861
FXUS63 KARX 031645
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1145 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 /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1145 AM CDT Wed Jul 3 2024

CIGS: mostly SKC/SCT conditions into the overnight with a gradual
increase in mid level VFR toward 12z Thu. Threat for MVFR cigs
increases moving into the afternoon with convection. IFR/MVFR
looking likely for Thu night.

WX/vsby: a few morning shra/ts are possible on Thu, although signals
are somewhat weak when it comes to forcing. However, several of the
CAMS are enthusiastic on at least scattered convection so will run
with VCSH for the moment. Scattered to areas of shra/ts are expected
for the afternoon into the evening. Various forcing mechanisms for
pcpn make timing challenging. Expect some refinement to the forecast
moving into tomorrow.

WINDS: a few gusts this afternoon (20kts) then a drop after sundown
and a swing to southerly by 12z Thu.

&&

.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.....Rieck
HYDROLOGY...Zapotocny