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.

UPDATE

Issued at 1215 PM CDT Fri Apr 17 2026

Continuing to see rapid evolution of the environment over the local area. Right now, the cold front is located near I35 in MN tailing southwestward into IA. Surface low pressure is southwest of Mason City at 17Z. To the east, widespread billow clouds indicate a stable boundary layer over the much of southeast MN and northeast IA. Closer to the front on I-35, the cumulus field looks primed to initiate.

Warm Front: A confluence region exists approaching the Miss river where some elevated echo is seen near Winona. SPC mesoa deep moisture convergence is along this axis N-S and the latest CAMS agree on more initiation over the next hour. That eastern environment is elevated still with 2000-2500 J/Kg MUCAPE. Development on this N-S axis is round 1 and as it heads east with the dewpoint surging northward, these storms have increasing chances at becoming surface-based in western WI. However, this is moving east at a pretty good clip and depending on the initiation of said storms, these might be surface-based and mature into central and southwest WI or east of the local area. The environment is rich, with effective helicity over 200 m2/s2 and 25-35 kts of 0-1km bulk shear now. If the low-level winds remained backed, the CAMS suggest 300 eSRH and 200 0-500m SRH in swrn WI by 2 pm. Supercells and strong tornadoes possible with probabilities higher to I90 and south in WI for this area.

Cold Front: The cold front will also initiate on the western edge of the warm sector and winds are all backed into the front right now with the surface low traveling north, over sern MN and nern IA, with loopy long hodographs there as well. The main question remains of how fast the storms becomes a line there. Initial discrete storms will work toward linear modes with 0-6 shear vectors and the 850-300 mb mean wind at 45 degrees to the front. Strong linear forcing /cold front/ seems like it may go linear than remain discrete. BUT, while discrete, the looping hodographs mean tornado potential is certain there in sern MN and western nern IA. First lightning strikes just seen along the cold front near I-35 per GOES GLM.

Tornado Watch is forthcoming.

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 /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1220 PM CDT Fri Apr 17 2026

A rapidly moving cold front is currently located just to the west of KRST as of 17/17Z. Visible Satellite and Radar Imagery show convection starting to develop along the front. MVFR to IFR Cu and stratus with gusty southerly flow up to 30 kt is being observed just east of this activity. Thunderstorms and convective showers will rapidly develop with TSRA chances with IFR or lower ceilings for KRST through 19Z. These storms will likely develop into a line with better chances for TSRA around 20Z for KLSE (+/- 1 hour). All modes of severe weather are possible including large hail, damaging winds, and lightning. General frontal turbulence is also likely. Winds will quickly shift to the NW and remain gusty behind the front shortly after. CIGs are likely to remain MVFR category are several hours and gradually lifting to VFR conditions with high clouds towards the latter end of the package.

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 through Saturday morning for WIZ042>044-053>055- 061. MN...None. IA...None.