NWS-La Crosse Forecast Discussion

						FXUS63 KARX 260344
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1044 PM CDT Fri Apr 25 2025

.KEY MESSAGES...

- The threat for widespread severe weather is increasing for
  Monday afternoon and evening with large hail, damaging winds,
  and tornadoes all a possibility. Details on the exact timing
  of the storms and the hazard potential will be fine tuned
  over the next few days.

- Weekend Outlook: Sunny and seasonable temperatures for
  Saturday with scattered showers and storms for Sunday into
  Monday morning.

- Monday will be the warmest day of the week with highs surging
  into the 70s before temperatures slide back into the 60s for
  midweek.

&&

.DISCUSSION...
Issued at 220 PM CDT Fri Apr 25 2025

Weekend Outlook: Sunny Saturday, Scattered Showers Sunday

Clouds clear overnight as today's rainfall departs to the east,
setting the stage for a beautiful start to the weekend. Cannot
fully rule out the possibility of localized fog towards
sunrise, but with modestly strong winds just off surface, did
not feel confident to mention fog in the forecast at this time.

Despite weak cold air advection on Saturday, ample sunshine
will result in temperatures topping out near seasonal norms for
this time of year in the low to mid-60s. Mid to high clouds
increase in the afternoon and evening as the 500-mb ridge slides
off to the east with a mix of sun and clouds for Sunday.
Southerly flow deepens for Sunday with steadily increasing
theta-e advection in the lower to mid troposphere possibly
fueling isolated to scattered showers, primarily north of I-90
where saturation in the 900-700-mb layer looks to be more
likely, though mid-level lapse rates do not seem conducive to
anything outside of showers and possibly a rumble of thunder.

Monday Afternoon/Evening: Severe Weather Threat

Moving past the weekend, all eyes remain fixed on the severe
weather potential for Monday afternoon and evening. A deep
longwave trough carves out the western CONUS by Sunday midday
before ejecting northeastward across the central High Plains by
midday Monday. At the surface, a broad warm sector becomes
established across the central CONUS with a surface low tracking
from the Colorado Front Range northeastward to the MN/ND/SD
border by midday Monday. A dryline/pre-frontal trough will
march into southwest Minnesota and western Iowa for Monday
afternoon and serve as the focus for convective initiation
during the late afternoon and evening hours.

As is typical at this time range from the event, fine scale
details with the thermodynamic and kinematic fields will play a
large role in how the severe weather threat evolves Monday
afternoon and evening. There still exists appreciable
variability (on the mesoscale) in the timing/position of the
dryline, how quickly the warm sector destabilizes, and the wind
profiles in the lower to mid-troposphere. Barring open warm
sector convective initiation, storms are likely to develop along
the dryline in southern Minnesota and advance eastward,
congealing into a QLCS through the evening. How quickly this
takes place will hinge on the orientation of the deep shear
vector and the degree of inhibition in the warm sector. A very
subjective view would favor discrete cells west of the
Mississippi River with a QLCS mode more likely east of the
river.

It is worth noting the strong consensus in the EPS and GEFS-
based AI aggregate severe weather probabilities that have only
been increasing over the last 24-48 hours. The latest 12Z
probabilities are all highlighting a 30-60 percent corridor for
the potential of severe weather (within 25 miles of a point) across
Iowa into southern MN, which is an exceptionally strong signal
for being 4 days out. The top 5 analogs from the CIPS guidance
are all large severe weather outbreaks over the Upper Midwest,
including several that produced longer tracked tornadoes (May
17, 1992, June 7, 2007, April 18, 2004, and April 10, 2011).

The details with this event will continue to be refined over
the next few days. Please continue to check for forecast
updates through the weekend.

Tuesday - Thursday: Seasonal Temps, Dry Midweek

The flow pattern quickly moderates in the wake of the trough
passage Monday night into Tuesday, resulting in temperatures not
straying too far below seasonal norms for midweek. Low-
amplitude ridging ripples along the Canadian border late Tuesday
into Wednesday, bringing a stretch of drier weather for this
period. By Thursday, the next northern stream wave approaches,
and while Gulf moisture is progged to remain south of the
region, scattered showers cannot be ruled out with this trough
passage.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1044 PM CDT Fri Apr 25 2025

VFR conditions are expected through the TAF period with very little
cloud cover. Winds will remain under 10 kts and will shift from
northerly to southeasterly throughout the TAF period.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Naylor