textproduct: La Crosse

KEY MESSAGES

- Showers and storms possible (40-60% chance) this afternoon across portions of northeast Iowa and southwestern Wisconsin. An isolated strong to severe storm cannot be ruled out with wind gusts of 40 to 60 mph.

- Gusty winds develop behind a passing cold front this afternoon and into the evening. A Wind Advisory is in effect for portions of northeast IA from 1 PM to 7 PM for wind gusts up to 45 mph.

- Seasonable to slightly below normal temperatures for Thursday and onward with some precipitation chances (20-60%) late Friday and early Saturday.

DISCUSSION

Issued at 117 PM CDT Wed Apr 2 2025

Rest of Today: Storms This Afternoon, Gusty Winds into this Evening

The overall threat for any strong to severe storms across northeast IA and southwest WI has been diminishing in recent model runs which have some subsidence present throughout the mid-levels this afternoon and suggest low-topped convection. As we head towards noon, a pre-frontal mid-level trough will swing northeast of the region, with a present TROWAL warming the 500-700mb layer and thus capping this layer as well. In addition to this, a strong low-level cap is already present this morning as warm advection begins to push into the region prior to warming surface temperatures. As surface warming takes place ahead of an incoming cold front, several of the CAMs have the cap breaking and allowing for convection to the top of the secondary cap at 500-600mb where our equilibrium layer is roughly 14kft as shown in the 02.15z RAP/HRRR. A couple points of consideration need to be realized in order for convection to manifest though. The first of these is that surface temperatures need to warm quickly over the next couple of hours from the low- level warm advection. If cloud cover mitigates this or the warm advection is not robust enough, the low-level cap will remain in place and convection simply will not occur. Second, with the mid- level subsidence and stable layer present, the question is if this would suppress any convection if the lift in the low-level is not sufficient.

Assuming that convection is able to take place, gusty to potentially damaging wind gusts would be the primary threat with the 02.15z RAP/HRRR having a low-level jet of 45 to 65 kts and winds atop the equilibrium layer reaching 80 to 100 kts. As a result, any subsidence with this convection could momentum transfer a stray 40-60 mph gust or two. This is seconded by the HRRR machine learning probabilities (5-10% chance) which only have severe probabilities for wind gusts in excess of 58 mph. Given the equilibrium levels top out at around 14kft, the top of the convective layer would only get to around -5C, as a result hail seems to be very unlikely and likely you would not see any lightning with these convective cells. This may change as you head further west into north-central IA where the depth of the convective layer in 02.15z HRRR/RAP soundings is slightly deeper and may be conducive for hail and lightning production. Cannot completely rule out a weak tornado with these as 0-3km CAPE ranges from 100-150 J/kg with residual 0-1km SRH as high as 200 m2/s2 that rapidly diminishes as the cold front approaches. As the cold front progresses through the region, we stabilize the low-levels quickly and thus ending any potential for convection.

Later into the afternoon and evening, low-level cold air advection will aid in momentum transfer of stronger low-level winds down to the surface as they mix into a 35 to 50 kt low-level jet. The strongest of these wind gusts generally will be over unsheltered areas of northeast IA where the 02.12z HREF has fairly high probabilities (40-70%) for wind gusts over 40 mph. Consequently, have continued the wind advisory for portions of northeast IA through 7pm this evening.

Thursday and Beyond: Calmer Conditions with Cooler Temperatures

Looking to the end of the work week, Wind Advisory until 7 PM CDT this evening for IAZ008-018-019- 029.the overall upper-level flow pattern features a broad trough situated over the western CONUS with upper-level ridging in the east and our area squarely in southwesterly flow. Some weak upper-level ridging sneaks its way in for late Thursday and Friday allowing in temperatures with the national blend keeping highs on Friday in the lower to middle 50s for the area, perhaps some upper 50s. As we head into Friday night and Saturday, a northern stream trough phases with the aforementioned broader western trough as a developing surface low progresses northeast through the Mid-Mississippi Valley. Depending on how these two upper-level features interact, deterministic models push the northern extent of the associated deformation zone of this low into the southern half of our area. The grand ensemble (GEFS/Canadian/EC ens) depict fairly high probabilities (50-80% chance) for measurable precipitation south of I-90 but unlikely (10- 40% chance) seeing more than 0.1" late Friday and into Saturday. Overall, does not look impactful but is likely our next realistic shot at any precipitation. After this system moves east of our region, a descending upstream trough will provide some cold air advection which will push temperatures below normal for Sunday and into Monday with highs in the 40s to lower 50s south of I-94 with some upper 30s for highs on Monday in north-central WI. Cannot rule out some snow showers (15-25% chance) as it passes overnight Sunday and on Monday morning but still quite a bit of uncertainty amongst deterministic and ensemble guidance as to how much moisture will be present, so have opted to hold with the national blend for precipitation chances for now.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1010 PM CDT Wed Apr 2 2025

CIGS: MVFR cigs expected through the night and the better part of the day Thursday. Short term models scatter it out Thu evening as weak high pressure moves overhead...and favored through Thu night. That said, a few models suggest some low level RH could linger, and perhaps re-manifest as low stratus Thu night. Going to stay the SKC/SCT for now and monitor.

WX/vsby: no impacts expected.

WINDS: west staying fairly breezy/gusty through Thu morning. Winds will be diminishing by mid/late afternoon Thu as high pressure starts to influence the region. Light north/northeast Thu night.

ARX WATCHES/WARNINGS/ADVISORIES

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