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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KARX 260403
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1103 PM CDT Thu Apr 25 2024

.KEY MESSAGES...

- Active weather pattern starts tomorrow and lasts through
  Monday. Several periods of strong to severe storms are
  expected with moderate to heavy rainfall as well.

- Both systems (Friday into early Saturday followed by Sat night
  into Monday) will pose challenges with timing, location and
  impacts. More details will be provided through messaging and
  briefings.

- Several waves impact area next week with two rounds of precipitation
  and breezy conditions; also briefly warmer.

&&

.DISCUSSION...
Issued at 231 PM CDT Thu Apr 25 2024

.Short Term /Tonight through Friday Night/...

Confidence Short Term:  Medium to High

Large scale changes well on the way. Subjective sfc analysis shows
Great Lakes High still near GRB this morning, but expected to move
out overnight as first piece of energy over Wyoming begins to
deepen. H850 subjective analysis clearly shows large scale increase
in moisture with significant plume of +10C to +15C dew points
advecting north from the Gulf. The H850 low and most northern warm
front is the current focus for isentropic lift/saturation into
western Iowa where showers have develop this morning. Farther
south, deeper moisture and stronger H850 flow has developed a
full blown MCS. This will continue to shift southeast today.
Model guidance has been in good agreement with the overall
arrival of the first round of showers and thunderstorms with
limited chances (30 to 40%) as we move to the 12z, early commute
time frame. Have trimmed PoP back a bit with the expectation
that the Great Lakes High will continue to slowly pull east
overnight. Coincident with the morning showers/storms arrival,
stronger southeast winds and increasing gusts will overspread
the forecast area from southwest to northeast during the day.
For now, winds should remain below advisory criteria with gusts
nearing or just exceeding 40 mph during the period. After 00z,
boundary layer should decouple somewhat though occasionally
higher gusts to 35 mph may overturn from time to time. Most of
the precipitation will be showers tomorrow, though some embedded
thunder will be possible. Highs have been trimmed slightly. The
larger area of precipitation will hold temperatures down until
the first slug of warm air advection pulls northeast our far
northeast by mid to late afternoon. Highs there will actually
happen prior to the arrival of the showers, with cooling temps
in the afternoon. Instability ramps up by late afternoon with
increasing shear at that time as well. This will pose a small
threat from some elevated hail/sfc wind gusts, but with the
generally saturated column, winds should be a bit harder to mix
down. GFS PWATs quickly ramp up by midday tomorrow with 1 to
1.25in values over the region by 00z. Some efficient rainfall is
anticipated west of the Mississippi with warm cloud depths
rising to over 10kft by afternoon. The first round of showers
and storms through 00z cloud easily drop up to an inch of rain
where repeated bands of precip occurs over northeast Iowa,
mainly. Though we normally would lower our guard for severe
potential during the mid to late evening hours, synoptic and
hires models continue to advertise a second significant push of
thetae advection and increasing low level jet from 00 to 06z
Saturday. Afternoon convection in southwest/western Iowa will
continue to shift northeast and may re-intensify over central to
northeast Iowa in the evening hours. The advancing warm front
into our area, combined with increasing late evening instability
will need to be monitored. We may see a brief period of sfc
based instability approaching our forecast border with Des
Moines with winds/hail/isolated tornados possible between 04 and
06z. This activity should lessen somewhat until the morning
hours around 12z as instability weakens and the front is pulled
through the remainder of our forecast area. Lows tonight will be
milder than last night and settle into the upper 30s east to
the mid 40s elsewhere. Our current SPC SWODY2 has a marginal
risk over the region through the night with a slight risk
nearing our southwest border. It is possible that a slight
northeast shift could occur by tomorrow. Tomorrow highs should
mainly be in the mid to upper 50s with overnight mins Friday
night nearly steady to slowly rising into Saturday morning as we
become fully a part of the warm sector.

.Long Term /Saturday through Thursday/...

Confidence: Medium

The first of the two major waves will pull northeast to Lake
Superior by 18z Saturday with it gradually filling and weakening
with time. Upper level winds remain parallel to the trailing
boundary which will result in the trailing cold front slowing and
becoming stationary during the day. After lingering showers/iso
storms in the morning, we will have a brief break from most of the
action until later Saturday evening into Sunday. Saturday will see
brisk southwest flow with warm and humid conditions for the day.
Highs will reach the lower to mid to upper 70s with dew points
rising to the upper 50s to lower 60s. The nearly stationary
boundary is forecast to lift a bit tomorrow into the overnight
hours as the second significant wave lifts northeast from
western KS to eastern NE by midday Sunday. The second system is
also weakening with time, but should have plenty of
dynamics/moisture to pose various threats across the region. By
Saturday night, thunderstorms will develop along the boundary
and with some potential for repeated rains over the same region
should increase our risk for some minor flooding. There remains
a wide variance in the synoptic model data with regard to the
evolution of the heaviest rainfall Saturday night into Sunday.
PWATs will again increase to 1 to +1.25 inches near the stalled
boundary in the south. Warm cloud depths over the south again
suggest the potential for efficient rainfall processes with
rates of 1 to 2 inches per hour in the heavier storms. As
previous packages have noted, we remain in a marginal risk for
excessive rainfall. Over the two storm period, there may be a
few regions that receive up to 2 to 3 inches of rainfall. We
will continue to monitor. The severe threat also remains with
one mid level speed max crossing the area mid to late Sunday
morning and another more significant upper level wind max
crossing the region Sunday night into Monday with some
additional threat for stronger to severe storms Monday.
Temperatures through the weekend will cool by Monday with highs
by Monday back in the lower to mid 60s. Northwest flow and drier
conditions with a return of some partial sunshine on Monday as
westerly/zonal flow returns.

For the remainder of the extended from later Monday to Thursday,
additional weak wave energy will rotate through the stronger upper
level trough over Hudson Bay Canada. This will provide periods of
possible showers/storms and passing clouds Tuesday and again late
Wednesday over our north. Highs through the period Tuesday through
Thursday will range from the 60s to the lower 70s with mins mainly
in the 40s to lower 50s.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
-- Changed Discussion --
Issued at 1040 PM CDT Thu Apr 25 2024

Initial quiet conditions with mostly clear skies will make way
to an impending cloud deck seen over western Iowa at TAF
issuance. Associated flight restrictions march northeast
overnight, reaching local counties in northeast Iowa near
26.12Z. A quick drop to IFR is expected after MVFR becomes
realized. The southwest to northeasterly trajectory brings MVFR
ceilings along the Mississippi River Valley by the early
afternoon.

Thunder potential remains in flux. While confidence within the
local area is moderate-high, nailing down the exact location
remains a challenge. Therefore, given the 18 hour forecast
window, have omitted any mention for thunder at 26.06Z TAF.

Ongoing Low Level Wind Shear expected to continue into the early
morning hours at KLSE. Models would suggest another bout
returning tonight, as surface winds calm and the low level jet
persists. Timing of frontal features and associated jet will
aide coming forecasts in subsequent LLWS potential.
-- End Changed Discussion --

&&

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

&&

$$

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