GFS suite (GFS/GEFS/AIGFS) has been more variable in the location of this heavy rain axis having favored a northward solution 24 hours ago, but has begun to trend southward towards the ECMWF solutions in recent runs. Currently thinking northeast Iowa into southwest Wisconsin are the most likely areas to see the highest rainfall totals given these trends, especially for those along and south of Highway 18. However, where this axis sets-up may still change further as there is the potential for the heaviest rainfall to largely remain south of the area and across central into eastern Iowa (20-30%). This is seen in the 28.12z LREF cluster analysis, but the EFI has also largely kept the greatest signal for anomalous rainfall potential to our south.

Regardless of where the heavy rainfall axis sets-up, currently expect the heaviest rainfall to occur Wednesday afternoon into Wednesday night as a strengthening 850hPa low level jet surges another plume of moisture northward, this time associated with the remnants of Hurricane Polo as it gets absorbed into the mean flow when the cutoff low across the Southwest lifts northward. RAP/HRRR soundings also suggest some elevated instability may be present during this period, generally around 300 J/kg, leading to some convective enhancement and locally higher rainfall totals. Current probabilities for amounts of 2+" sit around 40-60% in the 29.01z NBM with the potential for amounts of 3-4+" where convective enhancement occurs. Flooding concern remains through Thursday across portions of northeast Iowa, southwest Wisconsin, and southeast Minnesota, especially if higher end rainfall totals are realized. Urban and street flooding along with river rises are the main concerns although flash flooding is possible, especially where showers and storms frequent in rapid succession. Rainfall ends Thursday as a cold front moves through the region and high pressure slides southeastward into the Midwest.

Friday into the Weekend: Drier and Trending Cooler

In the wake of the cold frontal passage, temperatures should slowly trend cooler this upcoming weekend as broad scale troughing attempts to dig south over the Great Lakes region. Daytime highs are not forecast to be all that much cooler, with temperatures remaining in the mid 60s. Where the cooler air will be more noticeable will be overnight, with overnight lows drop into the lower 40s to mid 30s. Dry conditions are currently expected but there may be a low chance for light rain on Saturday if moisture moves in quick enough out ahead of another southern surge in the upper level trough.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 1230 AM CDT Tue Sep 29 2026

Outside of some patchy fog in low lying areas and some of the Mississippi River tributaries, VFR conditions are expected throughout the morning hours. Showers begin to move into the region from the west this afternoon, generally after 17z (70-90%), bringing MVFR visibilities under heavier rainfall. Ceilings also begin to lower towards 3-5kft as the showers enter the region and continue to lower through the end of the TAF period when IFR conditions look to return. Otherwise, light and variable winds overnight become predominantly southerly by the afternoon with some gusts around 20KT possible west of the Mississippi River.

ARX WATCHES/WARNINGS/ADVISORIES

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