textproduct: La Crosse

KEY MESSAGES

- Hail threat lowers late tonight and Sunday morning.

- Severe weather threat on Sunday and Sunday night shifts west.

- Severe weather threat from Monday into Tuesday morning is becoming more uncertain.

DISCUSSION

Issued at 124 PM CDT Sat May 16 2026

This Afternoon

A dry line will continue to slide south through northern Iowa, southern Wisconsin, and northern Illinois this afternoon. Surface dew points will fall into the 30s and 40s north of this dry line. This will lower the afternoon relative humidities into the teens and 20s for much of the forecast area. The dew points were lowered closer to the CONShort which seems to be handling this dry air the best. Even then, the dew points are not low enough south of Interstate 90.

Further south across northern Missouri and central Illinois is the outflow boundary from the storms that moved through our area last evening. With convection continuing along this boundary, it will likely remain quasi-stationary.

Tonight and Sunday

The boundary across northern Missouri will move slowly north as a warm front in response to the nocturnal jet tonight and the surface low lifting northeast out of the Central Plains on Sunday. While there is good 1 to 7 km shear located north of the warm front, the instability stays south of the warm front. As a result, we have seen a significant drop in the probabilities of seeing elevated hail for tonight and Sunday morning.

In an interesting development in the 16.16z HRRR, a MCS develops over eastern Nebraska and western Iowa tonight. As this system moves toward our area, it gradually weakens. One thing that will have to be watched is whether the rain-cooled air produced by this MCS will limit the northern extent of the warm front on Sunday. This could potentially keep the high temperatures in the lower to mid-60s north of Interstate 94. This would be 10 to 15 degrees cooler than the NBM.

Sunday night

Plenty of uncertainty is showing up in the CAMs. While many are in general agreement that storms will initiate over western Iowa, eastern Nebraska, southwest Minnesota and eastern South Dakota during the late afternon/evening, they then diverge on their solutions. Some show a line developing as their cold pools coalesce and others show clusters of storms. In either scenario, the storms are trending to weaken as they move into the Upper Mississippi River Valley due to weakening instability and shear. As a result, the Day 2 slight risk has been shifted west. It now is located along and west of the Mississippi River

Monday into Tuesday morning

Much of uncertainty exists on the timing of the cold front moving through the area. This ultimately affects the location of the warm sector. The GFS and many of its ensemble members move the front across the area on Sunday night or Monday. The 16.12z operational GFS was further west than its 16.16z run. It has 1500 to 3000 J/kg CAPEs in northeast Iowa, and southwest and central Wisconsin. While there is more instability, the better deep shear is post frontal. The operational ECMWF and many of its ensembles has its cold front moving through on Monday night. With this timing, the instability (up to 750 J/kg) is much less ahead of this cold front. Its has deep shear over 40 knots north of Interstate 94, so there may be some supercells in this area during the evening.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1256 PM CDT Sat May 16 2026

No impacts to aviation are expected through this afternoon. As we head into the evening hours, after 00z, showers and storms begin to move northward into northeast Iowa and southwest Wisconsin (20-40%), but confidence in how far north they can progress is low. They may only pose an isolated hazard into the overnight hours for those sites south of Highway 18. Northwest winds this afternoon begin to veer to the southeast tonight.

Confidence in showers and perhaps a thunderstorm increases Sunday morning as a line of showers/storms moves northward (60-80%). MVFR to IFR conditions are expected to accompany these showers/storms as cloud bases fall to near 1kft. Visibilities are expected to fall generally to 4-5SM, but more vigorous areas of precipitation may result in IFR visibilities. Winds will also increase during this period, with southeast gusts of 15-25KT for most while those west of the Mississippi River may see gusts upwards of 30KT.

HYDROLOGY

Issued at 440 AM CDT Sat May 16 2026

Storm chances increase daily, initially tonight through Sunday morning. The accompanying anomalously moist airmass raises heavy rain concerns with a widespread 1" to 2" of rainfall expected through Monday. While low lower confidence in specific repeat storm locations over the next 72 hours limits overall confidence in location of higher rainfall amounts, 2" to 3" will be possible should storms repeat over similar areas.

Resultant currently low confidence (10-20%) for minor river flooding impacts limited to flashier creeks and rivers in northeast Iowa such as the Turkey River at Garber and Elkader.

ARX WATCHES/WARNINGS/ADVISORIES

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