502
FXUS63 KARX 290007
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
707 PM CDT Sun Apr 28 2024

.KEY MESSAGES...

- Widespread showers continue this afternoon, gradually becoming
  more scattered this evening and pushing north of the area
  Monday morning.

- Cooler weather through Monday, with a rebound to above average
  temperatures for Tuesday and Wednesday. Seasonal temperatures
  for the remainder of the week.

- Several more rounds of showers and storms are lined up for
  Tuesday and late Wednesday through Friday. Overall severe
  threat looks low at this time, but details in the specifics
  still need to be sorted out.

&&

.DISCUSSION...
Issued at 230 PM CDT Sun Apr 28 2024

This Afternoon through Monday

A slowly filling 1003-mb surface low pressure system was
meandering towards the Siouxland region early this afternoon
with an attendant warm frontal structure arcing to the east.
This warm front is rather complex in nature given that an old
warm frontal boundary--evident as a 8-10 F cutoff in the surface
dewpoint values--lingers around the Highway 20 corridor in
northeast Iowa to along the WI/IL border. The surface flow over
Iowa does not turn more southerly until one reaches the I-80
corridor with the strongest moisture transport displaced farther
east towards northeast Illinois. All combined, this disjointed
nature of the kinematic and thermodynamic fields should result
in little movement of the low level warm sector into the
southern forecast area today. Lapse rates north of the warm
front are only around 5-6 C, resulting in almost no upright
instability to realize (despite plentiful deep shear and
elongated hodographs). This changes little over the course of
the afternoon and well-reflected in the shift of the SPC
Marginal Risk further south and the removal of most thunder
wording from the daytime forecast.

Broad 290K isentropic ascent over the lower tropospheric warm
front tapping into the western flank of the deeper Gulf moisture
has fueled widespread showers this morning and early afternoon
across the region. This deeper moisture shunts off to the east
late this afternoon and evening as the mid to upper tropospheric
dry slot wraps around the parent trough. While the widespread
showers shift to the east this evening, lapse rates steepen
enough that any perturbations within the mid-level flow could
instigate a line of showers or thunderstorms. Indeed, the
RAP/HRRR runs have over the course of the morning started to
suggest at a thin line of convection wrapping northeastward out
of Iowa during the late overnight hours and have kept slight
chance PoPs to account for this scenario.

The upper level trough separates from the mean flow and slows
over Minnesota for the day on Monday, resulting in showers
possibly lingering north of Interstate 90 through the day,
ending before sunset. Low stratus/stratocumulus likewise persist
throughout the day on Monday in close proximity to the surface
low, thus have kept temperatures on the cooler side, especially
over southeast Minnesota.

Tuesday:

There will be a lull in the precipitation Monday night through
the day on Tuesday as shortwave ridging slides on through in the
broad cyclonic upper level flow. A compact vort lobe races
eastward--pushed along by a 110-kt jet--across the the Northern
High Plains Tuesday and results in a narrow warm sector
expanding northward ahead of an advancing surface trough.
Temperatures on Tuesday should rebound into the 70s for highs as
southerly flow strengthens ahead of the trough.

The narrowness of the warm sector, combined with the fast
forward motion of the upper level wave, will limit the coverage
and duration of any severe weather--but there exists modest
spread in the timing of these features that will need resolving
over the next few days. At this point, the bulk of the guidance
points to any severe weather taking place west and south of the
forecast area with waning convection outrunning the warm sector
as they arrive in our area.

Wednesday through Saturday:

A zonally-oriented low/longwave begins to rotate and amplify
heading into the middle to latter part of the week. This results
in a broader and deeper downstream warm sector spreading
northward. The model guidance vary substantially on the
characteristics and timing of the upper level wave as it
amplifies, and likewise with the position of the various surface
features. The forecast has broad-brushed PoPs due to this
spread and uncertainty, but the likelihood is high that there
will be drier stretches during these time windows.

Temperatures also will trend cooler/seasonal owing to the
increasing clouds and rainfall potential. The trends in the
medium range guidance and ML analogs have been for any severe
weather on Wednesday or Thursday to stay south of the area. That
being said, there has been a steady increase in the rainfall
probabilities over the last 24 hours of model runs, with the
Grand Ensemble mean now around 60% for 0.50" and 40% for 1.00"
(up 30% from 24-36 hours ago).

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 645 PM CDT Sun Apr 28 2024

Another band of rain is heading toward the TAF sites this
evening, with additional scattered showers possible into the
early morning hours. The surface warm front stretches from west
to east from near OLZ to DBQ. The upper level low will continues
to lift north with the greater moisture transport east of the
TAFS. There is enough moisture convergence/warm
advection/forcing for this area of more widespread rain, however
activity will become more scattered. The confidence is low for
thunderstorms due to the lack of instability. MVFR/IFR
conditions with the storm system moving through. Some of the
guidance has some patchy LIFR conditions overnight, so will
continue to assess for this. East winds 5 to 15kts with some
variable gusts around the showers this evening. Winds become
south then southwest and increase for Monday 10 to 20kts with
gusts 20 to 30kts.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Zapotocny