973
FXUS63 KARX 262326
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
625 PM CDT Wed Jun 26 2024

.KEY MESSAGES...

- Rain/Storm Chances Return Early Friday and Continue for Friday
  and Friday Night. Probability for .1" of rain is 80-90%
  however, for 0.5" 30 to 60% with the highest toward DBQ.

- Cooler Weekend On Tap, Temperatures 0 to 10 Degrees Below Normal
  For This Time Of Year.

- Turning More Active as we Head into Early Next Week.

&&

.DISCUSSION...
Issued at 300 PM CDT Wed Jun 26 2024

Overview:

Water vapor satellite imagery, 500mb heights, and lightning showed a
500mb trough from the Great Lakes southwest toward the southern
Plains.  A ridge was over the Rockies and the Four-Corners area.
Another area of closed low pressure was heading toward the West
Coast. A couple of large mesoscale convective systems were over
the eastern Great Lakes/PA/NY and also from southern MO/MS/LA.
Sporadic convection was noted throughout the ridge and ahead of
the West Coast trough.

Locally, we are under northwest flow aloft with surface high
pressure building in.  Satellite imagery showed sunny to partly
cloudy skies. Winds were brisk northwest with temperatures in the
70s to lower 80s with dewpoints in the 50s.

Through Friday:

A back-door cool front will move southwestward across parts of
Wisconsin through tonight with surface high pressure building in.
RAP forecast soundings show patchy low level moisture.  With the
MVFR ceilings upstream. Clouds should decrease after sunset, but will
need to monitor any patchy low cloud development. Also, where
areas are more clear, there is the potential for valley fog
with the weak low level flow. With the drier airmass and short
nights, any fog looks to be patchy. Thursday morning, return
flow is on the increase on the western periphery of the high
pressure system and the ridge aloft tries to build. Moisture
transport is on the weaker side, however some 850mb moisture
warm air advection will continue to produce clouds and a few
sprinkles or a shower could occur Thursday afternoon into
Thursday night. The RAP/HRRR both show some activity in parts of
the forecast area by 01Z,however do not agree on the area.

Moisture transport increases across Minnesota after 06Z Thursday
night into Friday, shifting eastward across the forecast area and
tends to split north and south. Meanwhile a shortwave trough is
forecast to flatten the ridge and move through the westerly flow. As
a result, we see showers and thunderstorms become more widespread 80-
90%.  At this time, the severe weather threat appears low.  Forecast
soundings show limited instability with MUCAPE of 250-500J/kg
with weak mid level winds. 0-3km shear is substantial though.
Should instability increase, the severe weather threat would be
more elevated, thus as we approach Friday, we`ll need to watch
this. The surface front lags the short wave, thus showers and
thunderstorms continue into Friday night. The upper level trough
swings through and tends to focus the storms north and south of
the forecast area. Before the cold front clears the forecast
area, some instability pushes through overnight, but the flow
is more veered by then. The EC/Canadian/GEFS ensemble
probability for .1" of rain is 80-90% however, for 0.5" 30 to
60% with the highest toward DBQ.

The weekend into next week:

Saturday, the upper level trough is digging through the Upper
Mississippi Valley with surface high pressure building in the the
west.  Generally dry weather is forecast, however forecast soundings
show a spotty shower cannot be ruled out with the heating during the
afternoon/low level instability. Surface high pressure is
forecast to be over the area Sunday. High temperatures Thursday
through Sunday are forecast to be in the 70s to lower 80s, thus
near to 10 degrees below normal.

Active weather returns for early next week.  A trough over the West
Coast during the weekend heads toward the Upper Mississippi Valley
for early next week.  Southerly flow ahead of a couples of embedded
shortwave troughs should bring sporadic thunderstorm activity to the
forecast area through possible Tuesday afternoon.  Warmer
temperatures also return with highs in the 80s.

&&

.HYDROLOGY...
Issued at 300 PM CDT Wed Jun 26 2024

Several rivers fell below flood stage over the last 12 to 24
hours, but most of this water is still on route to the
Mississippi River. The Mississippi River is expected to keep
rising through the next 5 to 7 days depending on location and
which rivers route into that particular location. Minor to
moderate flood stages are expected from Lake City through
Guttenberg. While official forecasts from the River Forecast
Center do take into consideration all of the water that has
already fallen and is on its way into the Mississippi River, it
only takes the next 24-48 hours of forecast precipitation. Since
the next chance of precipitation for the local area is not until
Friday, this will not be accounted for in the official
forecasts. At this time, rainfall expected for Friday (0.10-0.70")
does not appear that it would substantially affect the
Mississippi River forecast.

The forecast rain on Friday will likely keep the river elevated
beyond the next 7 to 10 days and the river may initially fall
some before beginning to rise again as the additional rainfall
flows downstream this weekend into next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 625 PM CDT Wed Jun 26 2024

Cooler and drier regime holding us in VFR conditions for the time
being. Diurnal cumulus will clear this evening as winds slacken.
With this surface high building in, will be monitoring risk for any
valley fog towards morning but dry layer and wind direction not
helpful for anything extensive. Perhaps only other concern might be
approach of stratus layer from the north towards morning but even
that would keep us in VFR levels. There is a scenario where MVFR
ceilings develop but risk for that is 10-15%.

Return flow behind departing high will swing winds around and could
lead to more diurnal cumulus development later Thursday, as well as
increasing high clouds from convection to the west.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...Shea
HYDROLOGY...JW/Zapotocny