975
FXUS63 KARX 171658
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1158 AM CDT Wed Jul 17 2024

.KEY MESSAGES...

- Seasonably cool, dry weather will prevail through Friday, with
  slow warming into the weekend.

- Scattered shower/storm chances (20-40%) at times this weekend
  into early next week, but organized heavy rain/severe weather
  potential is quite low at this time.

&&

.DISCUSSION...
Issued at 210 AM CDT Wed Jul 17 2024

Today-Friday: Seasonably cool and dry

Canadian high pressure will build south across the Upper Midwest and
Great Lakes regions through Friday bringing seasonably cool temps
and dry conditions to the area. Diurnal cumulus development will
occur this afternoon with boundary layer warming beneath the
large upper trough/cold pool centered over the Great Lakes.
Highs today will be mainly in the 70s, about 10 degrees below
mid-July averages, with only slow moderation in temps by Friday
as the surface ridge slides east of the area. Humidity levels
will be very comfortable for mid-summer thanks to the Canadian
airmass in place keeping dew points in the 50s. Although the dry
air may be a limiting factor, with light boundary layer flow
under the ridge, will have to watch for any early morning fog
development over the next few mornings, especially in favored
low-lying areas and valleys.


Saturday-Tuesday: Slight warming with low shower/storm chances

Some moisture increase will occur heading into the weekend and early
next week back towards seasonal values as the surface ridge moves
east. However, a stagnant, blocking flow pattern will evolve
featuring an amplified western US ridge and broad, weak troughing
across parts of the central plains/Mississippi Valley. Scattered
diurnally enhanced showers/storms associated with weak lower
predictability waves within the broader trough are possible
during this time. However, there is not much signal for
organized severe weather in this pattern with minimal shear.
Little change in airmass will occur through this period with the
global ensemble suite showing relatively small day-to-day
temperature variation in the interquartile ranges. Highs
generally in the the mid/upper 70s to low/mid 80s are likely
this weekend through the middle of next week, near to a touch
below average.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1157 AM CDT Wed Jul 17 2024

VFR conditions expected. There are a few signals for valley fog
to affect the Mississippi river tonight and specifically the
KLSE airfield. Radiational cooling, a deep very dry layer
aloft, and light winds all are positives. There is some spread
in the depth of the light wind layer overnight with the HRRR/RAP
latest solutions showing the more aggressive 5kft depth. This
would favor valley fog formation but there are a number of CAMS
that suggest the layer is much shallower. Deeper mixing today
and the north wind /even light/ overnight works against fog
formation at KLSE per research and most of the guidance raw
T-Td spreads are over 7F at 03Z. So, right now, have decided to
leave valley fog visibility restrictions out of the TAF at KLSE
but it cannot be ruled out.

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

&&

$$

DISCUSSION...JM
AVIATION...Baumgardt