781
FXUS63 KARX 081728
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1228 PM CDT Wed May 8 2024

.KEY MESSAGES...

- There has been a southwestward shift to heavier rainfall tonight
  and also a slower onset today...making for a nice day! Heaviest
  rainfall and any severe/strong storms should remain southwest of
  the area.

- River rises continue across the area and additional rainfall in
  northeast Iowa will cause further in bank rises. Heavy rainfall
  over 2" could cause higher rises but this is a low probability
  outcome (less than 3%) at this time.

- Rain showers and isolated thunderstorms likely to shift through
  mainly Wisconsin Friday afternoon and night.

&&

.DISCUSSION...
Issued at 400 AM CDT Wed May 8 2024

Storm and Heavier Rainfall Chances Later Today and Tonight

Interesting forecast for later today with strong signals for a
deformation rain band in the I-90 to I-80 corridor with increasing
moisture inflow from the south for heavier rain amounts into Thursday
morning. Later today and into the evening, as the massive longwave
trough dominating much of the central CONUS shifts south, a surface/low-
level low pressure area travels along the frontal boundary /now from
the Ohio Valley into southeast OK/. GOES derived precipitable water
indicates the air mass of ~1.50" is beginning to shift north ahead of
the surface low on the KS/OK border. By later today, the surface low
in the 08.00Z SPC HREF is in southeast IA with mean dewpoints of 60F
there, with an inverted trough extending northwest across Iowa. This
sets up a nice rainfall environment with moderate to strong east-
west 850-600 mb layer frontogenesis, the inverted trough convergence,
and rich 1" precipitable water values traveling into that nearly
stationary lift region tonight. The 08.00Z HREF 24 hour rainfall
mean/PMM/LPMM suggest 1.5"/2"/3" respectively in northeast IA by Thu
7 pm, but most of that rain falls tonight. However, there is a
continued southward model trend noted /e.g., the 07.12Z HREF had this
max more on the I-90 corridor/. The latest few runs of the NBM QPF
are slowly shifting the rain south as well, possibly just south of
the forecast area. This trend toward the moisture inflow and
instability further south makes more sense and the synoptics have
trended south as has the SPC risk for severe storms in MO. Just
seeing the 08.06Z HRRR, it slows the timing even more into today (dry
day across the forecast area!) but then places the deformation rain
band 1" line from KDBQ to KMCW (to the south, 1"+), with noticeable
lighter amounts across the I-90 corridor in the drier easterly flow
in the 925-850mb layer.

Thus, have to step forward cautiously as the trend seems southward
yet, but rainfall amounts could be several inches in the forcing.
Right now, will have the greatest rain amounts through Thursday
across northeast Iowa with around 1" and thinking any heavier
rainfall will be southwest. With dry air in the low-levels in the
later afternoon think surface-based storms to leverage the looping
hodographs is unlikely. Dewpoints could mix out in the afternoon
further stabilizing the marginal risk area. MUCAPEs in the latest
08.06Z HRRR are in the hundreds. Thus, pea-sized hail, wind gusts
above 40 mph, or funnels look to be located out of the forecast area
and are of very low probability.

Friday Afternoon and Evening Showers and Isolated Storms

Good consistency remains in a highly dynamic shortwave trough rapidly
shifting through the area in northwest flow Friday afternoon and
exiting Friday evening or so. Model consensus on a 300-500mb QVector
dipole of forcing shifts through mainly WI with widespread showers
expected. While forcing is good and wind shear is okay, the system is
pretty instability starved with most indications of 100-200 J/Kg of
MLCAPE in the afternoon. So, thunder is probable but severe storms
are not expected. Rain chances are in the 20-55% range from northeast
Iowa to northcentral WI,respectively, however as models converge on
position and timing differences, would think those chances will
increase. Right now it looks like a showery period in at least WI.

Weather Trends Toward Above Normal Temperatures Next Week

Saturday begins a period dominated by a flatter ridge over the
northern states with periodic lower rain chances and warming
temperatures. The days should be pretty sunny with above normal
temperatures in the 70s and tranquil weather seen.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1228 PM CDT Wed May 8 2024

VFR conditions are expected for much of the period with MVFR cigs
sneaking into the region by the morning hours on Thursday. A
weather disturbance will approach the region this later this
evening bringing some chances for shower into the region.
Currently, short-term guidance has been continuing to trend the
heavier precipitation further south, suggesting that areas
along and north of I-90 will likely receive minimal rainfall.
With both TAF sites right on the sharp cutoff for more
substantial rainfall, opted to hold onto a VCSH with confidence
being lower on exact timing of any showers that may occur this
evening and overnight. Otherwise, as this system begins to
depart, some wrap around moisture will allow for cigs to drop
during the later morning hours on Thursday to MVFR. Winds will
remain fairly light throughout the TAF period at around 5-10
kts.

&&

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

&&

$$

DISCUSSION...Baumgardt
AVIATION...Naylor