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FXUS63 KARX 270423 AFDARX

Area Forecast Discussion National Weather Service La Crosse WI 1123 PM CDT Fri Apr 26 2024

KEY MESSAGES

- Scattered showers and storms continue tonight with gusty winds blowing 40-50 mph at times.

- Conditions trending drier on Saturday. Strong to severe storms could develop Saturday evening in northeast Iowa into far southwest Wisconsin. Large hail and damaging wind are the primary threats.

- Widespread showers and storms likely on Sunday with a healthy dose of rainfall on tap.

DISCUSSION
Issued at 315 PM CDT Fri Apr 26 2024

Showers and Storms Tonight:

Broad cyclonic flow exists this afternoon across the western and central CONUS with a compact negatively-tilted shortwave ejecting northeastward through the central Missouri River valley. A maturing 991-mb surface cyclone was positioned near Hastings, NE at 18Z and is progged to lift northeast into central Minnesota by sunrise on Saturday. The leading wave of showers rooted along the attendant 700- mb theta-e advection wing lifting through our area decayed through the morning as they outran the axis of supportive MUCAPE, but enough evaporative cooling occurred on the backside of these dissipating showers to transport a 50-60-kt LLJ down to the surface west of the Mississippi River, producing surface wind gusts of 45-55 mph in their wake.

The next round of showers rooted closer to 850 mb warm frontal nose have started developing over central Iowa and lift through the region this afternoon and evening, once again outrunning the axis of elevated upright instability but the possibly still putting out lightning as the move through given the low static stability.
Attention then turns to the line of linear/quasi-linear convection working northward from western Iowa towards the 03-06Z timeframe.
There is some uncertainty with how far north the surface warm front will lift and the exact timing of these storms in relation to this frontal progression will dictate the possibility any QLCS tornadoes over northeastern Iowa. Surface wind profiles and hodograph trajectories within the warm sector and along the front would be supportive of QLCS tornadoes--if the warm sector makes it this far north. The latest few runs of the HRRR have been hinting at a northward push of the warm sector ahead of this line, so the evolution of this mesoscale environment bears watching through the evening. The severe weather threat wanes steadily once the storms overrun the front north of Iowa.

The mid-level dry slot cuts across the forecast area in the wake of this overnight convection, bringing an end to the precipitation by sunrise on Saturday morning.

Severe Storm Threat Saturday Afternoon/Evening:

Confidence is increasing for a drier Saturday than previously forecast thanks to weak upper-level ridging and the aforementioned dry slot. Gusty winds hold on through the first half of the day, but gradually subside Saturday afternoon as the pressure gradient relaxes. Southeast gusts look to top out in the 30-35 mph range area wide with sustained winds of 20-25 mph.

Despite the broader synoptic-scale environment lacking organized ascent, thunderstorm chances increase across northeast Iowa into far southwest Wisconsin Thursday evening. Weak shortwave perturbations in the mid-level flow provide the best forcing with this system, as frontal boundaries in the area lack baroclinicity. Furthermore, an elevated mixed layer is present in model soundings. The CIN associated with the EML increases as you move northward across the forecast area. A few Dubuque, Iowa model soundings show the potential to tap into 1500-2000 J/kg of CAPE as the EML erodes in the evening, but as you move northward the chance of eroding enough CIN to freely convect decreases rapidly. The chance of convective initiation will be highly correlated with how much daytime heating is achieved tomorrow afternoon with differences remaining across models, especially when it comes to cloud cover.

If storms are able to initiate, a narrow corridor of CAPE 1500+ J/kg and deep-layer sheer over 40 knots across northeast Iowa into far southwest Wisconsin could lead to storms quickly becoming strong to severe. Large hail and damaging wind are the main threats, but a tornado or two cannot be ruled out if storms are surface based.

As the evening progresses, the atmosphere will stabilize putting an end to the severe weather threat. Showers and sub-severe storms will likely increase in coverage as potential vorticity advection increases across the Upper Mississippi Valley.

Soaking Rain Sunday:

Widespread showers and storms take hold on Sunday as a low pressure system approaches from the southwest. Heavy rain is the primary threat with NAFES precipitable water values at our above the 95th percentile for this time of year. This will result in 1-1.5" of rain area wide, with locally higher amounts possible. As it stands, the area can take this much rain with minimal impacts but we will continue to monitor the potential for urban street flooding and minor river/stream rises as this system approaches.

Looking Ahead To Next Week:

An active weather pattern continues into the new week with shortwaves propagating in the primarily zonal upper-level flow. This pattern supports periodic preciptiation chances and seasonable temperatures. More details to come after we get through this weekend's push of precipitation.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/
Issued at 1123 PM CDT Fri Apr 26 2024

MVFR/IFR cigs will persist overnight and into the morning hours across the region. Showers and a few storms will continue to push through the area overnight and into the early morning hours. As past sunrise, precipitation will exit allowing for some diurnal mixing to allow for some clearing of low cigs to MVFR or low VFR heights. Winds throughout the overnight will remain at around 12-20 kts with gusts up to around 25 kts across much of the region, maybe lower in valley locations that are able to decouple. As a result, some LLWS is possible in valley locations where winds subside with the strong 50 kt low-level jet.
Otherwise, winds will gradually lessen later in the day tomorrow to around 5-10 kts by the end of the TAF period and turn westerly.

ARX WATCHES/WARNINGS/ADVISORIES
WI...None.
MN...None.
IA...None.

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