textproduct: La Crosse

KEY MESSAGES

- A complex severe weather forecast for this afternoon as possibly two waves of storms are in the offing, with the strength and timing of the first wave of storms playing into how the second wave of storms evolves.

- Large hail up to 2-3" in diameter, damaging winds of 60 to 70 mph, a few tornadoes (some strong), and locally heavy rainfall are all on the table with these storms.

- Much colder for the weekend with low temperatures falling below freezing starting tonight through Sunday night. Daytime highs over the weekend will be stuck in the 40s. Can't rule out some flurries on Saturday.

- Dry conditions and warming temperatures expected for the start of next week.

DISCUSSION

Issued at 430 AM CDT Fri Apr 17 2026

Severe Weather Threat for this Afternoon

Even at the 11th hour, some critical details on the evolution of our afternoon severe weather threat remain to be nailed down. This is a situation where attention needs to be paid to the weather throughout the afternoon as the threats unfold.

The early morning surface analysis across the Upper Midwest shows our culprit cold front stretching from Sioux Falls, SD to International Falls, MN and marching steadily to the ESE, aided by an approaching seasonably strong upper tropospheric northern stream trough and strong CAA pursuing over the Dakotas. A 994-mb surface low pressure cell over eastern Nebraska will propagate quickly northeast along the front, lifting a warm front with it through the region this morning. Strong surface theta-e advection is being observed across western Iowa ahead of this low with dewpoints reaching the low to mid 60s just ahead of the front. This is a classic "nick of time" severe weather scenario where the favorable thermodynamic severe weather environment will arrive just ahead of storm initiation.

There are two windows of opportunity for storms to form later this morning into the afternoon, one wave along the leading warm frontal boundary and the second with the cold front coming through a few hours later. Rapid destabilization is progged to occur in the warm sector as the warm front lifts through the region between 16-19Z with warm sector MLCAPE values of 2000-2500 J/kg and deep shear profiles of 30-50+ kts. Low level hodographs along the warm front will feature very favorable profiles in the 0-1 to 0-6-km layer supportive of strongly rotating updrafts and tornadoes. This parameter space exists for several hours in the warm sector in the wake of the warm front until the cold front blasts through between 18-00Z today from west to east.

The timing of the storms will be absolutely critical today to determine how the exact severe hazards will unfold with several different scenarios in play. The latest runs of the HRRR are favoring elevated convection developing north of the triple point and kickstarting convection down the warm front as it lifts into the forecast area. However, the updraft strength in these model runs has not been overly strong with this round of convection at first, likely owing to the mean storm motions (for storms developing along the front) outpacing the boundary and resulting in the storms tracking into a much higher inhibition airmass ahead of the front before deviant right moving storm motions can be achieved. A few CAM solutions show more robust updrafts able to slow down and stay within the more favorable thermodynamic environment, so how the threat along this boundary evolves may have to be assessed on a storm by storm basis in real time.

Can't fully rule out more open warm sector convection lagging the warm front given the dynamic environment in place, which would result in storms posing an all hazards risk rather quickly. This looks to be more of a lower confidence, but potentially higher impact scenario.

Attention then turns to the cold front as it cruises to the ESE through the forecast area this afternoon. Recent CAM runs are hinting at a better risk for larger and more robust storms further south along what would now be more of a prefrontal trough versus a true warm front in Iowa. Should robust convection develop within this corridor--the inflow for our cold frontal storms--this may choke off development further north along the cold front. How far north remains to be seen. A linear upscale growth progression is expected along the front as the afternoon goes on, with the line advancing out of the region by 00Z. Both supercellular and QLCS tornadoes would be possible along the line given the favorable respective shear profiles for both setups.

Alot to happen in a short 6-7 hour window this afternoon and only small changes to the timing of any one of these variables could have a domino effect on the rest of the event.

This Weekend & Early Next Week: Drier & Cooler

As we head past today, cooler air works its way in within the broader troughing regime on Saturday with highs likely plummeting into the 40s to lower 50s for the weekend. Cannot rule out some flurries or light snow showers north of I-94 during the morning hours on Saturday. Overnight lows on Saturday night will likely fall below freezing for many spots so be sure to cover any plants you have set out! However, no accumulations would be expected. Otherwise, surface high pressure moves into the region for Sunday keeping us relatively precipitation free into early next week.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 545 AM CDT Fri Apr 17 2026

A transient band of LIFR stratus working into central Wisconsin should lift through the mid morning today. An MVFR CU field develops towards late morning and persists into the afternoon. Thunderstorms are expected to develop today along and ahead of a cold front that moves through during the afternoon, some of which could be severe and produce large hail and damaging winds. South to southeast winds increase through the morning to 10-20G25-30kts early this afternoon with winds backing to the northwest at 10-20G20-25kts behind the front for the evening and overnight. A band of IFR to MVFR stratus may follow in the wake of the cold front, becoming more scattered as the night goes on.

HYDROLOGY

Issued at 430 AM CDT Fri Apr 17 2026

A Flood Watch has been issued for Southwest Wisconsin and portions of West Central Wisconsin given the chance for additional showers and thunderstorms this afternoon and evening. These areas have already seen copious amounts of rainfall over the last week with many rivers experiencing signficiant flooding (enough to close some roads in multiple counties). A widespread 1/2 inch of rain is likely with localized pockets of 1-2 inches--any additional rain will be problematic and only exacerbate existing problems. Mudslides also look to be a problem as we haven't seen a full green up yet so roots have not taken a strong hold yet. Rivers across Wisconsin are likely to see additional rises, especially the Kickapoo, Yellow, Black, and Wisconsin.

ARX WATCHES/WARNINGS/ADVISORIES

WI...Flood Watch from 1 PM CDT this afternoon through Saturday morning for WIZ042>044-053>055-061. MN...None. IA...None.