812
FXUS63 KARX 200342
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1042 PM CDT Sat Apr 19 2025

.KEY MESSAGES...

- Widespread rainfall moves into the area Sunday afternoon and
  continues through Monday morning. Rainfall amounts of around
  1" are likely. Locally higher rainfall amounts 1.5" or more
  may occur (15-20% chance) and would suggest low-end chances
  (10% chance or less) to reach minor flood stage in some area
  rivers.

- Milder temperatures are expected for Tuesday and beyond with
  highs generally in the 60s. Additional rain chances linger
  throughout much of the week.

&&

.DISCUSSION...
Issued at 245 PM CDT Sat Apr 19 2025

Rest of Today - Monday: Widespread Rainfall Moves In Sunday Afternoon

GOES-19 water vapor imagery and 19.15z RAP 500mb heights shows
a slightly southwesterly mid-level flow pattern with a
pronounced 500mb trough over portions of the Four Corners region
this afternoon. As we head through the overnight and into the
day tomorrow, this trough will pivot quickly northward in the
southwesterly flow pattern along with a deepening surface low
pressure center. Overall the general idea will be an initial
round of precipitation will move in for Sunday afternoon
associated with a warm advective wing out ahead of the main
deformation zone. By evening, the surface low will approach the
area and the warm conveyor processes will be overhead with a
strengthening deformation area and TROWAL aloft. What may be
tricky is the exact placement of heavier rainfall amounts as the
19.15z RAP has a piece of low-level frontogenesis on the
gradient of the deformation zone which may locally enhanced
rainfall rates in the northwestern half of the local area.
Across portions of northeast IA and southwest WI, a sharp axis
of 850mb moisture transports will advect some, but very minimal
(up to 100-200 J/kg of MUCAPE), instability into the
southwestern WI. However, this would seem more embedded thunder
within the broader rain shield. With the low pressure center
moving fairly quickly through the area overnight and into
Monday, expecting much of the precipitation to be through the
area by afternoon on Monday. Cannot rule out some snow mixing in
across portions of north- central WI with sounding profiles in
the HRRR at Medford, WI showing near-surface cooling to around
the freezing mark. However, little accumulations would likely
take place.

As far as rainfall amounts are concerned, consensus in both the
19.12z HREF and NBM precipitation means have a general swath of
around 0.75" to 1" of QPF through the local area. Noting some
differences in the exact placement of the deformation zone amongst
the CAMs which pretty much spans the range of the local area. As a
result, thinking that the broad brushed mean QPF values in the HREF
are a function of the uncertainty and in reality there will be an
axis of slightly higher amounts depending on where the gradient of
the deformation zone sets up as much of the individuals CAMs suggest
higher rainfall amounts in the axis of heaviest rain. Regardless,
some river responses are likely with sites along the Black and Cedar
Rivers forecast to reach action stage. If the some locations see the
HREF maximum values for QPF over their respective basins, could not
rule out some sites reaching minor flood stage however the
probability for this remains low (5-10% chance) at this time.

Tuesday - Friday: Milder Temperatures with Periodic Rain Chances

Looking beyond this Sunday and Monday system, the 19.12z GFS/NAM/EC
generally agree on pushing some shortwave ridging within a zonal mid-
level flow pattern for Monday evening. As a result, temperatures
will likely moderate for some on Tuesday. However, another round of
showers is in the cards (40-70% chance) as the warm return flow will
aid for instigation of some showers overnight Monday and into
Tuesday along the axis of an 850mb moisture transport axis. Cannot
rule out some rumbles of thunder with these, however the fairly
pronounced warm nose in 17.12z NAM soundings keep MUCAPE values to a
minimum (under 100 J/kg).

As we head into the second half of the upcoming work week,
temperatures continue to trend mild with highs in the national blend
in the 60s to lower 70s through Friday with a zonal synoptic flow
pattern in place. Some uncertainty with precipitation chances for
Thursday and Friday as there are some pretty notable differences
between the 19.00z EC and 19.12z GFS. THe GFS solution has a
pronounced shortwave through that pushes southeastward out of
southern Canada and thus increasing our precipitation chances.
However, with dewpoints progged in the 40s to lower 50s, instability
in this solution remains minimal. This is contrasted by the EC
solution which has some southern flow and a moisture transport
surge which would help instigate some convection. However, with
no profound source of shear in this solution, the potential for
severe weather would appear low. This is seconded by the FengWu
and Pangu AI convective hazard forecasts which show no organized
severe potential across our area over the next 8 days.
Regardless, still seems like we will get some precipitation
across the area Thursday/Friday with high probabilities (60-80%)
in the grand ensemble (GEFS/EC/Canadian ensemble) for at least
some measurable precipitation in the region.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1033 PM CDT Sat Apr 19 2025

Primary concern remains impending rain Sunday afternoon and
associated reductions to MVFR and IFR and timing of said
reductions. Still looking on track to see MVFR by mid-afternoon
with IFR during the evening, but did adjust onset a bit later
for each. Otherwise, still highly confident winds will tend to
become mainly easterly by midday Sunday.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...Ferguson