409
FXUS63 KARX 011746
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1246 PM CDT Mon Jul 1 2024

.KEY MESSAGES...

- Two rounds of convection are expected Monday night into
  Tuesday night, bringing a threat for heavy rain. While the
  probability is low, some severe storms could occur late
  Tuesday afternoon and evening.

- Additional rainfall could occur Thursday through Sunday but,
  at this time, heavy rain and severe thunderstorm potential
  appears to be low during this period.

&&

.DISCUSSION...
Issued at 221 AM CDT Mon Jul 1 2024

Monday Night into Tuesday Night: Heavy Rain Threat, Potential Severe

Today, as a surface high, centered to our east as of 06z, will
continue to build eastward as an upper trough departs off the mid-
Atlantic coast. As this occurs, a second upper trough will advance
from the northern Rockies to the northern Plains. This combination
will allow for southerly return flow to resume, bringing plentiful
moisture to the CWA by tonight.

Late tonight into Tuesday morning, as a lead shortwave ejects
northeastward, expect widespread showers and thunderstorms to
develop given the 850mb moisture transport described above. Progged
PWAT values look to approach 2" as the night goes on, near or at
GEFS/ENS climatological maximums. Thus, while the system as a whole
looks to be progressive in nature - a welcome contrast compared to
the repeat generation of convection along a quasi-stationary front
seen June 21-22 - thunderstorms should be highly efficient rain
producers. Latest NBM 25th to 75th percentile values suggest around
0.5" to 1.5" of rain for most locations with the potential for
locally higher amounts as the distribution still has a long tail on
the high side, in line with what one would expect with convectively
driven heavy rain.

Moving ahead to the second main round late Tuesday afternoon into
Tuesday night, additional convection should develop ahead of a lead
shortwave progged to traverse southern portions of the CWA and ahead
of the body of the main upper trough farther north. Trend in
guidance has been for weaker moisture recovery following the initial
round of convection and a slightly farther south trajectory with the
shortwave and have reduced PoPs north of a Wabasha to Black River
Falls line accordingly. As for hazards, flooding risk seems similar
compared to the initial round, with highly efficient rain production
tempered by the progressive nature of the pattern and storm motions
of 35 mph or greater leading to an additional 0.5" to 1.5" of rain.
Severe thunderstorm risk, while potentially encompassing all hazards
given abundant deep and near-surface shear, appears increasingly
doubtful as leading edge of the best moisture return and instability
continues to trend to be just to the south of the CWA. Nonetheless,
will need to continue to closely monitor this potential.

Independence Day through Sunday: Additional Chances for Rain

Guidance is now in fairly good agreement that an upper low should
slowly churn east over SD to the Great Lakes, bringing the potential
for periodic showers and a few thunderstorms, in particular
Independence Day and Friday. As for hazards, Gulf moisture return
continues to be displaced south of the CWA as it has been for
several model cycles, so am not too concerned about flash flooding
or severe thunderstorms during the latter part of the week. That
said, any additional rainfall would serve to delay falls on any
rivers in flood due to Monday night into Tuesday night`s heavy
rain.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1247 PM CDT Mon Jul 1 2024

VFR conditions are expected into the evening with a trend
towards increasing TSRA chances and lower ceilings from 03->07Z.
A low-level jet will transport instability and moisture into
the area overnight with TSRA expected to break out across the
local area and airfields. Confidence was high enough to TEMPO
TSRA for a period overnight before the weather tapers off and
shifts east around sunrise.

A good portion of the morning hours will be MVFR, and currently
ceilings are forecast to be on the higher side in the TAFS
versus much of the model guidance. Most of the model guidance
suggests IFR ceilings are likely from 12-18Z Tuesday. With the
GLAMP/NBM guidance usually running too pessimistic, have kept
the forecasts in the MVFR range at this time where confidence
was higher.

&&

.HYDROLOGY...
Issued at 223 PM CDT Sun Jun 30 2024

Minor to moderate flooding continues along the Mississippi River and
will continue into this week. Additional rainfall is expected Monday
night into Tuesday evening. At this time...rainfall amounts of 1 to
2.5 inches are expected across the area. With the antecedent
conditions across the region...water levels will rise on area
rivers and flash flooding will be possible. Interests along
rivers...streams and creeks should monitor the latest forecast
and trends Monday night into Tuesday.

&&

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

&&

$$

DISCUSSION...Ferguson
AVIATION...Baumgardt/Kurz
HYDROLOGY...DTJ