014
FXUS63 KARX 121040
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
540 AM CDT Wed Mar 12 2025

.KEY MESSAGES...

- Well-advertised potent spring storm system remains on track to
  bring widespread rain for Friday late afternoon and evening.
  How the severe weather threat (damaging winds, embedded
  tornadoes?) unfolds hinges on the alignment of a multitude of
  thermodynamic and kinematic parameters.

- Southerly winds gust to 30 to 40 mph during the day on Friday
  ahead of the storms, followed by the potential for gusty
  northwest winds on the backside of the departing system over
  the weekend.

- Steadily rising temperatures for the remainder of this week
  with record highs (low to mid 70s) in sight for Friday,
  followed by a brief cool period for the start of next week.

&&

.DISCUSSION...
Issued at 325 AM CDT Wed Mar 12 2025

Today - Thursday: Warming Up

Split flow persists over the forecast area for today as the polar
jet remains displaced along and north of the U.S./Canadian border
and a southern stream trough meanders eastward across the southern
states. A cursory glance at verification of the last few days of NBM
deterministic output shows a solid 2-4 degree cool bias across the
forecast area, and thus have trended the highs to around the NBM
50th percentile (deterministic NBM is closer to the 25th
percentile). Weak flow today increases from the south on Thursday as
the southern trough departs, allowing for increasing warm air
advection and temperatures rising further into the 60s. It is not
out of the question that locales south of I-94 could make a run at
70 degrees with 10-20% of the NBM ensemble inputs eclipsing this
mark.

Friday: Windy and Warm! Record Highs in Sight

By Friday morning, the sharp, negatively-tilted shortwave that will
bring impactful weather to the Upper Midwest will be ejecting from
the Four Corners region with a 130-kt jet streak along its southern
flank. The upstream pressure gradient begins tightening in response
Thursday night, increasing from 5 ubar/km midday Thursday to 20-30
ubar/km by the same time on Friday. A 40-50 kt low-level jet expands
across the area by sunrise on Friday ahead of the 975-mb low pulling
off the Kansas Front Range. All put together, southerly winds
gusting up to 30 to 40 mph are likely during the day. Positive lower
tropospheric theta-e advection takes place overnight Thursday into
Friday morning, with 925-700-mb theta-e values rising by 10K between
Thursday evening and Friday morning. There is very strong agreement
amongst the NBM guidance that highs will surpass the 70 degree mark
Friday afternoon and look to top out very close to our daily
records. These ribbons of theta-e advection may spark some
morning showers in central Wisconsin, but any impacts should be
minimal.

Friday Afternoon and Evening: Severe Storm Potential

The bulk of our attention is focused on how the severe weather
threat will unfold on Friday late afternoon and evening. While
confidence in severe weather is certainly higher to the south of the
local forecast area towards I-80, several wild cards remain up
Mother Nature`s sleeve that make it difficult to pin down much for
details up here. What we can say with higher certainty is:

1. A linear storm mode is expected given the strong synoptic forcing
and a predominantly line parallel effective shear vector that will
result in rapid upscale growth to a QLCS.

2. Deterministic GFS profiles show a respectable pre-frontal cap in
place (CINH of -100 to -130 J/kg), backed by 60-70 percent of the
LREF members. Therefore, convective initiation will likely take
place near peak heating along the boundary over central Iowa with
rapid destabilization taking place right ahead of the line.

3. This arcing line of storms will be moving to the northeast at an
appreciable speed of around 35-45 kts (40-50 mph), derived by
extrapolating the motion of the initiating Pacific Front.

The two big wild cards at play are the characteristics of the warm
sector and the exact orientation of the QLCS with respect to the
environmental wind field.

First, surface dewpoints in the warm sector--at least as
currently resolved by the global models--are rather low when it
comes to a tornado risk. Typically, we tend to see dewpoints
creeping up into the upper 50s to low 60s for the risk of
tornadoes to be realized and some of our more recent early
spring/late fall QLCS events have hit this mark. The deeper Gulf
moisture looks to be hung up in southern Illinois, no doubt
being worked over by convection further SSE down the boundary.
That being said, from forecast profiles, LCLs and SBCAPE values
look favorable to support vortex stretching, so this alone does
not rule out the tornado threat. The warm sector will be
narrowing as the Pacific Front/QLCS race northward towards I-90
and there is uncertainty in the exact width of the warm sector
and at what point the QLCS will outrun its favored environment.

Moving on to the kinematic fields, the 0-3-km shear vector will be
oriented to the NNW, not an ideal orientation for a SW-to-NE-
moving line. Yet, a closer look at the boundary alignment does
show a potential window west of the Mississippi River where the
front lays out in an E-W orientation that would be more
favorable to realize both the 0-3-km and deeper shear vector. In
areas where the line races northeastward, the shear vectors are
not supportive of sustained surface-based updraft maintenance.
While the northern side is favored for tornadogenesis potential
from a kinematic standpoint, it is the furthest displaced from
the richer airmass.

Putting the pieces together, confidence is high we will have a line
of storms. Given the ambient environment and storm motion, damaging
wind gusts could occur along the entire line. The tornado potential
hinges on multiple factors that, at this stage in the forecast,
aren`t really lining up. Taking a peak at the various AI and analog
guidance, this thought process is well-reflected in their outputs
which are leaning on keeping the tornado threat south. This event
will start moving into the convective allowing model time window
later today/tonight and hopefully we will begin to answer more of
these critical questions.

Saturday - Sunday: Cooler, Lingering Rain/Snow

Polar air wraps around the backside of the occluded surface low
during the day on Saturday, keeping temperatures steady in the 40s to
low 50s through the morning before falling into the 30s and 40s by
sunset. While the deformation zone attendant with the main storm
system is progged to stay west of the region, mid-level cyclonic flow
remains in place and there is an increasing signal in the
GEFS/EPS members that we will see light snow showers move
through Saturday night into Sunday with at least one backdoor
perturbation. Snow amounts will be light (<1-2 inches);
nevertheless, gusty northwest winds of 20-30+ kts could lead to
localized visibility reductions with any snow showers.

The period of cooler weather on Sunday (highs in the 30s) is short-
lived, with the flow turning southerly once again by Monday ahead of
a progressive ridge that slides through to start next week. Highs
look to return to the 50s to possibly low 60s by Tuesday, with
another storm system looming in the wings for the middle of next
week.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 540 AM CDT Wed Mar 12 2025

VFR conditions are forecast for the TAF period with winds
gradually veering from the southeast this morning to the west
by this evening and back to the east by tomorrow at less than 5
kts.


&&

.CLIMATE...
Issued at 619 PM CDT Tue Mar 11 2025

Possible Record Warmth (Record / Forecast):

                           Maximum                Minimum
Thursday (03/13)
Rochester, MN          68 (2024) / 65              N/A
La Crosse, WI          71 (2015) / 69              N/A

Friday (03/14)
Rochester, MN          74 (2012) / 72        49 (2012) / 51
La Crosse, WI          75 (2012) / 75        49 (1990) / 50

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Skow
CLIMATE...JAR