887
FXUS63 KARX 150052
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
752 PM CDT Sun Jul 14 2024

.KEY MESSAGES...

- Storm chances persist for this evening into Monday evening. A
  few storms may be strong to severe with damaging wind gusts
  and localized flooding being the primary threats primarily on
  Monday. Large hail is a secondary threat.

- Hot and humid conditions continue into Monday with highs in
  the upper 80s to near 90. Widespread heat indices in the lower
  to upper 90s with some areas reaching 100 on Sunday.

- Cooler and drier conditions beginning Tuesday and into much of
  the upcoming week with highs in the middle to upper 70s.

&&

.UPDATE...
Issued at 750 PM CDT Sun Jul 14 2024

With the severe threat shifting south and southeast of our
northeast and north-central Iowa counties, we have dropped the
Severe Thunderstorm Watch.

&&

.DISCUSSION...
Issued at 304 PM CDT Sun Jul 14 2024

Humid With Storm Chances This Evening, Some Strong to Severe:

Muggy conditions are the name of the game this afternoon with
observed dewpoints across much of the region well into the 70s. Air
temperatures maintaining into the lower to mid 80s are maybe a touch
cooler than expected with some cloud cover around but still
relatively on track to see highs reach into the upper 80s in
spots by late afternoon and early evening with heat indices in
the middle to upper 90s in some locales.

As we head into the evening hours, our convective environment is
essentially a powder keg with guidance showing the potential for the
cap to erode across southern portions of our area during the evening
hours (shown in the KDBQ 14.13z RAP sounding at 6pm). When
considering MLCAPE values of 2000-3000 J/kg across our south would
expect storms to initiate very quickly with anything that provide
any kick in the atmosphere. Looking at GOES-16 water vapor imagery,
a remnant MCV can be noted across western and central MN that
is slowly trekking southeast accompanied with a subtle
shortwave. As a result, many of the CAMs rapidly develop a
cluster of storms across portions of northeast IA and
southwestern WI later this evening. Currently, the main question
with this is the aforementioned cap which tries to erode with
this weak forcing. How much the cap erodes will determine
convective coverage. Shear profiles appear fairly weak with
0-6km shear in the 14.16z RAP being fairly minimal, around 20-30
kts. As a result, would expecting a more multi-cell
configuration with individual cells struggling to maintain.
Consequently, would only expect to see hail with more robust
updrafts. However, with fairly robust DCAPE values collocated
with the MCV, would think that cores could collapse fairly
vigorously which could lead to isolated downburst gusts to 60
mph. Storms will quickly exit the region to the southeast with
the window for storms being likely a few hours (through about
8-9pm at the latest).

Severe Weather Potential Early Monday into Monday Evening:

As we head into the overnight, forecast soundings show in the 14.16z
RAP has low-level saturation and fairly light winds throughout
the low-levels. Consequently, with much of the recent rainfall
we have seen over the past few days, could see areas of fog
initially during the overnight, with questions remaining as for
how long it maintains as an incoming MCV approaches. As this MCV
and accompanying 500mb shortwave approaches, expecting to see
convection as it approaches the local area during the morning
hours. As it then progresses eastward a surface cold front is
draped across our region and pushes south with additional
convective initiation possible. As a result, with severe regimes
in play for Monday several different scenarios may be possible.

Starting with the morning MCV, as the shortwave moves through
the local area (Which also still maintains some uncertainty in
timing amongst deterministic guidance) during the morning hours,
modest MLCAPE of 2000-3000 J/kg would certainly provide enough
of a boost to get convection going. However, deterministic
soundings (14.15z RAP) would suggest a cap is still largely in
place during the morning hours which would be a serious
hindrance to convection as noted in the 14.12z HRRR which has
storms fizzling out as they approach the local area. However, if
storms can form an organized cold pool with the higher
effective bulk shear in the vicinity of the MCV. This type of
convective complex (as illustrated in the 14.12z ARW) would
likely pose some damaging wind threat if it can utilize the
instability gradient and get through the cap.

As we head into afternoon, the aforementioned cold front pushes
through the region during the afternoon. The key question as we
approach this time of day is how the cap will behave, the CAMs
vary considerably on this with the 14.12z HRRR keeping the cap
in place throughout the day leading to minimal convective
development as the front progresses through. This is contrasted
heavily by the most aggressive solution in the 14.12z ARW which
would favor an organized MCS along the boundary pushing south
with relatively uncapped thermodynamics and plenty of 0-3km
shear (roughly 35-50 kts in the 14.15z RAP) for forward
propagation and potential bowing structures. As a result, if
this scenario were to manifest would expect damaging winds to be
the primary threat with heavy rainfall potentially leading to
flash flooding concerns where soils are already very saturated.
Consequently, the SPC has a slight risk (Threat level 2 of 5)
across much of the local area.

The last caveat to this whole forecast is an organized MCS across
central IA that some of the recent CAMs have been trying to pick up
on. While much of the CAMs mainly keep it south of the area through
the afternoon and evening hours, fairly favorable 0-3km shear
profiles and MLCAPE of 3000-5000 J/kg in the 14.15z RAP would enable
a bow echo type structure to push east with the potential for
damaging wind gusts (perhaps approaching 70-80 mph).
Furthermore, this is highlighted in the 14.12z HRRR neural
network probabilities which has modest probabilities (40-50%)
for winds over 50 kts across portions of northeastern IA. With
some uncertainty in exactly where the MCS positions itself the
SPC has put extreme southern portions of our forecast area
across northeast Iowa and southwestern WI in an Enhanced risk
(Threat level 3 of 5) to address this potential. However, many
questions remain such has how any prior convection`s outflow
could affect the environment and available instability for this
MCS. As a result, will be very important to stay tuned to
upcoming forecast updates as mesoscale details become more fine
tuned.

Regardless, if storms materialize will want to watch the heavy rain
potential as well with precipitable waters of around 2" and fairly
respectable warm cloud depths (around 3.5 km in the 14.12 NAM)
which would lead to some efficient rainfall makers. While the
14.12z EC/GEFS ensemble generally only has medium probabilities
(30-60% chance) or over 0.5" through Monday night, the 14.12z
HREF ensemble max paints spots of 2-3" over the next 48 hours
which would suggest areas that receive repeat rounds of storms
could see locally heavy rainfall amounts. As a result, when
considering prior rainfall from yesterday of anywhere 2-5"
leading to saturated soils, localized flash flooding may be
possible.

Cooler and Dry Pattern Begins Tuesday:

Going into Tuesday, a broad scale synoptic trough pivots its way
into our region, aiding in the shove cold air advection further
south resulting in a cooling trend through the middle of next week.
As a result, the 14.00z grand ensemble (GEFS/EC ensemble/Canadian
ensemble) with an inter-quartile range of highs at La Crosse
for Wednesday of 71 to 76 with a median of 74. Median
temperatures in the grand ensemble generally trend in the middle
70s to lower 80s. Additionally, with increasing subsidence with
surface high pressure building in mid-week, would expect
minimal precipitation chances through Friday.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 707 PM CDT Sun Jul 14 2024

Much of the showers and storms will remain south and southeast
of the TAF sites for tonight. On Monday, there will be 2 areas
of showers and storms moving southeast through the region. The
first will move through the morning and the other during the
afternoon. Still some uncertainty on the strength of these
storms, so just went with vicinity showers at the tAF sites.

&&

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

&&

$$

UPDATE...Boyne
DISCUSSION...Naylor
AVIATION...Boyne