643
FXUS63 KARX 201046
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
546 AM CDT Thu Jun 20 2024

.KEY MESSAGES...

- Multiple rounds of heavy rain expected Friday and Saturday.
  How the bands line up will determine the local flooding
  threat. Confidence is lower on how the later storms Friday
  night/Saturday evolve.

- A few storms Friday and Saturday may be severe, with damaging
  winds the main threat.

- A brief respite from the active pattern for Sunday and Monday,
  but more storms loom for Monday night into Tuesday.

&&

.DISCUSSION...
Issued at 400 AM CDT Thu Jun 20 2024

Today - Tonight: Seasonably Cool, Scattered Showers/Storms

The early morning 08Z surface analysis shows the region trapped
between a 1030-mb high pressure cell centered over the
MN/Canadian border and a larger, retrograding synoptic high
pressure ridge extending from the Mid-Atlantic to the Southern
Plains. A ribbon of higher theta-e air straddles these two
ridges from Kansas northeastward into the southern Great Lakes
region. Weak isentropic ascent over this airmass has fueled
weak, disorganized convection across Iowa and Wisconsin over the
last 24 hours. Rain rates on the whole have not been overly
high, but the training trajectories of the storms have led to
1-2 inch totals in a few locations.

The upstream pattern over the High Plains begins to amplify
this afternoon and heights build over the southern Missouri
River valley. These two factors combined should nudge this
theta-e ribbon northward every so slowly today. Synoptic forcing
remains weak and output from the various convective models
shows scattered, disorganized convection at best throughout the
day. Therefore, have toned back PoPs for today, which may be
still too aggressive based on the latest HRRR runs. Bottom line,
today looks more dry then wet. Low clouds streaming
northeastward from the KS/NE region reach the region this
morning and linger for at least the morning hours. Highs today
will hinge on high quickly clouds can clear, but given the lack
of surface flow, expect similar temperature values to yesterday.

Friday - Saturday: Heavy Rain/Severe Weather Details

The most impactful period in the forecast exists from Friday
morning through Saturday afternoon, with multiple rounds of
heavy rain and severe weather on the docket. Surface to 500-mb
moisture transport increases over Nebraska and South Dakota
tonight in response to the upstream troughing digging over the
Four Corners. This results in a large MCS developing along this
moisture convergence axis and propagating ENE along the warm
front lifting through Iowa and southern Minnesota. The 00/06Z
CAM suite show this MCS bowing out and becoming more orthogonal
to the flow as it reaches central Minnesota. While rainfall
amounts of 1-2 inches are certainly within the realm of
possibilities, the orientation of this first complex is not
favorable to a widespread flash flood threat. Given the expected
storm evolution, cannot rule out pockets of damaging winds as
it arrives in the morning on Friday.

The strength of this initial convective cold pool will play a
huge role in how far south the near-surface baroclinic zone
shifts for the daytime Friday into Friday night. Theta-e
advection will be increasing during the afternoon on Friday
along the boundary, resulting in a west-to-east line of
convection developing by the mid- afternoon hours. The steering
flow will be slightly offset north of the lower tropospheric
boundary orientation, so there is some uncertainty in the degree
of training with these cells. A second MCS rolls out of South
Dakota early Saturday morning and once again drops a quick 1-3
inches of rain with its passage. Again, damaging winds would
also be possible with this early morning convection.

Finally, with the upper level trough ejecting through the
Dakotas and Minnesota during the day on Saturday, one last round
of storms develop for Saturday afternoon. How quickly the
morning convection can clear will drive the degree of
destabilization and severe weather threat. All modes of severe
weather appear on the table at this point, with any tornado
potential likely focused on the residual outflow boundary laid
out from the morning storms.

The environment will certainly be supportive of heavy rainfall
with daily record PWAT values of 2 inches expected from the EPS
by Saturday morning. The EPS Extreme Forecast Index shows this
axis of heaver QPF with EFI values of 0.8 to 0.9, but the axis
is shifted farther north than what the initial runs of the CAMs
are indicating, likely owing to the global model not correctly
assessing the cold pool strength of Friday morning`s storms.
Total QPF values could exceed 5-6 inches by the time the event
ends Saturday evening, but where this falls and how widespread
these values are will be hashed out in future forecast updates.

There you have it. Three rounds of storms in 36 hours. The
evolution and tracks of Friday night and Saturday`s storms will
be heavily influenced by the storms preceding them. The
short/medium range guidance are fairly well locked in on the
initial heavy rain axis early Friday laying out north of I-90,
with the second round of heavy rain being favored to fall more
along I-90 Friday night into Saturday morning. It remains too
early to determine where the third and final round of heavy rain
will fall on Saturday afternoon. Where these axes of heavy rain
line up will see an increased risk for river and flash
flooding.

Sunday - Monday: Brief respite from the Storms

With the main energy associated with the weekend upper level
trough lingering along the Canadian border and zonal flow
upstream, the cold air advection behind Saturday`s front will
not last long. Scattered showers in central WI and daytime
cumulus will be the main sensible weather for Sunday before the
1010-mb surface high pressure cell slides through Sunday night.
Monday morning will start on the cooler side, with progressive
flow ushering the upper tropospheric shortwave ridge axis east
of the region by Monday afternoon, allowing warm, moist air to
stream northward once again.

Monday Night - Mid-Week: Another Round of Storms, Then Cooler

A pair of shortwave troughs ripple along the MN/Canadian border
Tuesday and Wednesday and will serve as the focus for shower
and thunderstorm development. The best potential for severe
weather will be with the first round of storms Monday night into
Tuesday before the warm sector is shunted south of the region.
It remains too far out to nail down the details in the
convective evolution and threats given the differences in the
wave characteristics and timing amongst the global ensembles
(resulting in broad lower end PoPs in the forecast). Additional
showers may move through Wednesday with the second shortwave.

However, there is higher confidence in longwave ridging
building west of the region for mid to late week that will bring
quieter weather for the Wednesday to Friday timeframe. The
exact placement of the ridge axis will drive the temperature
forecast, but indications are for near or even slightly below
normal temperatures for that period.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 545 AM CDT Thu Jun 20 2024

MVFR ceilings spread northward this morning but may become more
scattered in nature in the afternoon hours before filling back
in this evening, lowering to IFR after 03-06Z. Isolated to
scattered showers are expected for the period with a lower
threat for a thunderstorm. A larger complex of thunderstorms
approaches southeast Minnesota towards the end of the TAF
period. Winds will be light from the east to northeast through
Friday morning.

&&

.HYDROLOGY...
Issued at 400 AM CDT Thu Jun 20 2024

Multiple rounds of heavy rain are forecast for Friday morning
through Saturday afternoon. Total QPF values may locally exceed
5-6 inches with more widespread amounts of 2-4 inches becoming
increasingly likely. Given the breaks between each round of
storms, any major flash flood threat will occur where the axes
of heavy rain overlap. These rains on top of already saturated
soils will result in river flooding concerns along the
Mississippi River and its tributaries. Many locations along the
Mississippi River will exceed flood stage towards next weekend
(60-90% chance) if rainfall unfolds as forecast. Should the
heavy rain line up over the smaller river basins, there is a
10-20 percent chance that these basins could see periods of
flooding at moderate flood stage.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Skow
HYDROLOGY...Skow