135
FXUS63 KARX 111039
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
535 AM CDT Thu Jul 11 2024

.KEY MESSAGES...

- Generally quiet and seasonal weather for today and Friday. A
  stray shower or rumble of thunder possible for some today.

- Temperature warm for the weekend with highs around 90 and heat
  indices nearing 100 by Sunday.

- Still assessing how the severe weather risk for Saturday
  evening through Tuesday will unfold. The forecast remains
  complex and of a lower confidence.

&&

.DISCUSSION...
Issued at 400 AM CDT Thu Jul 11 2024

Today - Friday: Seasonal Temperatures, Mostly Dry

Mid to high clouds attendant with a persistent MCV centered
over eastern Iowa have lingered for longer than forecast across
much of the region overnight, tempering the risk for fog for
most locales. Still cannot rule out fog with visibilities under
a mile forming west of Highway 52/63 where heavier rain fell
yesterday, burning off shortly after sunrise.

For today, the energy lobe with this final shortwave wrapping
through the longwave trough stays east of the forecast area,
though there may be just enough cyclonic flow in place to pop a
few showers along a corridor near I-90 between drier air to the
north and stronger 700-600-mb capping to the south. However,
forcing and shear are weak and SBCAPE values are <750 J/kg. The
steering flow remains light and storms are not expected to move
much, therefore any areas under these cells could pick up a
quick 1/2 to 1 inch of rain as shown in the max QPF fields from
the 11.00Z HREF--but these areas will be VERY localized in
nature. With general height rises occurring through the day on
Friday ahead of shortwave ridging and deeper moisture still
lingering to the south, have stayed with a dry forecast for
Friday but did increase sky cover above the NBM "clear" grids as
we look to at least reach convective temperatures for the
afternoon on Friday.

The stagnant airmass lingers for one more day before southerly
flow sets in for Friday, thus have forecast highs very similar
to yesterday and started nudging highs a few degrees warmer for
Friday.

Saturday - Sunday: Slowly Warming, Severe Weather Details

As surface high pressure slides further to the east, a warm
front surges northeastward Friday night and Saturday morning.
With a weak upper level shortwave moving through at this same
time, cannot rule out elevated convection developing along this
corridor of theta-e advection, but these storms should be weaker
in nature with the stronger jet displaced to the west. However
as we go through the afternoon and evening, SBCAPE values
increase to over 3000 J/kg under a deepening EML and shear
profiles becomes more conducive for organized storms. That being
said, there is concern that we could remain capped over the
forecast area and differences exist in the timing of the
instigating upper level wave, with some solutions lingering the
impulse more into Sunday morning. The Slight Risk from the SPC
for Saturday afternoon/night is quite conditional in nature and
will require further assessment over the next few days,
especially as we move into the range of the CAMs.

For Sunday, the zonal +21 to +24 C thermal ridge axis builds
into the region, which is shaping up to be the hottest of the
stretch with highs around 90 and dewpoints in the low to mid-70s
pushing heat indices into the upper 90s to near 100. There
exists minimal spread (2-3 degrees) in the inter-quartile range
of the NBM inputs for max temperatures on Sunday, thus forecast
confidence is on the high side. That being said, if we somehow
have an MCS that comes through late Saturday night into Sunday
morning, highs on Sunday may need to be adjusted downward in
areas affected by convective debris or the cold pool.

The severe weather threat on Sunday hinges on what transpires
on Saturday night. The northwesterly flow during the day on
Sunday is rather laminar in nature and there isn`t much of a
signal to latch onto for determining how the convective threat
will evolve. There is a possibility that we find ourselves
between systems with the next MCS arriving Monday morning.

Monday - Wednesday: Storms and Warmer Temps Shifting South

The global models diverge quickly on the timing of Monday`s
shortwave and subsequent waves on Tuesday/Wednesday. The thermal
ridge should be shunted southward with the passage of Sunday`s
or Monday`s storms, but confidence is low on any further
details. The warmer air and storm threat pushes south of the
area by Tuesday, resulting in a very pleasant and dry forecast
for midweek as highs fall back into the upper 70s.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 535 AM CDT Wed Jul 11 2024

CIGS: thinning/exiting high clouds then some afternoon VFR cu.
Expect SKC conditions tonight. If valley fog develops at KLSE, IFR
cigs would be realized by 09-12z Sat.

WX/vsby: isold shra/ts chcs this afternoon - but a lot more miss
than hit, and confidence low in development. Will leave out of
forecast. Mostly clear skies and light winds will favor river valley
fog again. Impacts at KLSE would be likely - vsby and cigs. Some
hints in HRRR/RAP soundings that near sfc winds could stir enough to
prevent fog from spreading across KLSE, but would still favor IFR
cigs. For now will add in BCFG.

WINDS; light and variable for much of the day, but looking more
south/southeast moving into the night time hours (but still light).

&&

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

&&

$$

DISCUSSION...Skow
AVIATION.....Rieck