756
FXUS63 KARX 040005
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
705 PM CDT Wed Jul 3 2024

.KEY MESSAGES...

- July 4th has shower and thunderstorm chances increasing from
  west to east as the day progresses, with most areas seeing
  rain at some point during the afternoon and evening. There was
  another uptick to the severe storm threat with the area now
  in a slight risk /level 2/ for wrn WI, nern IA, and sern MN,
  mainly in the afternoon.

- Showers still on track for Friday with the most rainfall
  occurring north of I-90. Amounts should remain behaved with
  little impact.

- Unsettled weather into the weekend and early next week with
  periodic rain chances and below normal temperatures. Heavy
  rain and severe weather chances look minimal.

&&

.UPDATE...
Issued at 700 PM CDT Wed Jul 3 2024

A somewhat conditional fog forecast exists for tonight. The
first hurdle to overcome are clouds with multiple bands of
upstream mid to high level clouds present. However, these are
expected to dissipate later this evening before clouds thicken
up after 3-6am, presenting a window of opportunity for fog
formation. ADJLAV and GLAMP are quite aggressive with the fog
formation along and south of I-90 (even showing some 1/4 sm
values towards sunrise), but stand apart from the various model-
driven visibility progs from the CAMs that are devoid of any
fog. Forecast soundings, if we stay clear, may indeed support
fog with winds expected to become light under a weak surface
ridge and soils still saturated from recent rainfall. For now,
have added patchy to areas of fog to the grids and the coverage
of the fog will need assessing through the night.

&&

.DISCUSSION...
Issued at 245 PM CDT Wed Jul 3 2024

Periodic Showers and Storms on July 4th, Severe Storms Possible

While there remains some differences in the details of the forecast,
the bigger picture is becoming more clear with consistent signals of
a dynamic mid-level trough deepening into the longwave trough over
the central U.S. with surface cyclogenesis deepening a low over
the area through Friday. GOES water vapor and lightning strike
data is impressive over MT right now with this incoming trough.
This system leverages the moist pool to the south with warm
advection and modest moisture transport convergence during the
day Thursday. Instability is becoming more agreed on which is
also modest at 750-1000 J/Kg of MLCAPE. With increasing mid-
level flow impinging on the area, there is a slight uptick in
the severe storm potential. SPC has upgraded to a slight risk
/level 2/ for near and west of the Miss river. While there
might be isolated severe storm risk, there are still questions
on the low level wind shear profile /weaker?/, shortening the
hodograph, and storm organization potential. But, with dynamics
coming in, good wind shear above 2km, and surface-based instability
growing in the afternoon, severe storms are possible. Damaging
wind and hail to 1" would be possible /higher freezing level
doesnt help/. Rainfall rates will be good, but certainly not
the high-end rates per the more tropic-like air mass we were in
Tuesday...with rain amounts around 1/2 inch most probable.

The morning may have a few showers around before clearing,
mainly west and north of La Crosse. Some inconsistency in the
CAM solutions for what appears to be only a weak mid-level shortwave
trough in the southwest flow ahead of the main larger low. This
is spawning the TSRA activity currently over SD/NEB. There isnt
much low-level support for the rain chances.

During the evening of the 4th, tried to build some better details
into the precipitation chances and honor some of the CAMS suggesting
a drying trend coming in from the west. While there is still spread
in the CAM solutions for Thursday night, it appears WI will be
the place for higher rain chances with more scattered to widely
scattered activity over sern MN/nern IA.

Showers Continue Friday

One noticeable trend in the guidance is to clear the weather
eastward as Friday afternoon progresses and having a dry
evening in most areas. Until the low pulls east and the clearing
occurs, a well-established deformation band wrapping northeastward
around the low will favor the northern forecast area for
heavier rainfall amounts The I-94 corridor and north amounts
look to be 0.25-0.50" Friday, diminishing southward /less than a
tenth/. Instability is negligible so showers will prevail and
there are still some differences on how this pivots as the low
ejects east...the southern forecast area looks like it could
have lower rain chances than currently forecast. It will be a
much cooler day in clouds and showers...built for frogs...with
highs in the 60s mainly.

More Rain Sunday Looking Likely, Showers into Next Week

Longwave troughing over the central CONUS is persistent into next
week with a series of shortwaves troughs shifting through. The next
is shifting in Saturday night and Sunday making for what looks like
another shower and thunderstorm day. More consistency in the timing
is occurring with good agreement on the system ejecting in from the
southwest with low-level moisture transport and warm advection.
Signals are low for over an inch of rainfall Saturday night and
Sunday (5-20%) at this time. Into early next week, more shortwave
trough activity shifting through the longwave trough means
slightly cooler than normal with shower chances. These do not
look like heavy rain makers at this time.

A pattern shift is seen in the extended forecasts by mid-end of next
week to possibly warmer and drier.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 700 PM CDT Wed Jul 3 2024

Immediate aviation concern for the near-term revolves around fog
potential from 06-12Z, which will be driven by cloud cover and
possibly winds--with both expected to decrease through the
evening. Best fog potential looks to exist south of the TAF
sites, but did add a BCFG to LSE to account for the possibility
near the river. Showers and thunderstorms move through during
the day tomorrow, but confidence on timing at the TAF sites is
low. Have continued the mention of VCSH through the day with an
added PROB30 for the best window for thunder potential in the
afternoon. Winds will generally be from the west to south at
5-10 kts. Ceilings do start to decrease to MVFR in the
afternoon, mainly north of the TAF sites and do continue to fall
to IFR/low MVFR overnight.

&&

.HYDROLOGY...
Issued at 245 PM CDT Wed Jul 3 2024

The main changes from this river forecast cycle were to refine the
Kickapoo river levels. Most river forecasts remained the same,
however, it should be noted that ALL of the rainfall expected
Thursday and Friday was NOT in the river forecast. Thus, it is
expected that this evenings forecast will see a bump in the
river levels as Thursday rainfall is accounted for. In reviewing
the tributaries, it seems 2-3" is needed to move these rivers to
higher level flooding...and these are above current forecasts.
The 03.12Z HREF suggests a 10-20% chance of 2"+ through Friday
morning, on and north of I-90. Current forecasts are 1-1.25"
for that same area.

&&

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

&&

$$

UPDATE...Skow
DISCUSSION...Baumgardt
AVIATION...Skow
HYDROLOGY...Baumgardt