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