361
FXUS63 KARX 060837
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
337 AM CDT Sat Jul 6 2024

.KEY MESSAGES...

- Showers and storms tonight and Sunday (60-80% chance).
  Rainfall amounts of around 0.1" to 0.25" expected with locally
  higher amounts possible.

- Remaining unsettled for first half of upcoming week with
  periodic chances for showers and storms from Sunday night
  through Wednesday.

- Temperatures fairly consistent through mid-week with highs
  slightly below average.

&&

.DISCUSSION...
Issued at 337 AM CDT Sat Jul 6 2024

Drier, Below Normal Temperatures Today:

GOES-16 water vapor imagery early this morning shows our region
sandwiched between two upper-level features caught in the general
northwesterly flow. The first wave situated to our northeast that
was responsible for our convection over the past couple of days with
northwesterly flow behind keeping our temperatures below normal. A
second shortwave trough can be noted moving out of the Canadian
Rockies and about to eject into the Dakotas. Weak surface and mid-
level divergence ongoing tonight and into early tomorrow will allow
for modest clearing of cloud cover which when combined with
relatively light winds and recent rainfall could result in some
patchy fog formation, especially in river valleys and west-central
Wisconsin. As we head through the day, upper-level northwest flow
should keep us seasonably cooler with highs in the upper 70s to
lower 80s with scattered clouds.

Showers & Storms Tonight and Sunday:

As we head into tonight, the aforementioned upper-level shortwave
moves towards our region bringing the likelihood for showers and
storms (60-80% chance) by late evening and into the overnight hours.
Mid-level Q-vector convergence towards the exit region of the trough
would enable some lift which combined with waning MLCAPE in the
06.09z RAP of 500-1000 J/kg would suggest weakening convection
moving into the region. Shear profiles seem fairly meager with the
06.06z RAP showing under 25 kts of 0-6km bulk shear overnight.
Consequently, minimal severe threat would be expected, maybe an
isolated strong storm with gusty winds and small hail in the evening
when instability and low-level lapse rates are marginally more
favorable. Showers and storms then move through the region
throughout the overnight and into the morning hours.

Rainfall amounts during this period currently trend lighter as the
environment with this shortwave is not as primed for more efficient
rainfall with precipitable waters in the 1.4-1.6" range, marginal
warm cloud depths of 3-3.5km and a relatively weak 850mb moisture
transport nose associated with the convergent forcing. Consequently,
probabilistic guidance has very low probabilities for 0.5" or
greater (around 10% chance in the 06.00z HREF) with only medium
probabilities for 0.1" or greater (40-70% chance in the 06.00z
HREF). As a result, current thinking for amounts is generally in the
range of 0.1 to 0.25" with some locally higher amounts of up to 0.5"
through Sunday morning, possibly in southwest WI/northeast IA where
the 850mb moisture transport nose is situated. Cyclonically induced
showers and storms likely occur during the afternoon and evening as
the wave departs northeast with the recent 06.06z CAMs suggesting
this potential, particularly across north-central Wisconsin.
With roughly 1000-1500 J/kg of MLCAPE and low- level lapse rates
of 7.5-8 C/km, cannot rule out some small hail with stronger
updrafts.

Additional Shower and Storm Chances into Next Week:

As the aforementioned primary shortwave pushes northeast, a secondary
shortwave perturbation is noted in deterministic guidance (06.06z
GFS/NAM/EC) late Sunday and into Monday morning as a part of a
broader synoptic trough. As this shortwave approaches the local area
from the south, low-level convergence with this wave will help
instigate a broader area of precipitation. Still some uncertainty
amounts deterministic guidance with this wave regarding both
placement and timing of its approach so holding onto chance
precipitation mention for now (30-50%) across southwest WI for
Sunday night and Monday. Regardless, probabilities in the 06.06z
GEFS/EC ensemble for 0.5" or greater with this wave are very low
across southwestern WI (10% or lower) at this moment. Furthermore,
with fairly limited instability and shear profiles, not anticipating
much of a severe threat.

As we head through the first half of the week, additional shortwave
perturbations can be noted in this complex synoptic pattern which
will continue chances for showers and storms through Wednesday.
Overall the 06.00z grand ensemble (GEFS/EC ensemble/Canadian
Ensemble) has an inter-quartile range of 0.5" to 1.25" for QPF
across the region from now through mid-week which is a sensible
spread considering the convective nature of precipitation and
generally muted moisture profiles. Towards the end of next week,
guidance pivots the aforementioned broader trough eastward resulting
in more quasi-zonal to northwesterly flow with a building ridge to
the west. Consequently, early indications suggest warming
temperatures to near or even slightly above average by the end of
the upcoming week and perhaps trending drier as well.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1112 PM CDT Fri Jul 5 2024

Main TAF concern in the short term still revolves around monitoring
the potential for fog overnight. Model guidance has backed off
some from previously noted runs, with higher focus more east of
the MS River. Have leaned towards the mention at KLSE, but there
is still some variability in short term model guidance and
uncertainties on winds just off the surface. Will continue to
monitor trends and observations through the night. Expect short
term adjustments will be possible. Winds remain light through
the period, turning more west/southwest through the day
Saturday. Otherwise, look for shower and storm chances to
increase again Saturday night.

&&

.HYDROLOGY...
Issued at 337 AM CDT Sat Jul 6 2024

Rainfall amounts with scattered showers and storms today
generally were generally around 0.25" or less with pockets of
higher amounts of 0.5" to even over 1" at Sparta. Some river
responses have been noted over many western Wisconsin basins
and the Mississippi tributaries in light of rainfall that has
fallen over the past couple of days. However, general recovery
in river stage is expected in some basins with lighter rainfall
expected into the upcoming week.

Off and on shower/storm chances are expected tonight and Sunday
which could yield additional rainfall amounts of 0.1" to 0.25"
with localized higher amounts. An unsettled pattern is expected
to persist through the first half of next week however amounts
are generally expected to remain light at this moment, likely
around 0.1" or so with each wave that passes through.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...EMS
HYDROLOGY...Naylor