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000
FXUS63 KARX 280412
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1112 PM CDT Sat Apr 27 2024

.KEY MESSAGES...

- Isolated severe storm chances in the 4-9 pm window today south
  of Highway 18 in swrn WI and Clayton/Fayette counties in IA.
  Large hail and damaging wind are the main threats.

- Heavy rain axis has shifted south of the area. A more
  widespread rain of 0.75-1.25" is expected through Monday, with
  most falling Sunday and Sunday evening. Storms will be around
  but the risk of those being severe is quite low after this
  evening.

- An active week is ahead with periodic rain and storm chances,
  with severe storm probabilities looking low at this time, but
  stay tuned. Later Tuesday is the next opportunity.

&&

.DISCUSSION...
Issued at 206 PM CDT Sat Apr 27 2024

Severe Thunderstorm Potential This Afternoon/Evening:

At 1 PM, surface observations show southwesterly winds persisting
area wide. The hi-res models are in very good agreement for winds to
slowly begin turning to the west throughout the afternoon for much
of the area except far southwest Wisconsin and parts of eastern Iowa.
This will create a weak convergence zone along an axis from the Strawberry
Point, Iowa area towards Lone Rock, Wisconsin. With a lack of synoptic-
scale forcing due to weak shortwave ridging aloft, this will be the
primary forcing mechanism for ascent. South of the aforementioned
convergence axis, a reservoir of 700-1200 J/kg of mixed layer CAPE
is progged to continue to increase with daytime heating. HRRR model
soundings support the CAPE axis growing to 1500- 1800 J/kg in the
southern parts of Clayton and Grant Counties by late afternoon. In
previous forecasts and model soundings, this instability was capped
by an elevated mixed layer 2-3 km off the surface, but recent HRRR
soundings at Dubuque have reduced the strength of the warm nose aloft
leading to less capping. Day Cloud Phase satellite imagery shows
hints of this as well, with bubbly cumulus cloud tops along and south
of Highway 151, and some glaciation and orphan anvils further south.
The lack of a stable layer aloft could lead to airmass thunderstorms
developing as daytime heating continues to fuel the environment.
Thunderstorms that form before the weak convergence zone sets up
would be rather pulsy in nature and may decrease the potential of an
organized severe weather threat this afternoon/evening.

If the cap does develop this far north this afternoon and inhibit convection
until convergence is present, isolated to scattered severe storms are
possible in southern Clayton and Grant Counties. The seasonably strong
CAPE mentioned above combined with 35-40 knots of deep-layer shear supports
supercells capable of producing large hail and damaging wind. A tornado
or two cannot be ruled out with around 20 knots of 0-1 km shear present
and a very brief window of helicity greater than 150 m^2/s^2 in model
soundings, but generally speaking hodographs are straighter in nature
for much of the event making large hail the primary threat.

With the lack of strong forcing this evening, thunderstorms will succumb
to a stabilizing atmosphere after sunset this evening. The forecast for
tonight is much drier than previous forecasts with all of the stronger
forcing remaining south of the forecast area until early tomorrow morning.

Rain and Storms for Sunday into early Monday

As the current four-corners deep low ejects out of the southwest
and tracks northeast across the region, lift increases Sunday
on a brush-by of QVector convergence /strongest northwest/ and
the Highway 20 warm front trying to shift north as the surface
low tracks through Minnesota. Low-level moisture advection shifts
north during the morning hours and continues to supply the area
abundant moisture /precipitable water values at the 95th percentile
to near record by late day/. One change is the moist air stream is
bifurcated, keeping the very juicy air in the northeastward stream
from IL->MI /precipitable water 1.50"+/. While the surge in moisture
occurs, the model consensus surface warm front remains near the
WI/IL border and west into IA as the low occludes, limiting CAPES
to 500J/Kg, reducing severe storm potential. This triple point
will evolve northward overnight Sunday ahead of the cold front,
with some surface-based instability /500 J/Kg/ dragging northeastward
into WI per the latest RAP forecasts. Wind shear is still pretty
impressive for this time frame with long,straight hodographs. Right
now, this RAP migration of the triple point and surface instability
is more of an outlier compared to the HREF CAMS. It is something
to monitor Sunday night. The current SPC marginal severe storm risk
appears appropriate with such reduced instability. So, waves of
showers and storms continue until Monday where cold and dry air
advection stabilize the atmosphere from southwest to northeast post-
low. Rainfall amounts look to be in the 0.75-1.25" range from Sunday
to Monday, with the 27.12Z HREF having about 20-35% chances for 1"+.

Active Weather Next Week, Monitoring for Severe Storm Potential

Tuesday: Increasing mid-level winds ahead of a deeper longwave trough
evolving over the northcentral/northwest CONUS Tuesday will lead to
the monitoring for our next chance of severe storms. An open Gulf of
Mexico flow will provide a moisture tongue into IA/MN ahead of a north-
south oriented cold front Tuesday afternoon. By evening, the 27.00Z
Grand Ensemble mean places the cold front near I-35 with 70-80% probabilities
of 40 kt 0-6km bulk wind shear. Forecast soundings from the 27.12Z GFS/NAM
for 7 pm Tuesday show some moderate capping in place and the area being
on the northern extent of the instability tongue. The ensemble probability
of SBCAPE over 500 J/KG is only 15-20% just west of the Miss river. So,
the kinematics are in place for severe storms, but instability looks to
be a limiting factor at this time. The 27.00Z CSU severe probabilities
from machine learning indicate as axis centered on I-35 from DSM-DLH of
5-15% late Tuesday. So, will continue to keep an eye on this potential.

Mid-Late Week: Swift zonal flow continues across the CONUS with various
solutions and large spread on the shortwave troughs moving through with
a frontal zone and instability in the region. Forecast confidence is quite
low on any details beyond Wednesday but big picture says periodic rain/storm
chances and breezy at times into the weekend with near normal temperatures.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1112 PM CDT Sat Apr 27 2024

Conditions will deteriorate with MVFR/IFR cigs by morning as a low
pressure system begins to track northwest of our region. As it
progresses, showers will envelope the region during the morning
Sunday. A few storms are possible later into the day on Sunday,
however confidence is very low in how they may manifest at this
time so will opt to keep any mention of VCTS out of the TAF for
now. CAMs try to reduce coverage of shower activity close to 06z
but will hold onto shower mention through the evening. Winds
will begin the TAF period under 10 kts before increasing during
the morning as the surface pressure gradient tightens with wind
speeds of around 15 kts by the afternoon.

&&

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

&&

$$

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