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375
FXUS63 KARX 191022
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
522 AM CDT Wed Jun 19 2024

.KEY MESSAGES...

- Quieter and cooler through tomorrow with periods of showers,
  mainly in NE Iowa/SW Wisconsin today and areawide on Thursday.

- Warmer and wetter weather surges back north for Friday and
  Saturday. Locally heavy rain and a few strong storms possible
  north of I-90.

- Another brief respite from the warmth and rainfall Sunday into
  Monday, but the potential for active weather still looms for
  midweek.

&&

.DISCUSSION...
-- Changed Discussion --
Issued at 400 AM CDT Wed Jun 19 2024

Today - Thursday: Cooler with Periods of Showers

Despite ample atmospheric moisture, the lack of favorable forcing
kept the threat for heavy rain tempered yesterday evening with only
portions of south-central Minnesota and north-central Iowa seeing
over 0.50" of rain. A 40-kt LLJ overriding the near-surface front
has led to some redevelopment along the advancing cold front early
this morning in northeast Iowa, with another 0.5" to 1" of rain
expected from these training cells before winding down around
sunrise.

The nebulous surface front drops through the region this morning and
eventually sets up from southern Iowa to northern Illinois. However,
while the surface wind shift migrates well to the south, there
will be very little baroclinicity along the boundary with 60-65
degree dewpoints lingering well north of the frontal axis. The
lower tropospheric frontal surface slopes back across northeast
Iowa and southwest Wisconsin today and while forcing for ascent
will be weak and mesoscale in nature, multiple rounds of showers
are possible along this surface through the day. HREF PMM
rainfall totals for today and tonight are quite low and
generally less than 0.5" of rain is expected today and tonight
(once this overnight convection fades).

Low clouds linger for this morning, giving way to mid/high clouds
for the afternoon north of the lower tropospheric front. Coupled
with the cold air advection behind the front, highs today should
remain confined in the 70s.

The boundary moves little today and tonight with weak 500-mb height
rises to the north and a retrograding synoptic ridge to the south.
By Thursday morning, the upstream pattern gradually amplifies
and allows the 925-700 mb front to lift back northward. Synoptic
moisture transport along the front remains minimal today and
tomorrow, limiting the risk of any heavy rain to mesoscale
features/forcing. There does exist some timing differences for
how quickly this boundary lifts northward, which will dictate
how soon the heavy rain threat discussed below will begin
(possibly as early as Thursday evening).

Friday - Saturday: Warm and Muggy, Heavy Rain Threat Returns

The front lifts into central MN/WI for Friday as upstream troughing
over the west coast amplifies and ejects a series of perturbations
across the north-central CONUS. A series of training thunderstorm
complexes may work along the front during the day on Friday into
Saturday morning. This synoptic setup favors heavy rain and possibly
flash flooding. The 19.00Z NBM membership of global models is pretty
locked into the heavy rain corridor from central Minnesota to
central Wisconsin with >80% of the members showing over 2" of rain
and 30% of the members showing over 4".

A look at the individual members shows that an axis of heavy
rain exists in nearly every one. Therefore, it is not so much a
matter of if, but where, the heavy rain occurs. The highest
EPS/GEFS members have 5-6 inches of rain. Remember that these
are global models and oftentimes under-perform in convectively-
driven rainfall events, so it is very possible that these
amounts are realized in some locales. However, each round of
storms will be influenced by their predecessors and the axis of
heavy rain could easily shift in space and time (or fail to
occur).

By Saturday afternoon, the west coast trough lifts into the northern
CONUS and ushers a cold front southeastward through the region. It
is a little soon to ascertain the severe weather threat with
this frontal passage, but there is a 5-15% severe weather signal
from the GEFS ML products.

Sunday - Tuesday: Briefly Cooler and Drier

A shot of cooler Canadian air arrives in the wake of the cold front
for Sunday, dropping dewpoints below 60 degrees and highs to
around 75 to 80 degrees. Diurnal showers in the post-frontal
cyclonic flow results in lower PoPs on Sunday, but otherwise
this period features the best potential for drier conditions.
The surface ridge passes through by Monday midday and return
southerly flow sets in for Monday afternoon into Tuesday.
Warmer air streams northward again ahead of the next shortwave
in the zonal flow pattern, but the medium to longer range
solutions diverge quickly on the timing and strength of this
wave to delve into any details yet. Such a patter would once
again favor more rounds of showers and storms.
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&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
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Issued at 521 AM CDT Wed Jun 19 2024

MVFR conditions this morning in conjunction with a band of showers
and isolated thunderstorms shifts to south of a CCY to ISW line by
late morning and lifts to VFR by early afternoon. Ceilings south of
that same line may start to fall back to MVFR overnight with showers
starting to spread back northward. Winds will be light at 5-10 kts
and gradually veer from the NW to the N by early this afternoon and
then the ENE by the end of the TAF period.
-- End Changed Discussion --

&&

.HYDROLOGY...
Issued at 400 AM CDT Wed Jun 19 2024

There will be a break from the heavy rain threat today and
Thursday, but another round of heavy rainfall (2-5 inches) looms
for Friday and Saturday across central Minnesota into central
Wisconsin. This heavy rain across already saturated soils will
result in continued rises along the Mississippi and any
tributaries affected by this heavy rain. Many locations along
the Mississippi River will approach flood stage towards next
weekend (50-70% chance) if rainfall unfolds as forecast.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Skow
HYDROLOGY...Skow