544
FXUS63 KARX 030944
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
444 AM CDT Wed Jul 3 2024

.KEY MESSAGES...

- Seasonable temperatures today. Most areas will remain dry, however
  an isolated shower or storm could develop this afternoon.

- Shower and thunderstorm chances increase through the day on
  July 4th and continue overnight and into Friday. 24hr
  probability for 1" or more of rain at least 35 to 45%. Severe
  weather potential will be conditional on the instability.
  Current risk level is 1 of 5.

- Seasonably cool for the weekend into next week. Rain chances
  increase again for Sunday, however the probability for .5" or
  more is 20% or less.

&&

.DISCUSSION...
Issued at 400 AM CDT Wed Jul 3 2024

Overview:

Water vapor satellite imagery, 500mb heights, and lightning showed a
large 500mb trough from the Northern and Central Rockies into the
Upper and Mid-Mississippi River Valley. A large area of high
pressure was over the Mid-Atlantic and Southeast U.S. States.
Clusters of storms were noted over eastern Colorado, parts of
the Southern Plains and a MCS (mesoscale convective complex)
over central Missouri. By 08Z, the cold front had pushed east of
PVB, however dewpoints were still in the 60s for much of the
local area. The deep moisture with 2.2"+ precipitable water axis
was in the vicinity of the outflow boundary/convective complex
from central MO into central IL.

24-hr Rainfall amounts across the region per the widespread and
impressive.  MRMS radar estimate showed .5-2.5" covering a large
portion of MO/IA/central and southern WI and northwest/wcentral
IL...with pockets of 2.5 to 4".  LSE missed their daily rainfall
record of 2.05" with 2.01" and RST just missed their daily rainfall
record of 1.34" with 1.32".  Here at the NWS office, we received
2.24".  In the wake of the rainfall, low clouds and fog developed
with dense fog especially for parts of northeast Iowa and ridgetops.

Seasonable temperatures today. Most areas will remain dry, however
an isolated shower or storm could develop this afternoon:

Some fog will linger into the early morning hours until the
westerly winds can scour out the low level moisture.

A series of troughs move through the flow today and there are
hints by the various hi-resolution models/HREF that an isolated
shower or storm could develop. Some weak moisture transport,
temperatures warm into the 80s, steepening lapse rates and weak
vorticity advection occurs. Most areas remain dry, but did
include some low pops during peak heating to account for this.

Shower and thunderstorm chances increase through the day on
July 4th and continue overnight and into Friday. 24hr
Probability for 1" or more of rain at least 35 to 45%. Severe
weather potential will be conditional on the instability.
Current risk level is 1 of 5:

Return flow increases across Minnesota tonight north of the warm
front as weak shortwave trough energy works east in the westerly
flow ahead of the approaching 500mb trough. Several of the hi-
res cams hint at isolated precipitation, thus have some low pops
to cover this. During the day Thursday the 4th of July, the
500mb trough closes off over South Dakota, shifting eastward
across Minnesota. This slow-moving storm system continues over
the Upper Mississippi Region into Friday, gradually exiting the
area Saturday morning.

Although precipitable water values increase to the 1.6 to 1.8"
range, the very high 2" plus PWAT value appear to hold farther south
across MO/IL.  In addition, forecast soundings generally show lower
warm cloud depths of 3.5km vs. 4.5km with the Tuesday system.  The
EPS/GEFS forecasts show a bit of spread with the 03.00Z EPS having
nearly half the members with less than 0.10" and nearly half with
over 1" for a 24 hour period by Friday morning. The slow storm
movement and storms repeating over the same area could lead to
greater totals though. The HREF mean does not go out far enough
yet to cover the event. Will continue to mention the potential
for locally heavy rain and the potential for localized flash
flooding due to the saturated soils and 1-2"+ local amounts. The
bulk of the 1"+ area is mainly over southeast MN and western
and central WI, however pockets could occur farther south.

With the closed area of low pressure aloft and the triple point,
warm front/cold front, there is a conditional severe weather threat
depending on the instability present.  Forecast soundings show some
potential for strong to severe storms with favorable shear, however
instability may be limited.  CSU machine learning probabilities are
increased south of the local area and we are currently in a marginal
risk; risk level 1 of 5.  For now, this seems reasonable, however
increased instability would increase the potential.

Seasonably cool for the weekend into next week. Rain chances
increase again for Sunday, however the probability for .5" or more
is 20% or less:

Saturday night into Sunday and into early next week, the unsettled
weather pattern continues with a ripple then a deepening trough
working into the area.  At this time, rain amounts look to be more
of the nuisance variety versus the heavier rains we`ve seen of late.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1133 PM CDT Tue Jul 2 2024

Showers and storms have mostly pushed east of the local area.
While satellite and observations show some clearing in the
west, MVFR to IFR conditions still linger across portions of
the area. Have also noted some patchy fog has started to develop
with lingering moisture. There are some differences in model
guidance on how this evolves, but will likely introduce a period
for fog overnight to the latest forecast. Will continue to
monitor trends though and make short-term adjustments if
necessary. Otherwise, light winds tonight become more west and
increase to around 10 kts into the day. VFR conditions look to
prevail for the day Wednesday.

&&

.HYDROLOGY...
Issued at 400 AM CDT Wed Jul 3 2024

Flooding continues on the Mississippi river with 24hr rainfall
heaviest across parts of northeast Iowa into southwest WI,
resulting in higher river levels at McGregor and Guttenberg.
Some flooding was also noted on the Kickapoo Tuesday evening at
Ontario with rises at Readstown and La Farge.

Additional rainfall with locally heavy rain possible Thursday
afternoon/night into Friday. With soils being saturated, we`ll
continue to evaluate the need for a Flood Watch where the
flooding risk is greatest.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...EMS
HYDROLOGY...Zapotocny