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000
FXUS63 KARX 251034
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
534 AM CDT Thu Apr 25 2024

.KEY MESSAGES...

- Active weather pattern starts on Friday lasting into Monday.
  Rain and some thunder moves in Friday, delayed a bit from
  previous forecasts.

- Probabilities increase for more impacts from weather later on
  Saturday through Sunday night. Severe thunderstorms are
  possible across northeast IA into southern WI Saturday
  afternoon and eve, transitioning to periods of rain, possibly
  heavy into Sunday night. Risk of flooding has taken a small
  step up.

&&

.DISCUSSION...
Issued at 337 AM CDT Thu Apr 25 2024

Active Weather Begins Friday

Another nice day is in store today as high pressure remains
over the area. Temperatures will be in the upper 50s to mid 60s
with mostly sunny skies expected. Winds will be light, 10 to
15mph, with gusts up to 25 mph. Relative humidity values will
be between 20 and 30%, with the lower values east of the
Mississippi. With an approaching weather system, southeast winds
will increase overnight Thursday into Friday, just below Wind
Advisory criteria /30mphG45mph/ in MN/IA Fri afternoon. Some
adjustments were made to the onset of precipitation due to dry
air in the lower levels throughout the morning hours. With this
in mind, for areas west of the Mississippi, precipitation is
expected to begin early to mid morning on Friday. With modest
instability of hundreds of elevated CAPE, and low-level moisture
transport convergence increasing and shifting east, precipitation
will spread east into WI mainly in the afternoon. Forcing and
instability increase on the low-level jet Friday night with
widespread rainfall expected. Most probabilities suggest
0.50-1.0" is likely by Saturday morning.

Severe Storms Possible Saturday East/South of La Crosse

Saturday still looks to be drier than Friday or Sunday, with a
fairly nice afternoon for most as shortwave ridging builds in
after the departing Friday wave. Probabilities in the 24.18Z
and 25.00Z ensembles are starting to suggest a cold front
shifting slowly southeast Saturday, arriving in northeast IA and
southwest WI by late afternoon. Instability /MLCAPE/ with rich
low-level moisture grows to around 2000 J/Kg near and south of
the front, with steep lapse rates in an elevated mixed layer
aloft, and supercell wind shear. This would provide a threat for
damaging wind, large hail, and tornadoes. Questions remain
however on the frontal location 65+ hours out and also the
upper level forcing/lift with shortwave ridging building through
and the next low pressure area far away in western Kansas. This
could limit the coverage of storms. Stay tuned as this still
has plenty of time to change, and thus decided not to heavily
message this quite yet. For Saturday afternoon, have decreased
the rain chances northwest of the front (a nice day!).

Heavy Rainfall Chances Increase Saturday night to Sunday Night

The cold front is expected to stall in the vicinity of northeast
Iowa and southwest Wisconsin, possibly south, in advance of the
next stronger shortwave trough shifting toward the area. An
increasingly moisture rich southerly flow will encounter the
front, isentropically lifting unstable air and converging on the
northern nose of the low-level jet. Depending on how well the
ingredients come together, there could be an elevated storm
large hail threat overnight and definitely a heavy rainfall
threat. Precipitable water values in this incoming warm sector
will be in the 1.25-1.50" range per ensemble means from the
latest ECMWF/GFS ensembles...which is near or record for late
April. Should MUCAPE remain ~1000 J/Kg and the low-level
moisture transport convergence moderate to strong (suggested in
25.00Z NAM), rainfall rates could be 1-2" per hour, with a
preference for areas south of I-90. The latest ECMWF ensemble
suggests a bit further north solution and a more widespread rain
solution and no so focused near the warm front. With the
ensemble consensus suggesting the low-level low center track to
the northwest of the area, and prolonged southwest flow of near
record precipitable water values, the area could be in a Maddox
frontal-type heavy rain archetype. Thus, the threat of heavy
rainfall and possibly flooding (~10%) has increased some in this
forecast. The WPC excessive rainfall outlook for the weekend is
in the marginal category, but could grow should confidence in
these ingredients change. Something to keep an eye on.

Above Normal Temperatures With Continued Rain Chances

The forecast for the remainder of the week looks of lower
confidence with above normal temperatures likely and periodic
rain chances.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 529 AM CDT Thu Apr 25 2024

VFR conditions expected. Winds will be increasing today and
through the night with gusts expected as low pressure draws
closer to the area. Moisture will be on the increase Friday
after 12Z with increased probabilities for lower LIFR and IFR
ceilings.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Frost Advisory until 8 AM CDT this morning for WIZ053-054-061.
     Freeze Warning until 8 AM CDT this morning for WIZ055.
MN...None.
IA...None.

&&

$$

DISCUSSION...Baumgardt/Cecava
AVIATION...Baumgardt