945
FXUS63 KARX 280058
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
758 PM CDT Thu Jun 27 2024

.KEY MESSAGES...

- Showers with isolated storms tonight into Friday morning. Storm
chances for Fri afternoon - evening. Conditional strong/severe
threat with a lot of uncertainty. Expect updates/refinement to these
concerns over the next 12-24 hours.

- Cooler, drier Sat night into Monday.

- More rounds of showers/storms Mon N-Tue and again next Thu-Fri

&&

.UPDATE...Tonight amd Friday Morning
Issued at 753 PM CDT Thu Jun 27 2024

Adjusted the rain chances for tonight and Friday morning. Warm
air advection ahead of an approaching shortwave trough has
resulted in more shower coverage across the forecast area this
evening, so raised the rain chances up considerably for areas
east of the Mississippi River. These showers should become
scattered later this evening and early overnight and then become
more widespread for the remainder of the overnight and Friday
morning as a shortwave trough, over South Dakota, moves east
through the region. There may be even a few isolated storms.

&&

.DISCUSSION...
Issued at 225 PM CDT Thu Jun 27 2024


* SHOWERS/STORMS CHANCES TONIGHT/FRI - conditional strong/severe
  storm risk Fri afternoon/evening

Broad area of low level warm air advection, along with the nose of
the low level jet/moisture transport and ripples of energy a loft,
helping to produce a broad area of showers with a few embedded
storms across SD, southwest MN into northwest IA early this
afternoon.

The lift/fuel will continue to slide eastward to across the local
area tonight, with an expansive area of rain reaching the
Mississippi River toward midnight (per latest CAMS models).
Instability is meager and elevated, but enough for at least low end
(10-20%) thunder chances. While there could be pockets of moderate
(perhaps heavy) rainfall, overall amounts tonight through Fri
morning trending mostly from 1/4 to 1/2" - generally from northwest
WI southwest into northeast IA. The rain will continue to ease off
to the east over the eastern 1/2 of the badger state for the
afternoon.

What happens westward of this rain shield is a tricker question to
answer. All the meso guidance points to a potentially deep (4kft)
stratus layer with the showers, that lingers well into the
afternoon. The GFS provides additional support for this. Clouds
would keep it cooler, reinforcing the low level inversion and
limiting the potential instability. Where the westward edge to the
low cloud deck hangs, and how quickly it shifts east aren`t clear.
CAPE pool (2k+ J/kg SCAPE per RAP in MN) progged to develop in the
clear slot to the west of the clouds during the day. Deeper shear
lies to its west, behind a cold front, although 0-3Km shear could be
enough to provide some storm organization. There will be various
forcing mechanisms that could kick off storms in the warm sector -
from a sfc cold front to bits of upper level energy/MCV, cloud-sun
thermogradients, left over outflows, etc. Not much clarity in how
this will play out. Could be some strong/severe risk where the
elements all come together, but hard to pin point where that
could/would be. Expect refinements to expectations over the next 12-
24 hours.


* DRIER, COOLER SAT NIGHT INTO MON

Cooler air set to drop in from Canada post the cold front Sat with
850 mb temps progged to drop from 16 C at 00z Sat to around 7 C by
18z Sun. A sfc high will be meandering across the region Sun while a
loft the shortwave ridge axis zips in Mon. Below normal temps should
result (5 to 10 degrees on the cool side) with mostly low 70s for
highs. Also looking at a break from the rain, at least for a couple
days.


* MON NIGHT/TUE: next round of showers, storms

Long range guidance in solid agreement with driving an upper level
shortwave trough from the PAC NW east to the northern plains
Mon/afternoon-eve and then across the upper mississippi river valley
Tue.

Broad area of low level warming and PWs around 2" will provide ample
fuel/lift for widespread rain chances ahead of the shortwave and its
attendant cold front. How much instability will be around to
increase the thunder threat will be dependent on timing - and there
are some differences (see next paragraph). Comes earlier - more
instability...later, not as much. However, the warm cloud depth (4k+
km per GFS) and the aforementioned juicy PWs will favor warm
cloud/heavy rain threat. The system looks to be transitory, helping
lower a flash flooding risk. However, depending on where the
heaviest rains fall, will work to keep river levels elevated, slower
to abate.

Some differences in the models with timing - the GEFS generally
trending a bit slower than the EC. While some of the EPS suite
suggest the related shower/storm threat could push in by Mon
afternoon, latest operational run sides with the GFS as does the
bulk of its (EPS) members. NBM chances a bit more aggressive/quicker
with onset. While slowing the system down looks like a solid trend,
will let the NBM detail the timing for now - given the system is
still several days out.


* NEXT THU/FRI: more rain chances

The progressive upper level flow currently favors driving another
shortwave trough across the northern CONUS, spinning over over the
local area in the Thu/Fri time frame. A lot of consensus in the EPS
suite of members while the GEFS has more variability. The projected
system would still have a juicy, summery airmass to work on -
highlighting widespread rain (potentially locally heavy) chances.
Too far out to fine tune any details but another period to keep an
eye on.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 640 PM CDT Thu Jun 27 2024

A series of weak shortwave troughs will move through the area
tonight and Friday. The best chances of showers will be from
tonight into Friday morning and during the mid and late
afternoon. Ceilings will be VFR tonight and then become IFR/MVFR
early Friday morning and then continue through the day. While
visibilities might become MVFR at times due to showers on
Friday, confidence was not high enough to include them in the
TAFs at this time.

&&

.HYDROLOGY...
Issued at 401 AM CDT Thu Jun 27 2024

While rivers continue to fall below flood stage, most of this
water is still on route to the Mississippi River. As a result,
the Mississippi River is expected to keep rising through the
next 3 to 5 days depending on location and routing of tributary
rivers. 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.

The next chance of precipitation returns late tonight lasting
through Friday. While widespread amounts of 0.5" to 1.0" are
not expected to immediately or substantially affect ongoing
flooding concerns, an additional widespread 1" to 2" of rainfall
upstream will prolong residence of elevated river
levels. As a result, the Mississippi River levels may initially
slightly decrease before increasing again as the additional
rainfall flows downstream this weekend into next week.

Elevated rivers with ongoing minor to moderate flood stages
from Lake City through Guttenberg are expected to persist beyond
the next 7 to 10 days.

&&

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

&&

$$

UPDATE...Boyne
DISCUSSION...Rieck
AVIATION...Boyne
HYDROLOGY...JAR