695
FXUS63 KARX 171818
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
118 PM CDT Wed Jul 17 2024

.KEY MESSAGES...

- Quiet and comfortable weather for the next week with slowly
  rising temperatures to near normal /80-85F/ by the weekend
  into next week.

- Rivers will get some much needed time to fall areawide with
  little rainfall runoff expected through the middle of next
  week.

&&

.DISCUSSION...
Issued at 118 PM CDT Wed Jul 17 2024

The large scale pattern over North America will stay nearly
steady over the next week under an Omega block over the eastern
Pacific and North America per Grand Ensemble forecasts. The
area will remain under north-northwest flow which will keep
temperatures below or near normal. With some cool air aloft and
mild instability, chances of afternoon showers or a storm begin
Saturday.

GOES water vapor imagery is showing shortwave trough energy
rotating through the U.P. of Michigan and northern WI as of 18Z,
bringing broken skies and cooler than forecast temperatures in
north central WI. Ridging will build in overnight and dominate
Thursday and Friday. Some fog is possible tonight along the WI
river and possibly into northcentral WI with cooler highs
observed today and clearing expected.

Another weak weather impulse in the northwest flow "generally"
phases with the remnants of the low currently over the Pacific
NW for late week. GOES water vapor has an impressive low lifting
north through OR/WA at this time...which has a well-agreed on
guidance track over the ridge position and into the northcentral
U.S. This carves out a weak, broad trough position over WI into
the deeper low center over western IA/swrn MN. This will
promote afternoon showers centered on that trough axis beginning
Saturday as the surface ridge weakens and shifts east, lasting
through Wednesday. Instability is modest with MLCAPE of 300-750
J/Kg. Would expect much of the area to remain dry in the
afternoons with clouds increasing with heating. Precipitable
water values are about normal for mid-July at 1.25" suggesting
normal rainfall intensity. Runoff should be minimal over the
week allowing rivers to fall.

&&

.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...Baumgardt
AVIATION...Baumgardt