738
FXUS63 KARX 311048
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
548 AM CDT Fri May 31 2024

.KEY MESSAGES...

- Showers and a few thunderstorms later today could produce 1-3"
  of rain along a corridor running from roughly Albert Lea to
  Eau Claire. Rainfall risk lessens as one travels further east.

- Potential for stronger storms early Monday morning and again
  Tuesday evening. Details of these systems are still being
  hashed out.

- Near seasonal temperatures through the 7 day forecast with
  the warmest days in the lower 80s for early next week.

&&

.DISCUSSION...
Issued at 320 AM CDT Fri May 31 2024

Today/Tonight: Rainfall Details

A weak surface cold front stretched from the Lake of the Woods
region SSW to near Sioux Falls and is gradually acquiring an
increasing positive tilt as the parent upper level shortwave
energy lifts northward into Canada. The front continues to pivot
along the southern axis point over the course of the day and
lays out along a line from the Siouxland region to west-
central Wisconsin by this afternoon. The warm sector ahead of
this front is quite narrow owing to a sharp upstream ridge axis
centered over eastern Wisconsin keeping a drier southeasterly
fetch in place up across the MN/WI border region.

Weak 300-310K isentropic ascent just ahead of this front has
kept a band of semi-discrete convection ongoing through the
night, but with only weak moisture transport within the warm
sector and MUCAPE values of < 500 J/kg, the convection has been
gradually decaying through the night as evident by the warming
cloud tops and consolidation of higher reflectivity values on
radar. As the front lays out and the flow becomes more parallel
to the isobars, the convection is expected to weaken further and
have reflected this in the precipitation forecast with a lower
of PoPs through the morning along this axis.

Increasing theta-e advection over the frontal surface commences
this afternoon as a second shortwave impinges from the
northwest. This will fuel reinvigorated convective development
along this boundary through the afternoon and evening with
training cells propagating northeastward along the front. There
has been a fairly consistent signal in the HREF that a 10 to 20
mile wide corridor of 1 to 3 inches of rain will fall during
this period--the big question has been where that band will set
up. The latest 31.00/06Z members are favoring an SW to NE axis
roughly from Albert Lea to Eau Claire, with the guidance spread
by up to a county on either side of this line.

Despite this favorable synoptic setup, deeper moisture will be
confined well south of the region with PWATs only pushing around
1.5 inches and 925-700-mb mixing ratios topping out at 6-8g/kg.
Thus, this increasing lift along the boundary will only be able
to realize the limited moisture which has pooled in the
immediate vicinity ahead of the front. The potential rainfall
amounts from the HREF are not extraordinary for this time of
year, but may lead to localized flooding in areas of poor
drainage.

Saturday and Sunday: Occasional Showers

The surface flow pattern becomes weak for Saturday as low to
mid-tropospheric shortwave ridging builds in for the day. A
residual 925-700-mb theta-e gradient should still exist across
the forecast area, serving as the focus for weak pulse showers
and thunderstorms during the day. Moving into Sunday, decaying
convection from the previous night out in the Dakotas may sneak
into the region during the morning, but more likely than not we
will see dry conditions during the day with increasing warm air
advection and capping strengthening during this timeframe.
Highs push into the lower 80s as the southerly flow strengthens.

Monday and Tuesday: Two Rounds of Stronger Storms

A zonal flow pattern that sets in on Sunday ushers two
shortwaves in quick succession for the first half of the new
week. Convective initiation with the first wave takes place
well upstream in the central and eastern Dakotas Sunday
afternoon, growing upscale into an MCS Sunday night over
central or southern Minnesota. Medium range solutions have
slowed the arrival time of this system and it now looks to reach
our region after midnight on Monday morning. We look to reside
on the far eastern side of the warm sector (MUCAPE values of
less than 1000 J/kg) and indications are that this complex will
be undergoing a weakening trend when it arrives. Cannot fully
rule out some stronger winds given the expected storm mode, but
the severe threat looks to stay mainly to the west.

A second shortwave amplifies as it races along the
U.S./Canadian border Monday night into Tuesday within a 120-kt
jet streak. This system possesses better kinematics compared to
the first shortwave with an open Gulf warm sector of 65+ degree
dewpoints that can be easily advected northward. There remain
modest differences in the medium range solutions as to the
characteristics/timing of the wave given that the vort lobe is
still over the data-sparse western Aleutians. This spread should
narrow as we go through the weekend and the system becomes
better sampled, but this complex for Tuesday afternoon/evening
looks to be our main focus for severe weather potential for this
forecast period.

Mid to Late Week: Quieter Weather

The evolution of the weather pattern for the latter half of the
week remains in a state of flux depending on the degree of
amplification with our midweek shortwave. A slower and deeper
trough would result in the risk for showers to linger through
the end of the week under the upper low, whereas the more
progressive solutions usher in upper level ridging faster and
dry out the forecast after Wednesday. Either way, temperatures
will cool to some degree under such a pattern, with highs either
in the 60s or 70s--the warmer temperatures tied to the more
progressive solutions.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 545 AM CDT Fri May 31 2024

Predominantly VFR conditions are expected for today, though
there is an increasing risk for localized visibility
restrictions through the day as showers and thunderstorms
increase in coverage west of an OLZ to ISW line. These showers
spread eastward overnight with ceilings lowering to high
MVFR/low VFR levels towards sunrise on Saturday. Winds will be
from the south around 5-15 kt, but could have localized speed
and direction variability around any storms.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Skow