NWS-La Crosse Forecast Discussion

						FXUS63 KARX 262351
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
651 PM CDT Fri Apr 26 2024

.KEY MESSAGES...

- Scattered showers and storms continue tonight with gusty winds
   blowing 40-50 mph at times.

 - Conditions trending drier on Saturday. Strong to severe
   storms could develop Saturday evening in northeast Iowa into
   far southwest Wisconsin. Large hail and damaging wind are the
   primary threats.

 - Widespread showers and storms likely on Sunday with a healthy
   dose of rainfall on tap.

&&

.DISCUSSION...
Issued at 315 PM CDT Fri Apr 26 2024

Showers and Storms Tonight:

Broad cyclonic flow exists this afternoon across the western and
central CONUS with a compact negatively-tilted shortwave ejecting
northeastward through the central Missouri River valley. A maturing
991-mb surface cyclone was positioned near Hastings, NE at 18Z and
is progged to lift northeast into central Minnesota by sunrise on
Saturday. The leading wave of showers rooted along the attendant 700-
mb theta-e advection wing lifting through our area decayed through
the morning as they outran the axis of supportive MUCAPE, but enough
evaporative cooling occurred on the backside of these dissipating
showers to transport a 50-60-kt LLJ down to the surface west of the
Mississippi River, producing surface wind gusts of 45-55 mph in
their wake.

The next round of showers rooted closer to 850 mb warm frontal nose
have started developing over central Iowa and lift through the
region this afternoon and evening, once again outrunning the axis of
elevated upright instability but the possibly still putting out
lightning as the move through given the low static stability.
Attention then turns to the line of linear/quasi-linear convection
working northward from western Iowa towards the 03-06Z timeframe.
There is some uncertainty with how far north the surface warm front
will lift and the exact timing of these storms in relation to this
frontal progression will dictate the possibility any QLCS tornadoes
over northeastern Iowa. Surface wind profiles and hodograph
trajectories within the warm sector and along the front would be
supportive of QLCS tornadoes--if the warm sector makes it this far
north. The latest few runs of the HRRR have been hinting at a
northward push of the warm sector ahead of this line, so the
evolution of this mesoscale environment bears watching through the
evening. The severe weather threat wanes steadily once the storms
overrun the front north of Iowa.

The mid-level dry slot cuts across the forecast area in the wake of
this overnight convection, bringing an end to the precipitation by
sunrise on Saturday morning.

Severe Storm Threat Saturday Afternoon/Evening:

Confidence is increasing for a drier Saturday than previously
forecast thanks to weak upper-level ridging and the aforementioned
dry slot. Gusty winds hold on through the first half of the day, but
gradually subside Saturday afternoon as the pressure gradient
relaxes. Southeast gusts look to top out in the 30-35 mph range area
wide with sustained winds of 20-25 mph.

Despite the broader synoptic-scale environment lacking organized
ascent, thunderstorm chances increase across northeast Iowa into far
southwest Wisconsin Thursday evening. Weak shortwave perturbations
in the mid-level flow provide the best forcing with this system, as
frontal boundaries in the area lack baroclinicity. Furthermore, an
elevated mixed layer is present in model soundings. The CIN
associated with the EML increases as you move northward across the
forecast area. A few Dubuque, Iowa model soundings show the
potential to tap into 1500-2000 J/kg of CAPE as the EML erodes in
the evening, but as you move northward the chance of eroding enough
CIN to freely convect decreases rapidly. The chance of convective
initiation will be highly correlated with how much daytime heating
is achieved tomorrow afternoon with differences remaining across
models, especially when it comes to cloud cover.

If storms are able to initiate, a narrow corridor of CAPE 1500+ J/kg
and deep-layer sheer over 40 knots across northeast Iowa into far
southwest Wisconsin could lead to storms quickly becoming strong to
severe. Large hail and damaging wind are the main threats, but a
tornado or two cannot be ruled out if storms are surface based.

As the evening progresses, the atmosphere will stabilize putting an
end to the severe weather threat. Showers and sub-severe storms will
likely increase in coverage as potential vorticity advection
increases across the Upper Mississippi Valley.

Soaking Rain Sunday:

Widespread showers and storms take hold on Sunday as a low pressure
system approaches from the southwest. Heavy rain is the primary
threat with NAFES precipitable water values at our above the 95th
percentile for this time of year. This will result in 1-1.5" of rain
area wide, with locally higher amounts possible. As it stands, the
area can take this much rain with minimal impacts but we will
continue to monitor the potential for urban street flooding and
minor river/stream rises as this system approaches.

Looking Ahead To Next Week:

An active weather pattern continues into the new week with
shortwaves propagating in the primarily zonal upper-level flow. This
pattern supports periodic preciptiation chances and seasonable
temperatures. More details to come after we get through this
weekend's push of precipitation.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 652 PM CDT Fri Apr 26 2024

MVFR/IFR cigs are expected through the morning hours on Saturday as
low stratus hangs onto the region. Light showers will continue to
progress north of the region through rest of the evening hours.
The next set of TAF concerns comes overnight with much of the
deterministic CAMs pushing a round of showers and storms through
the region. Still a fair amount of disagreement in exact
coverage but opted to hold onto -SHRA and VCTS mention to
account of this potential. As morning approaches, conditions
should begin to improve with MVFR cigs, perhaps low VFR in
spots.

Winds through the TAF period will remain fairly breezy at around 15-
20 kts with gusts of up to 30 kts from the southeast. There remains
some questions in valley locations for LLWS, with some
deterministic soundings showing the potential for decoupling
overnight allowing for wind gusts to diminish. As a result, will
need to monitor to reintroduce LLWS into the TAF for KLSE if
this potential is realized. Otherwise, winds will swing to
westerly by the end of the TAF period.

&&

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

&&

$$

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