821
FXUS63 KARX 210911
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
411 AM CDT Fri Jun 21 2024

.KEY MESSAGES...

- Repetitive rounds of heavy rain fall across much of the region
  this morning through Saturday afternoon. General rainfall
  amounts of 2-4 inches expected with localized amounts of 5-6+
  inches, the higher amounts dependent on the degree of training
  between each round of storms.

- Widespread river flooding likely as well as localized flash
  flooding due to this heavy rain. River flooding may last well
  into next week, especially along the Mississippi River.

- Potential also exists for severe thunderstorms this afternoon
  and again Saturday, but this threat will be conditional in
  nature. Damaging winds look to 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 Fri Jun 21 2024

Heavy Rain Details for Today and Saturday

A large MCS developed as forecast across southeast South Dakota
and northeast Nebraska last night and is plowing down along the
I-90 corridor in south-central Minnesota as of 09Z along a
broad surface warm front. As the convective models hinted at
yesterday, the storm track is about 50-70 miles further south
than the global models have indicated for days (and are still
indicating). In fact, the current trajectory of the MCS--almost
due east along the Corfidi vectors--is farther south than the
convective models are depicting. Additional convection is
feeding into the inflow side of the system, which will only
serve to reinforce this easterly push. Have adjusted the QPF
forecast to reflect this reality for today, but even this
adjustment may not quite be far enough south with hints that the
Corfidi vectors turn southeast into Wisconsin later this
morning.

Rainfall amounts within this lead MCS have been in the 2-4 inch
range in southwest Minnesota and southeast South Dakota with
localized pockets of 4-5 inches being reported. There will be
some degree of moisture transport into this system as it moves
through south-central into southeast Minnesota, but the overall
forcing should slowly wane through the morning and the MCS
rainfall efficiency should reflect this trend. Still am
expecting 1-2 inches with pockets of 3 inches not out of the
question as shown by the 21.00Z HREF PMM guidance, though
ongoing trends would shift this model QPF axis about 40 miles
south to near I-90.

This southern shift in the initial round of precipitation that
none of the models have fully captured casts a shadow on the
rainfall forecast for the next 24 hours and whether the entire
forecast needs a southern shift. The cold pool with this lead
MCS is not terribly strong at the moment over northwest Iowa and
there is still the upper level trough/forcing well upstream,
have stayed the course with expecting convective redevelopment
along the outflow boundary lifting northward to along I-90
throughout the afternoon in conjunction with a the ribbon of
the higher 925-850-mb theta-e advection. These cells may train
if the orientation of the warm frontal axis lines up with the
steering flow, which has been depicted by multiple solutions.

A second MCS rolls out of the Siouxland region on the nose of a
50-kt LLJ overnight, hauling east along a similar trajectory as
this morning`s setup. It is interesting to note that the 21.06Z
HRRR has shifted this second MCS way back north to a trajectory
similar to what the global models were laying out yesterday for
right now. At this point, such a solution makes little sense
given the expected development ahead of the complex, which would
shunt its trajectory southward.

Finally, convection is expected to develop once again Saturday
afternoon along or ahead of the surface cold front that marches
southeastward during the day. The threat for heavy rain is not
as high with this final wave given the orthogonal motion of the
storm cells with the frontal motion vector. However, if these
cells track over areas that have been hit hard in the preceding
24 hours, a flash flood threat could quickly ensue.

The overall total rainfall forecast has changed little in
magnitude over the last 24 hours, just shifted southward. The
run total HREF PMM QPF output through Saturday afternoon has a
solid corridor of 2-4 inches laying out along a W-E axis (north
of I-90, but in reality it is more likely to occur along I-90),
with the run max values pushing 5-8 inches. These higher values
are quite possible in more localized areas given the convective
nature of the precipitation. Flash flooding isn`t as much of a
concern with this lead wave of rain, but the soils will be
unable to handle much more rain with the next rounds this
afternoon and tonight, which is where a flash flood threat may
arise.

Severe Weather Details for This Afternoon and Saturday

Confidence in the severe weather threat for the next two days
is on the lower side given the convective contamination of the
environment. For this afternoon, if we can get enough
destabilization on the warm side of the front (not a sure-fire
bet given the upstream cloud shield), we could see a ribbon of
1000-1500 J/kg of SBCAPE along the boundary by peak heating.
Forecast hodographs are not terribly impressive with weak mid-
level flow that quickly becomes a mess once convection
initiates. There is some backing of the wind field along the
front that gives the hodographs some cyclonic concavity, but
these features also appear transient. Expect that any severe
weather threat will be very localized in nature and will need to
be assessed through the day.

Saturday afternoon`s severe weather threat location hinges very
heavily on the evolution of the MCS Saturday morning.
Indications are that the cold pool with the overnight convection
will be stronger than today`s system, possibly resulting in a
damaging wind threat with the morning wave of storms per the 00Z
HRRR NNN. This cold pool may also shunt the entire warm sector
farther south than the current outline of the SPC Day 2 Outlook.

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).

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 and amplitude of the ridge axis will drive the
temperature forecast and when the next round of rainfall occurs,
but indications are for near or even slightly below normal
temperatures for that period.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 955 PM CDT Thu Jun 20 2024

Increasingly challenging Aviation forecast. In the nearest term,
upstream observations have varied between LIFR and VFR with
thunder slowly trudging east through central Minnesota late
tonight. Has been drastically slower with every radar loop
however. Confidence increases through Friday for precipitation
moving west to east. Confidence remains low on exact location
of heaviest rainfall and convective storms. Therefore, have
continued VCTS at both TAF sites but needs further refinement as
forecast hour nears.

&&

.HYDROLOGY...
Issued at 400 AM CDT Fri Jun 21 2024

Have expanded the flood watch eastward to encompass western
Wisconsin with the latest QPF estimates from the convective
models. The greatest threat for heavy rain still exists along
and west of the Mississippi River, but confidence in the exact
location of the heavy rainfall remains on the low side given how
sensitive the forecast to even small shifts in the movement of
the storms and their effects on subsequent storm morphology.
However, confidence is high in a widespread corridor of 2-4
inches along I-90 with pockets of 5-8 inches of rain not out of
the question.

River flooding concerns will last well into next week and
multiple rivers could exceed moderate flood stage if the
rainfall lines up correctly in their basins. Have also issued
new flood watches for river flooding at various locales along
the Mississippi River and its tributaries where the highest
rainfall totals look to be lining up.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Flood Watch from 7 AM CDT this morning through Saturday
     afternoon for WIZ032>034-041>044-053>055.
MN...Flood Watch from 7 AM CDT this morning through Saturday
     afternoon for MNZ079-086>088-094>096.
IA...Flood Watch from 7 AM CDT this morning through Saturday
     afternoon for IAZ008>011-018-019.

&&

$$

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