textproduct: La Crosse

KEY MESSAGES

- An MCS moves southeastward from the Great Plains tonight bringing the low potential for a stronger wind gust to the Upper Mississippi River Valley though this potential will largely remain across central to east-central Iowa.

- Additional showers and storms are expected tonight into Friday morning, bringing the potential for heavy rainfall to portions of northeast Iowa and southwest Wisconsin (40-60%), though the heaviest rainfall looks to be south of the area.

- Periods of showers and storms are expected through the weekend, though overall severe potential remains low.

DISCUSSION

Issued at 1230 AM CDT Thu Aug 13 2026

This Morning: MCS Potential

An MCS taking shape across Nebraska will continue propagating eastward this morning as a 500hPa shortwave trough traverses nearly zonal flow across the central to eastern United States. Expectations are for this MCS and its associated impacts to remain south of our area, generally along and south of the I-80 corridor as higher dewpoints and instability reside over these areas, south of a surface boundary situated across northern Iowa. That said, some shower and storm activity is possible across northeast Iowa/southwest Wisconsin during the morning hours (60-80%) as the upper wave moves through the region. If the MCS were to track further northward, those south of Highway 18 would see some potential for a stronger wind gusts given DCAPE values of 500-1000 J/kg, PWATs around 1.6in, and 0-3km shear of 20-30kts oriented nearly perpendicular to the MCS per the 13.03z RAP, though this a low probability scenario (< 10%) given the more favorable environment will exist south of our area.

Outside of the low MCS potential, patchy fog has begun to develop across southwest Wisconsin which may extend into northeast Iowa. However, increasing cloud cover overnight will act to limit the amount of radiational cooling across the region and the western extent of fog development. Fog will begin to dissipate through the morning hours.

This Afternoon - Friday Morning: Showers and Storms, Heavy Rainfall

Some uncertainty exists regarding how the rest of today will play out after the MCS moves out of the area. Remnant outflow and differential heating boundaries associated with the morning convection look to set-up across portions of the Upper Mississippi River Valley, though where exactly these situate themselves is tricky to pinpoint. Mid-level PVA is also depicted in the deterministic models which would provide synoptic-scale ascent atop the various surface boundaries. Another point of uncertainty regarding storm development during the afternoon is the influence of cloud coverage from the morning convection and how much instability develops in the afternoon. 13.00z model soundings do suggest that MUCAPE on the order of 700-1000 J/kg develops this afternoon, primarily focused across the Minnesota/Iowa border and into far southwest Wisconsin which would support convective initiation given the forcing mechanisms in place. If storms develop, they would likely remain sub-severe given the weak instability though some elongation in the hodographs suggests a stronger updraft can't be ruled out.

Late tonight, a strengthening 850hPa low level jet and associated moisture transport noses into the surface boundary across Iowa, providing a source of lift for convective development through Friday morning. Within this region of ascent, mid to upper 60s dewpoints will be in place along with PWATs of 1.8-2in, warm cloud depths in excess of 10kft, and MUCAPE values of 1000-1250 J/kg. Cloud layer winds also look to be nearly boundary parallel per the 13.03z RAP which would suggest some training of storms is also possible. These parameters indicate the environment will be primed for heavy rainfall along the boundary as we head into Friday morning.

Where exactly the boundary will reside is somewhat uncertain given the overnight time period and rainfall cooling the atmosphere ahead of the boundary though expectations would favor a southern solution given this conceptual model. Most of the CAM solutions including the 13.00z HREF/REFS indicate the boundary will primarily remain to the south of our area, along with the axis of heaviest rainfall, which fits said conceptual model. As such, probabilities for 1 inch or more of rain are highest across central into east-central Iowa with only a 10-30% chance for those south of Highway 18 in our area. Given these current expectations, flooding potential looks to largely remain south of our area though if a northward trend begins to develop localized flooding may become more of a concern. This appears to be an unlikely scenario though as there has been a southward trend in the 13.00z REFS.

Saturday - Tuesday: Periodic Showers and Storms

With zonal flow remaining situated over the central United States, several mid to upper level shortwaves will have the opportunity to traverse the Upper Midwest through the weekend. Each of these waves brings the potential for showers and storms, primarily early Saturday morning (60-80%) and Saturday night into Sunday morning (40- 60%). There's quite a bit of uncertainty with how these waves evolve, so will need to monitor for potential impacts over the next few days though the overall severe threat is low at this time.

Apart from the periodic shower/storm chances, slightly cooler temperatures in the upper 70s to low 80s are expected into the beginning of the new week with drier conditions possible as high pressure looks to sink southward into the Great Lakes region.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1230 AM CDT Thu Aug 13 2026

VFR conditions currently present across the region with light and variable winds. A large area of showers and thunderstorms will move into the region later this morning, with better chances across Northeast Iowa and Southwest Wisconsin. As probabilities for precipitation along and north of the I-90 corridor remain below 50 percent, have opted for PROB30s at this time since rain is not likely to start until after 12Z. Rain chances for the terminals will continue through the morning hours, decreasing into the afternoon hours. Dry conditions should prevail from mid/late afternoon through the end of the forecast period. Winds later today will remain light at less than 10 kts out of a general east/southeasterly direction.

ARX WATCHES/WARNINGS/ADVISORIES

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