954
FXUS63 KARX 151654
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1154 AM CDT Sat Jun 15 2024

.KEY MESSAGES...

- Showers increase today with storms developing this afternoon
  and tonight, moving out Sunday. A few strong storms possible
  tonight into Sunday. Heavy rain will also accompany the
  storms.

- Highest shower/storm chances shift farther northwest late
  Sunday through early next week then increase with a cold front
  Tuesday night.

- Hot and humid Sunday through Tuesday. The rain from tonight
  may hold temperatures down somewhat. 30-60% probability of
  temperatures above 90 degrees Sunday.

&&

.DISCUSSION...
Issued at 345 AM CDT Sat Jun 15 2024

Overview:

Water vapor satellite imagery, 500mb heights, and lightning showed a
large mesoscale convective complex over eastern Nebraska and
northern Kansas.  This was ahead of a shortwave trough over the
Plains.  Another closed area of low pressure was noted over the
Pacific Northwest.  Convection ahead of this feature were scattered
out ahead of it.   Surface high pressure was over the local area
with the warm front over southern Kansas. The MPX 15.00Z sounding
was dry; 0.59 inches of precipitable water (PWAT).  The latest WSR-
88D radar imagery has and areas of showers across southern MN and
northern Iowa.  Cloud heights were generally fl080-l120 with 1hr
rain amounts of a trace to 0.03".  Farther west, with the line of
thunderstorms, 6hr rain amounts were mostly 0.25 to 1.25".

Through Sunday night:

A shortwave trough will move through the Plains to the Upper
Mississippi Valley with a warm front lifting north.  The mid-level
moistening, mid-level warm advection, and upper level jet support is
supporting the lead showers. Strong moisture transport and a 40 to
50kt low level jet with vorticity advection within the instability
axis is supporting the thunderstorms. Strong moisture transport
continues through today across western Iowa with the
instability axis shifting eastward. Tonight the strong moisture
transport continues across the local area, however as the
instability axis shift into the local area it weakens. PWAT
values increase to around 2". Some MUCAPE of 1500-2500 could
reach parts of the area. The low level jet continues Sunday
morning. Will include gradually increasing rain chances today
with the higher pops in the northwest (SE MN/WC WI) and the
lower pops over southwest WI. Tonight rain overspreads the
entire area and exits Sunday. Will need to watch Sunday morning
though as the LLJ remains strong in the morning with increasing
instability for backbuilding. Surface dewpoints in the 50s this
morning increase into the 60s Sunday morning and 70s Sunday, so
noticeably muggier.

The Extended:

A building ridge over the southeast U.S. and a longwave trough over
the western U.S. will make for active weather across parts of the
Upper Midwest Monday and Tuesday.  A frontal system from South
Dakota into central Minnesota is currently the focus for storms and
heavy rainfall.  The instability axis Monday morning is across our
northern forecast area and is forecast to lift north during the day.
Greater rain probabilities are across our northern counties with 10-
40% pops farther south Monday and 10-30% probabilities Monday night
through midday Tuesday.  A shortwave in the flow is forecast to
lower heights across North Dakota into Manitoba dragging a cold
front into the local area Tuesday night into Wednesday with showers,
thunderstorms, and the potential for locally heavy rain.

We continue to carry rain chances into next week.  These will vary
on how far south the front makes it Wednesday and perturbations in
the upper level flow Thursday and Friday.

Hot Temperatures:

With the warm front lifting north, we have the potential Sunday
through Tuesday to experience hot temperatures in the 80s to the 90s
each day. These highs can be affected by showers, thunderstorms,
convective cloud debris, and outflow boundaries.  Current forecasts
have maximum heat index values of 90 to around 100 degrees.  The EC
Extreme Forecast Index (EFI) shows that 70 to 85% of models are
greater than the climate max where we could see records highs and
warm lows.  The Shift of Tails still low saying it is not an extreme
event. Today we have highs in the 70s to lower 80s with 80s to
lower 90s Sunday-Tuesday.


Potential for heavy rainfall/severe weather:

Both tonight and Tuesday night, precipitable water values increase
to around 2" or 175% of normal with warm cloud depths of 4km+. Storm
motions are 20 to 30kts, but appear to be parallel to the front.
Thus, the storms will be efficient rain producers.  The warm front
tonight lifts north and the cold front Tuesday night pushes south.
Both of these are progressive, however storms could repeat over the
same area.  Sunday night into Monday, it will depend on how far
north the front make it as a strengthening low level jet develops.
The current forecast has this north of the area.  Through tonight,
we see rainfall amounts vary from a .25" to 1.50".  The 15.12Z NAM
is a bit of an outlier with a swath of 3-5" in northeast Iowa with a
stronger wave moving a little further south.  Some of the CAMs have
this as well.  Looking at where this wave is now over northeast
Colorado, there is a 3-5" bullseye there too, however although there
is rain there, 6-hr rainfall was more like 0.10-1.0", thus overdone.
As is typical, where storms repeat over the same area, higher
amounts will be possible. The HREF mean is .25 to 1" in our
southeast to .75 to pockets of 1.75-2.5 inches in our northwest
counties of parts of southeast MN/northern Iowa.  The HREF max has
much larger values o 1.25-5 to 7" with the higher amounts in stripes
over parts of northeast Iowa and southeast MN.  The instability axis
is somewhat limited when the shear is greater and as the instability
increases, the stronger shear is out ahead of it. 6-hr flash flood
guidance is generally 2 to 3 inches across the forecast area. For
heavy rain we have the strong moisture transport, warm cloud
processes, efficient rain as positives, and the potential for repeat
storms.  On the negative side, would like to see higher instability
feeding into the region and slower storm movement.   Our range of
.25"-1.50" looks good, however local spots could see a bit more.

With the higher freezing levels it will be tougher to get large
hail.  Forecast soundings show skinny CAPE with sufficient shear.
Could see some gusty winds along with the rain. Sunday night into
Monday morning the instability axis looks to be across our northern
counties and is near the 2" PWAT axis with deep layer shear of 40 to
50kts. The MDZ sounding has some moderate to high elevated CAPE
Sunday night. With the hot temperatures/deep moisture...will
need to assess the severe weather potential each day.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1154 AM CDT Sat Jun 15 2024

VFR conditions are expected this afternoon with mid-level
ceilings and areas of mainly light showers. More widespread
showers and thunderstorms will spread across the region late
this evening through early Sunday morning, with a period of
MVFR/IFR conditions possible within storms and in the post-
storm saturated air, before skies clear during the morning on
Sunday. A period of LLWS could occur this evening as a low-level
jet increases, especially at KLSE, with gusty south/southwest
winds into Sunday, particularly at KRST.
 &&

.HYDROLOGY...
Issued at 345 AM CDT Fri Jun 14 2024

At this time, it appears the local area can take the rainfall
amounts expected through tonight. That being said, due to the
higher rainfall potential over local areas, flood
advisories/warnings could occur with repeated storms over the
same area. The FFG of 2 to 3 inches for tonight would likely
decrease after the rain tonight, so will need to assess for the
multiple rounds of rainfall next week. Through Wednesday night,
a large portion of MN could see 3-4"+ of rainfall. This would
likely keep the Mississippi River elevated for the next couple
of weeks. There is a 30% chance that the Mississippi River
reaches Minor Flood Stage over the next 10 days through Winona
(and 40% chance at Wabasha).

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...
HYDROLOGY...Zapotocny