975
FXUS63 KARX 151926
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
226 PM CDT Tue Sep 15 2026

.KEY MESSAGES...

- Staying dry and cool today and into the daytime on Wednesday
  with highs in the upper 60s to middle 70s.

- Shower and storm chances (20-50%) Wednesday evening and
  overnight

- Widespread rainfall likely (70-95% chance) Friday into
  Saturday. Probabilities are increasing (40-70%) for rainfall
  amounts of over 1" near and north of the I-90 corridor with
  localized rainfall amounts of 2-3"+ possible. Localized
  flooding and river rises may occur where heaviest rainfall
  amounts fall.

&&

.DISCUSSION...
Issued at 226 PM CDT Tue Sep 15 2026

Rest of Today - Thursday Night: Cooler Than Normal, Shower and Storm
Chances Late Wednesday

Zonal to slight southwesterly upper-level flow with accompanying
surface high pressure will dictate our weather conditions for the
rest of today and throughout much of the daytime on Wednesday.
Consequently, expecting fairly dry conditions for the rest of today
and through the daytime hours on Wednesday with northwesterly flow
today aiding in keeping temperatures cooler than normal with highs
in the upper 60s to middle 70s. Cannot rule out some fog tonight,
particularly north of I-94 where mid-level cloud cover manages to
clear out, however confidence is low for areas south of I-94 to see
much for fog at this point. Otherwise, continued dry conditions
during the daytime hours on Wednesday will give way to some shower
and storm chances Wednesday evening and into the overnight as some
weak ripples in the 500mb flow push from west to east through the
area. As a result, some of the CAMs generally indicate some shower
and storm development where forcing may be marginally better. Given
the fairly weak MUCAPE (only a couple of hundred J/kg) and
precipitable waters of around 1.25-1.75" as this precipitation moves
through, not expecting overly heavy rainfall producers with this
particular round of rain. However, the grand ensemble
(GEFS/EC/Canadian) has some modest probabilities (30-60%) for
rainfall amounts over 0.25" so would not rule out some healthier
rainfall amounts.

Friday - Saturday: Widespread Rainfall Likely, Locally Heavy
Rainfall Potential

The most impactful period over the next 7 days appears to be Friday
and into Saturday as a shortwave trough rolls off of the rockies and
approaches the region. As this occurs, model guidance generally
agrees on surface cyclogenesis taking place over the Great Plains
with the resultant low pushing eastward into the Upper Midwest. With
this low pushing towards the local area, a profound axis of 600-
800mb frontogenesis sets up over the region. While some uncertainty
remains on where this axis sets up, however the heavy rain parameter
space where this better forcing is present is quite substantial with
NAM/GFS preciptable waters of around 1.5-2.0" and warm cloud
depths in excess of 4km. Additionally, some weak MUCAPE present
atop this frontogenetic layer would likely aid to enhance lift
and precipitation production further.

In response to this fairly robust forcing regime, the grand ensemble
(GEFS/EC/Canadian ensemble) has fairly high probabilities
(40-70%) already for rainfall amounts over 1" across the region,
mainly near and north of I-90. Additionally, the recent EC
Extreme Forecast Index paints a shift of tails of 0 to 2 over
the region, suggesting some members reaching the 99th percentile
of model climatology for rainfall. This is being driven
primarily by the 95th percentile of the EC having members in the
3-5"+ ballpark within the heaviest axis of rainfall so cannot
rule out these amounts in some locations. As a result, areas
that receive any rainfall in this range will be susceptible to
localized flooding and river rises. As far as severe potential
is concerned, much of the warm sector with this system currently
is favored south of the local area and is displaced from the
favorable shear with the surface cyclone. As a result, thinking
that the severe potential at this point is fairly low for the
local area which is generally reflected in the AI/ML guidance
only having some 5% probabilities at the most across the local
area.

Overall, the main point of uncertainty with this setup is exactly
where the heaviest rain will fall as there still remains some
discrepancies between deterministic and ensemble models, including
AI guidance, as to where the axis of heaviest rainfall will be.
However, confidence is certainly increasing for fairly healthy
rainfall amounts where this corridor of rain occurs. Consequently,
areas that saw heavy rainfall in days prior may be susceptible to
localized flooding concerns depending on where the heaviest rain
falls.

Early Next Week: Staying Cooler Than Normal, Additional Rain Chances

As the rainfall from Friday and Saturday departs the local area,
northeasterly flow around the surface low will likely keep
temperatures slightly below normal into Sunday with median high
temperatures in the NBM hovering in the middle 60s to near 70. The
main point of forecast uncertainty as we head into next week will be
revolving around a potential cut-off upper-level low which guidance
has substantial disagreements on how to handle at this point in the
forecast cycle. Consequently, the NBM maintains some low-end
precipitation probabilities (15-40%) at this point from Sunday
through Monday which seems reasonable given the wide range of
solutions with this system. However, given the presence of this low
in the region into early next week, thinking that below normal
temperatures will maintain with median high temperatures in the NBM
hovering in the upper 60s to around 70 degrees.


&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1208 PM CDT Tue Sep 15 2026

VFR conditions are expected through the TAF period with sct to bkn
middle to upper-level clouds. Cannot fully rule out (under 10%
chance) some fog reaching the I-90 corridor early Wednesday
morning, however lingering cloud cover currently is favored to
limit this potential. Winds will begin the TAF period at around
8-15 kts from the northwest before diminishing and switching to
the east overnight.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...Naylor