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243
FXUS63 KARX 181858
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
158 PM CDT Tue Jun 18 2024

.KEY MESSAGES...

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- Widespread showers/storms will spread east this evening with localized
  rainfall amounts of 1-2"+ and damaging winds the primary
  threats, especially west of the Mississippi River.

- Periods of showers/storms continue over the next several days.
  Localized higher rain amounts and the cumulative impact of
  multiple rounds of rain will lead to some river rises and
  possible flood concerns.

- Drier, somewhat cooler conditions expected for early next
  week, although this break may be short-lived.
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&&

.DISCUSSION...
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Issued at 158 PM CDT Tue Jun 18 2024

This Evening-Tonight:

Early this afternoon a shortwave trough was ejecting into the
northern high plains with a cold front extending southwest from a
surface low in northern Minnesota. Ahead of the front a very warm to
hot, humid airmass persists across the region with gusty southerly
flow. Some gusts around 40 mph could occur through the afternoon for
open, unsheltered areas to the west. In addition, a few
showers/storms cannot be ruled out through the afternoon/early
evening as broad low-level moisture transport interacts with an
uncapped atmosphere.

The higher storm chances (>80%) arrive towards evening as the front
approaches. With a deep warm cloud layer and stronger deep layer
shear lagging the front, storms likely will have the tendency to
become outflow dominant as they progress towards the Mississippi
River through the evening, with localized higher wind gusts
(especially with any bowing segments) and high rain rates the
primary hazards. The hail/tornado tornado risks, while non-zero,
are considerably lower. The HREF 24-hr PMM rainfall amounts
exceed 1" for some areas west of the Mississippi River with low
(10-30%) chances for 2" or more. Low-level shear will be
stronger, but overall, confidence in sustained, balanced cold
pool development is lower given the weaker deep layer shear. As
a result, the highest risk for severe storms likely will be
within the first few hours of initiation near and west of I-35,
which agrees well with the area of SPC`s slight (level 2 of 5)
risk area today, and with the mid-level HREF UH fields.

Wednesday-Friday:

The front will sag south on Wednesday and slosh north/south through
late in the work week with an unsettled weather pattern continuing.
Wednesday will be cooler and cloudier in the post-frontal airmass
with little instability present to support much of a thunder threat,
but some showers are possible. The boundary likely will remain close
to the area within southwest flow leading to periods of
showers/storms through Friday and potential for localized heavy
rains. Global ensembles continue to indicate 40 to 60 percent
chances for 2" of rain this week (through Friday) from northeast
Iowa into central Wisconsin, although mesoscale details will impact
where higher rain amounts occur. See the hydrology section below for
more information as flood risk could increase with repetitive rounds
of storms.

Saturday:

There is a somewhat stronger signal for a deeper trough and cold
front to pass across the Upper Midwest this weekend. Ahead of the
front, temps may climb back well into the 80s late this week,
although frontal placement/timing and associated cloud/precip trends
lead to higher forecast spread. Some strong/severe storm risk is
possible ahead of the trough with sufficient flow on the southern
flank of the trough and potential for destabilization ahead of the
front, but considerable uncertainty exists in the evolution of this
system yet. The GEFS-ML probabilities show 15% chances for severe
weather centered on Wisconsin on Saturday.

Sunday-Tuesday:

In the wake of the frontal passage, global model guidance indicates
at least a transient period of drier weather comes early next week
as the flow transitions northwest and a somewhat drier airmass
builds in. However, stronger mid-level flow across the northern US
into next week within a quasizonal flow could result in a continued
active pattern with periodic shortwave passages and shower/storm
chances.
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&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1250 PM CDT Tue Jun 18 2024

A cold front, with showers and storms associated, will progress
through the region during the evening and overnight hours
bringing MVFR/IFR conditions along with it. A few showers and
storms are possible (15-25% chance) ahead of the front this
afternoon, however little consensus on how these may manifest is
seen in recent short-range guidance. As showers and storms move
out of the region, low-level post frontal clouds will envelop
the area during the morning hours on Wednesday bringing MVFR/IFR
cigs as shown in the 18.16z NBH guidance before mixing out by
the late morning and afternoon. Southerly winds will start the
TAF period at 15-25 kts with gusts approaching 35 kts in
unsheltered areas of northeast IA and southeast MN. As storms
approach during the evening hours today, higher wind gusts will
be possible. With the passage of the cold front during the
overnight, winds will diminish to around 5-10 kts by Wednesday
morning and will be generally west/northwesterly.

&&

.HYDROLOGY...
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Issued at 158 PM CDT Tue Jun 18 2024

Multiple rounds of storms capable of heavy rain remain in the
forecast for tonight and then from Thursday through Saturday.
Localized rainfall amounts of 1-2+" are possible with each
round of storms. There exists a 50 percent chance that any
given location in southeast Minnesota and west-central Wisconsin
will see storm total rainfall amounts exceed 3 inches by
Saturday and a 20 percent chance of those totals exceeding 5
inches. Some of these areas have received 2 to 4" of rain over
the past 3 days and will be more susceptible than areas to the
south and east to flooding. Each successive round of rain will
increase the risk for flash flooding and river flooding. River
flooding concerns, especially along the Mississippi River and
its Minnesota tributaries will need to be monitored closely.
Many locations along the Mississippi River will approach flood
stage towards next weekend (50-70% chance) if rainfall unfolds
as forecast.
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&&

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

&&

$$

DISCUSSION...JM
AVIATION...Naylor
HYDROLOGY...Skow/JM