textproduct: La Crosse

KEY MESSAGES

- Widespread rainfall persists through Saturday morning with rainfall totals of 1-3" likely for much of the local area. Locally higher amounts of 4-6"+ will be possible. Flash flooding and rivers rising into flood stage may occur where these heaviest amounts fall. A Flood Watch is in effect for much of the area south of I-94 to address this flooding potential.

- An additional round of storms will be possible (40-70% chance) in southwest WI and portions of northeast IA on Saturday afternoon/evening. Locally heavy rainfall will be possible with these storms which may result in further flooding potential.

- Rain chances (20-50%) persist through Monday before trending drier for the remainder of the upcoming week with cooler than normal temperatures expected.

DISCUSSION

Issued at 220 PM CDT Fri Sep 18 2026

Rest of Today - Saturday Morning: Widespread Rain, Heavy Rain Leading to Flooding Concerns Overnight

Rainfall has spread into portions of the region as stratiform rainfall from the deepening surface low we have advertised over the last couple of days is pushing eastward. With some dry air intrusion through portions of Iowa, have noticed a lull in precipitation generally near and south of I-90 this afternoon, which much of the CAMs suggest will persist into the early evening. The key mechanism we are watching for which will come later this evening and into the overnight is a fairly profound surge of 850mb moisture transport as the nocturnal low-level jet enhances and interacts with a quasi-stationary boundary situated over the region. As this occurs, the recent RAP generally agrees on advecting some more favorable MUCAPE into the region which in turn switches precipitation mode from stratiform to convective as illustrated uniformly across all of the CAMs. Consequently, heavy rain and flooding concerns begin to increase across much of the area during the overnight as precipitable waters approaching 2 inches and warm cloud depths over 4km would suggest a substantially primed environment for heavy rainfall with fairly strong sloping frontogenesis where the instability can reside atop this. In addition, the moisture transport vector may eventually turn more westerly which may aid in allowing for some backbuilding convection overnight and into the early morning hours, however the CAMs disagree to what extent this occurs as the stronger 0-3km shear vector present in the RAP would suggest some MCS organization which could keep convection more progressive.

As far as amounts are concerned, the inter-quartile range in the NBM for rainfall amounts over generally ranges between 1-3" across much of the region. When looking at the 90th percentile to ensemble maximum of the NBM, some pockets of 4-6"+ begin to show up. As a result, would think that the general story with this event will be a widespread 1-3" of rainfall but localized higher amounts where storms manage to traverse the same locations. Consequently, have expanded the Flood Watch to encompass the area where these higher amounts may be possible. If a more northern solution does take place with this system, portions of northeast IA may underachieve for rainfall amounts overnight with a few members in the lower percentiles of the NBM having amounts under 1". For hydrological impacts across the region in light of this rainfall, refer to the hydro section for additional details.

Aside from the rainfall impacts, cannot fully rule (under 10% chance) out some stronger storms overnight as well as the aforementioned instability pushes into the region. In general, shear profiles in RAP hodographs are not overly impressive, however modest mid-level lapse rates of around 6-6.5 C/km and MUCAPE values approaching 2000 J/kg may allow for some small hail or gusty winds with any descending cores, granted the heavy precipitation environment here would likely limit hail size substantially. In any case, storms will likely exit the region by early Saturday morning with the heavy rain and strong storm threat quickly diminishing after sunrise.

Saturday - Monday: Cooler, Additional Heavy Rain Possible Saturday Afternoon/Evening in Southwest WI and Northeast IA

After we get past this system, continued rain chances remain across portions of the area as an upper-level trough/low slowly pivots through the Great Plains with the aforementioned boundary still in place. Consequently, additional periods of rainfall remain possible through Monday with zonal flow in place with some 850mb moisture transport running overtop this. The most reasonable shot for additional rainfall would likely be Saturday afternoon and into the evening for portions of southwest Wisconsin as a more profound axis of 850mb moisture transport pushes north into southwest Wisconsin as shown in the NAM/GFS. Will need to watch this as convective initiation with this forcing begins to develop in far southwest Wisconsin and northeast Iowa by afternoon which could lead to additional heavy rainfall. Overall, confidence is low in how this would manifest, but some of the CAMs suggest some pockets of additional amounts of around 1-2" could be possible where storms form which could certainly result in flooding issues considering tonight's rainfall.

