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Monroe Center, Wisconsin Weather Forecast Discussion

908
FXUS63 KARX 220731
AFDARX

Area Forecast Discussion National Weather Service La Crosse WI 231 AM CDT Mon Sep 22 2025

.KEY MESSAGES...

- Showers and storms today will slowly shift southward into the evening. Some strong storms and an isolated severe storm (large hail) are possible. Locations northwest of a Mason City to Black River Falls line should receive the most rainfall /0.50-1.00" possible/, higher with repeat storms.

- Still unclear how the week will unfold but the higher probability outcome would have rain chances diminish into the 20-30% range for Tuesday through Thursday, then dry for the weekend. Temperatures will be in the 70s, 80F possible for the weekend.

&&

.DISCUSSION... Issued at 230 AM CDT Mon Sep 22 2025

Some Strong Storms Possible Today with Isolated Large Hail

GOES water vapor imagery at 22.05Z showing the large scale low over the Upper Miss Valley and western Great Lakes. Mid-level flow is enhanced from MSP southward and some splitting supercell activity was seen last evening and large hail. A good overlap of instability (1000 J/Kg MLCAPE) and wind shear (effective shear ~35kts) was present last evening off to the west and northwest of the area (Minneapolis) where up to 2" hail was reported. Today the upper low will continue to shift northeast toward Lake Superior with a more northerly push into the overnight. Most unstable CAPE is building east across southern MN and 1000 J/Kg is now right near I35 at 07Z.

Radar indicates showers and storms continuing across southcentral MN into northwest WI at 2 am on the nose of 850 mb moisture transport and 500-1000 J/Kg of MUCAPE. The instability pool over southwest MN will shift east this morning under southwest low-level flow with increases to perhaps 1000 J/Kg over sern MN by 7 am. Fair to Good 850 mb moisture transport convergence is suggested in the latest many RAP runs peaking around sunrise, along with moderate isentropic upglide, fueling morning showers and storms across much of the I-90 and north area through the morning (60-80% chances). A decrease in shower/storm activity is expected late morning into the early afternoon as the transport convergence wanes. Cannot rule out a pulse thunderstorm producing severe hail to quarters this morning, but wind shear is not supportive of longer lived updrafts.

During the afternoon, instability looks to grow, however for areas near I-90, this will be contingent on the amount of coverage and persistence of showers and storms from the morning. A more defined frontal boundary will develop in the low-levels and provide a convergent boundary in what appears could be an uncapped air mass by late afternoon from roughly RST-AUW per HRRR soundings. This has slowed with successive model runs. The 22.00Z HREF would suggest that as the upper low shifts northeast, deeper shear wanes to the ~25kt range by afternoon. With about 1200-1500 J/Kg of MLCAPE (T 72F/Td 63F) near that boundary for shower and thunderstorm development (45% chances after 4 pm). So, more of a pulse thunderstorm environment with hail possible, and an outlier 1-1.25" hail stone possible. The SPC marginal risk seems on track.

The front will slowly shift south during the evening and the shower and storm chances will mainly be found over northeast Iowa and southwest WI by mid-evening (40-50% chances). A more weather-free day allows for instability to grow and linger into the evening there. Overall, have increased the rain chances area wide for the forecast through 3 am Tuesday.

Rainfall: A pretty impressive 3-4" band has fallen just north of the area near Hastings/River Falls with the MRMS rainfall estimates tracking well per observations (3.90" just south of River Falls). 24-hour rainfall amounts will generally be highest with the instability area northwest of a line from Mason City IA to Black River Falls WI. Some pockets of 1-1.5" are suggested by the 22.00Z HREF LPMM as outlier high amounts there, with the average closer to 0.50".

Weather on the Decrease This Week, Seasonal, Shower Chances South

There are still uncertainties as the week moves forward as a strong shortwave trough, currently digging through the northern Intermountain West per GOES water vapor imagery, will develop/track through the nation`s mid-section. Portions of southern Wisconsin and extreme northeast Iowa may be under the very northern extent of light rain shield. The challenge is the varying track of the slow moving low pressure area traveling from roughly MO through the Ohio Valley. Should it take a northern track with a deeper low (~40% chance per 21.12Z LREF clusters), light rain chances would increase south of La Crosse Tuesday night through Thursday. The large scale pattern is undergoing big changes with ridge building in the western U.S toward Hudsons Bay, allowing for an unsettled low track across the central U.S. For now, rain chances are forecast in the 20-30% range southeast of La Crosse.

There is 80% agreement in the 22.12Z LREF clustering that the longwave ridge will build in by Friday and for the weekend, with temperatures above normal in the 70s-80F and dry.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1238 AM CDT Mon Sep 22 2025

Showers and storms will be the initial TAF concern through the remainder of the overnight and into the morning hours as an axis of increasing moisture transport will continue to develop showers and storms near I-90. MVFR to IFR vsby reductions will be likely with more robust showers and storms. As these showers and storms exit during the morning, low-level moisture will build in across the local area resulting in a period of MVFR cigs until some afternoon mixing increases ceiling heights above MVFR levels. Later into the afternoon and evening, an area of low-level convergence will push south towards the I-90 corridor. As this occurs, much of the CAMs develop some convection along this. However, confidence remains low on exact how this will manifest this far out but MVFR to IFR vsby reductions would be likely with any storms. Consequently, have included some prob30 TSRA mention in the TAF issuances during this period but will continue to monitor forecast trends and adjust as needed.

&&

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

&&

$$

DISCUSSION...Baumgardt AVIATION...Naylor

NWS ARX Office Area Forecast Discussion

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