420
FXUS63 KARX 302005
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
305 PM CDT Tue Apr 30 2024

.KEY MESSAGES...

- Showers and storms with a decaying severe weather threat from
  west to east this evening. Large hail and damaging wind gusts
  will be the primary hazards.

- A secondary system will bring showers and storms resulting
  heavier rainfall beginning Thursday morning into the overnight
  hours. Rainfall amounts of 1 to 2 inches are possible. Flood
  prone areas should be alert for localized flooding potential.

- Additional chances for showers and storms persist into the
  weekend and early next week.

&&

.DISCUSSION...
Issued at 305 PM CDT Tue Apr 30 2024

Thunderstorms This Evening Through Tonight:

We continue to closely monitor the potential for strong to severe
thunderstorms west of the Mississippi later this evening, especially
in parts of southeast Iowa. At 130pm, day cloud phase satellite
imagery shows a swath of showers and isolated storms moving towards
northeast Iowa. Convection associated with this first push of
preciptiation is located in a low CAPE, high CIN environment and is
not expected to produce a severe weather threat.

Further to the west near the Nebraska/Iowa border, a ML CAPE pool is
building on the order of 1500-2000 J/kg as strong warm air advection
works into this region ahead of an approaching cold front. Despite
showers and storms in this area earlier in the day, rapid clearing
and surface temperature recovery is underway. This is also where
satellite imagery shows convective initiation taking place. CAMs
suggest initial development along the Nebraska/Iowa border, upscale
growth to a lines or clusters of storms quickly moving west across
Iowa. These are the storms that pose a level 2 out of 5 (slight)
risk locally for severe weather this evening. Confidence for severe
weather is much higher to the south and west of the local forecast
area, with medium confidence for a decaying severe weather threat
locally.

Model soundings and guidance show an environment primed with a lot
of low-level and deep-layer shear this evening with a deepening
shortwave trough sliding from southwest Minnesota up towards
northwest Wisconsin. Despite the surface low moving northwest of the
area, much of the CAPE associated with this system is progged to
advect southeast of our northeast Iowa counties. This leaves much of
our forecast area in between  these two forcing features muddying
our severe weather risk locally. With plenty of shear present, we
cannot rule out a few stronger to severe storms, but organized
severe weather may be hard to come by without more CAPE to tap into.
Large hail, damaging wind, and a tornado or two are all on the table
during a narrow time window between about 6 to 9pm. The meager
instability present peters out after sunset.

Non-severe storms continue eastward across the forecast area late
this evening, exiting to the east by sunrise. Given the progressive
nature of this system, rainfall amounts will remain under a half
inch for most and do not pose a flooding/flash flooding threat.

Wednesday - Thursday Night: Heavy Rain & Storm Chances Thursday

As we head into the middle of the week, a secondary wave will
progress eastbound out of the MT/WY region. With its general pivot
northeast as it resides in the Upper Midwest, another round of
showers and storms will be likely across much of the area on
Thursday. The general surface setup amongst guidance suggests a
northward progressing warm sector with strong 850mb moisture
transport in the 30.12z NAM/GFS. Consequently, a notable
precipitable water surge can be seen in guidance Thursday afternoon
with values across deterministic guidance and their members reaching
to 1.1" to 1.4" which is well above the 90th percentile for climo
and approaching daily maximums for early May. With the higher warm
cloud depths approaching 3000-3500 meters or greater, could
certainly see some efficient rainfall when also considering
skinny instability profiles. As a result, the WPC has a Slight
Risk for our entire forecast area for excessive rainfall on
Thursday. The 30.12z EC ensemble showing fairly respectable
probabilities (50-70% chance) for over an inch of QPF. When
considering that 3-hr flash flood guidance is around 1.5" to 2"
for many spots, will have to watch for some localized flooding
concerns in susceptible locations.

Otherwise, there is some marginal severe potential as storms
progress into our region on Thursday as well. The 30.12z NAM shows a
degrading axis of MUCAPE pushing into portions of northeast IA and
towards I-90 into the afternoon hours associated with the
aforementioned 850mb moisture transport. As an MCS evolves along the
warm front maybe some gusty winds or small hail could materialize.
However, the hail threat seems to be a lesser concern with the
previously stated deeper warm cloud depths and skinny instability
profiles. However, with a stronger low-level jet to around 50-55 kts
in place, could not rule out a couple of 50-60 mph wind gusts with
any storms that can utilize it.

Friday - Tuesday: Additional Shower & Storm Chances Into Next Week

Deterministic long-range guidance in combination with cluster
analysis into the weekend and early next week signal the possibility
that we remain in a somewhat active pattern with additional embedded
upper-level waves progressing into our region through the weekend
and into next week. As we head into Saturday, the first of these
waves pushes into the region as a parent upper low centered across
Manitoba will help pivot the weak shortwave into the Upper Midwest.
Still some disagreement in the placement of this weak wave so will
keep chances for showers and storms low in accordance with the
recent NBM for now as much of the 30.12z GEFS/EC ensemble
probabilities have very low chances for any significant instability
ahead of this wave. So generally not expecting any substantial
precipitation with this system.

Guidance begins to exhibit a fair amount of spread after the weekend
with cluster analysis showing a couple of different scenarios in the
30.00z grand ensemble (EC ens/GEFS/Canadian ens). The majority of
the ensembles (roughly 70% of members) show some form of a wave that
ejects out of the rockies towards our region to in the later Monday
into Tuesday timeframe. The remaining members (roughly 30%) favor
keeping a ridge that guidance moves into place on Sunday. As a
result, have kept modest probabilities for precipitation across the
region during this period in accordance with the NBM. Mode of how
showers and storms will develop along with any severe threat  is
unclear at this time with any surface features and thermodynamics
remaining unresolved.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1236 PM CDT Tue Apr 30 2024

VFR conditions will prevail through much of the daytime as winds
trend SE ahead of a cold front. Some shower/isolated storm
potential could occur by 22-00Z at KRST with higher chances for
bands/clusters of showers/storms to move west to east through
the evening. Erratic, gusty winds and IFR/MVFR conditions could
occur with any storms. Skies will gradually clear in the wake of
the storms with winds shifting westerly, possibly gusting up to
25 to 35 kts at times, depending on mixing depth.

&&

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

&&

$$

DISCUSSION...KAA/Naylor
AVIATION...KAA