697
FXUS63 KARX 151744
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1244 PM CDT Mon Jul 15 2024

.KEY MESSAGES...

- One more very warm, sticky today day today before drier and seasonably
  cool temps return for much of the rest of the week.

- Severe storms are possible, especially late this afternoon or
  early evening, but confidence in storm evolution is low today
  due to potential impacts from morning showers/storms.

&&

.DISCUSSION...
Issued at 305 AM CDT Mon Jul 15 2024

Today-Tonight:

With the clear skies, moist boundary layer, and light flow, fog
have formed, even outside of more typical valley locations.
This fog will gradually erode through mid-morning.

Otherwise, early this morning convection was ongoing across the
northern plains ahead of an upstream shortwave trough. It will be
yet another challenging forecast for storms today with multiple
factors at play. Confidence is low exactly how storms will
evolve. In general, convection to the west should continue to
drop southeast this morning, likely weakening in time as it
approaches, given capping in place and weak moisture transport
into the area. However, this evolution is not entirely clear,
especially with these storms riding along the gradient of an
ample instability environment. Stronger storms could not be
completely ruled out even during the morning, especially
westward.

One possible scenario would be for additional storms to fire along
the edge of the outflow from the morning convection or on the
cold front, which could organize along the gradient in
instability (from northern Iowa into northern Illinois) into a
forward propagating MCS during the afternoon/evening, especially
across parts of Illinois/Iowa and perhaps far southern
Wisconsin. This is the area SPC has included in a level 3 of 5
risk for severe storms. Additional storms could form along the
cold front farther north during the later afternoon/evening with
some assistance from forcing from the main shortwave trough
passing along the Canadian border. However, the effects of any
residual cloud cover/capping could impact storm
coverage/intensity along the front to north, keeping confidence
in these low. Deep layer shear profiles certainly would support
storm organization/severe storms with 30 to 50 kts of westerly
shear and 20 to 30 kts of 0 to 3 km shear. Damaging winds would
be the main hazard if/where a linear convective system
organizes, otherwise, some hail would be possible if
sufficiently strong updrafts form along the boundary where deep
layer shear profiles are oriented at an angle to the cold front.
Any storms should exit south and east during the evening.

With precipitable water values at 150%+ of average and relatively
deep warm cloud depths, any storms will be efficient rain producers.
While widespread heavy rain is not likely with this system being
fairly progressive, localized amounts of 1 to 2"+ are certainly
possible in this environment.

Tuesday-Sunday:

An extended period of mainly dry, seasonably cool weather is likely
for the middle and end of the week as sprawling Canadian high
pressure drifts south across the Midwest under northwest flow
aloft. A few showers could not be ruled out on Tuesday as a
trough deepens over the Great Lakes ahead of the high. High
temps will be mainly in the 70s to low 80s with lows in the 50s,
and even some 40s in typically cooler locales.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1236 PM CDT Mon Jul 15 2024

A cold front will approach the area from the NW this afternoon
bringing shower and thunderstorm chances across the area. Due to
the cloud cover this morning, uncertainty continues on if we
will see thunderstorm chances impact LSE or RST directly. Will
plan on amending TAFs as needed for thunderstorms.

Tonight, there is another chance for fog to impact the river
valleys and LSE, but chances for dense fog are around 10 to 20%
at this time. Boundary layer winds look to be strong enough to
stave off widespread fog issues, but fog chances would increase
considerably if skies clear and winds go light until cold front
swings through and brings in drier air.

&&

.HYDROLOGY...
Issued at 305 AM CDT Mon Jul 15 2024

A swath of 2 to 4 inches of rain fell this weekend along the
Mississippi River in west-central Wisconsin into central Wisconsin
with river rises/flooding along portions of the Kickapoo and
Trempealeau Rivers. Although the environment today will be favorable
for high rain rates with a moist, unstable environment, confidence
in organized heavy rainfall is low today with the progressive
shortwave/front. However, localized heavy rainfall amounts (1 to 2")
are possible with any storms. Given saturated conditions and low
flash flood guidance in the areas that received higher amounts over
the weekend, convective trends will need to be closely monitored
today. However, confidence at this time is not high enough for a
flash flood watch.

&&

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

&&

$$

DISCUSSION...JM
AVIATION...JAW
HYDROLOGY...JM