Otherwise, the quasi-stationary boundary responsible for all of this will gradually sag south into Sunday and Monday so would expect precipitation potential to gradually diminish into Monday. The NBM suggests some uptick in precipitation probabilities into Monday as the aforementioned main upper-level trough/low pivots over the area on Monday. However, probabilities in the grand ensemble (GEFS/EC/Canadian ensemble) for amounts over 0.25" are fairly low (10-30%) during this period, so not expecting much for impacts with this round of precipitation.

Tuesday - Next Friday: Drier with Below Normal Temperatures

As we head into Tuesday, high pressure situated near the Hudson Bay and northeasterly surface flow will aid in advecting some drier and cooler air into the region throughout next week. Consequently, not seeing any significant signals within the NBM and other ensemble guidance for precipitation chances during this period. Cloud cover will likely linger with the aforementioned departing upper-level low/trough on Tuesday, however as some upper-level ridging pushes into the region by Wednesday morning, cloud cover will clear. Will be curious to see if we can manage to get some frost on Wednesday morning, particularly in more typical cold spots of west/north-central Wisconsin as the lower percentiles of the NBM have lows falling into the middle to upper 30s in these areas on Wednesday. However, as this ridging builds into the region, would expect a gradual warming of temperatures through the end of the work week.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

Issued at 624 PM CDT Fri Sep 18 2026

While a few more hours will see MVFR ceilings in many locations, IFR conditions will predominate over the next 24 hours as rain continues and stratus lowers overnight. Thunderstorms are expected to roll eastward between 02z and 10z. With timing and exact location still uncertain, have included TEMPO groups with TS at RST/LSE. Expect IFR/MVFR to continue for many hours beyond the end of the period with VFR possibly returning midday Sunday.

HYDROLOGY

Issued at 220 PM CDT Fri Sep 18 2026

Widespread rainfall amounts of 1-3" will likely result in numerous rivers to rise into action stage in addition to a few sites expected to rise into minor flood stage such as the Gays Mills, WI and Viola, WI along the Kickapoo River and Charles City, IA along the Cedar River. The key question that remains where corridors of locally heavy rainfall may fall which may result in more significant flooding depending on the basins they fall into. Basins that see 4- 6"+ of rainfall may see rises in moderate or perhaps major flood stage (under 10% chance), particularly the quicker responding basins such as the Trempealeau, Black and Kickapoo River Basins.

As far as urban and flash flooding is concerned, the areas most susceptible at this time would be along a line from Fillmore County, MN to Adams County, WI as these areas received 2-3" of rain earlier this week. In order to get flash flooding, repeat thunderstorms would be needed given the somewhat progressive nature of the complex overnight. However, given some tendency in the HREF PMM to show rainfall amounts in the 4-6" range overnight in some locations, this coupled with the fact that 6-hr RFC Flash Flood Guidance is around 2-3" would suggest that flash flooding is feasible where these locally higher amounts occur. Overall, there has remained a somewhat consistent signal for the majority of the members to favor these higher amounts near I-90 in southeast MN and east-southeastward into western WI. However, given that various CAMs and deterministic guidance has some deviation by a couple of counties from this aforementioned axis, have opted to expand the Flood Watch to include the entire local area near and south of I-94.

ARX WATCHES/WARNINGS/ADVISORIES

WI...Flood Watch through Saturday afternoon for WIZ032>034-041>044- 053>055-061. MN...Flood Watch through Saturday afternoon for MNZ079-086>088- 094>096. IA...Flood Watch through Saturday afternoon for IAZ008>011-018-019- 029-030.