610
FXUS63 KARX 081918
AFDARX

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

.KEY MESSAGES...

- Heaviest rainfall for tonight continues to shift south with the
highest rainfall amounts across northeast Iowa where 1 to 2 inches
are expected. Probabilities for 0.1" or greater drop dramatically
north of I-90.

- Scattered showers, a few storms, tracking southeast across the
area Fri afternoon/evening, mostly for WI.

- Periodic rain chances from the weekend into next week, but mostly
"low end" (20-40%) at this time. Temps trending 5 to 10 degrees
above the seasonable normals.

&&

.DISCUSSION...
Issued at 220 PM CDT Wed May 8 2024

* REST OF TODAY AND TONIGHT: Storms, Heavy Rain Shifting Further
  South

Pleasant conditions are expected for the rest of the afternoon as
fairly robust diurnal mixing will allow for some clearing in sky
cover which should allow temperatures to warm into the 70s for much
of the region. As a surface low moves south of the region, a mid-
level frontogenesis band will entrain itself across the southern
portion of the local area which when combined with some 850mb
moisture transport and precipitable water values of just over 1" in
northeast Iowa as shown in the recent deterministic model runs
(NAM/GFS/RAP). This will allow for fairly efficient rainfall where
the frontogenesis forcing is maximized. One recent forecast trend
that has created some challenge is the CAMs beginning to shove much
of the higher probabilities for heavier rainfall further south.

Currently, probabilities in the 08.12z HREF show modest
probabilities (40-70% chance) for 1 inch of QPF or greater across
southern portions of the local area in northeast Iowa. Additionally,
the probabilities in the 08.12z HREF of 0.1" or greater sharply
drops off north of I-90 (roughly 40% at La Crosse) where much of the
08.18z CAMs show a tight gradient in the northern edge of the
frontogenesis band. As a result, confidence remains low with how far
north more substantial precipitation will get but areas where the
forcing is best realized across northeast Iowa and points south
could see 1 to 2 inches of rainfall with locally higher amounts
possible. The strong to severe storm threat with convection appears
to be fairly minimal across our region with forecast soundings
showing only a few hundred J/kg of MUCAPE, so while some rumbles of
thunder are likely, much of the severe threat will be outside the
local area.


* FRIDAY/FRIDAY NIGHT: another round of showers, isold storms

The medium/long range guidance has been in good agreement with
dropping a shortwave trough south/southeast out of southern Canada,
sliding it across the upper mississippi river valley Friday
afternoon/night. Preceding warm air advection is weak with not well
defined Fgen along the leading edge of the shortwave. The system
brings enough saturation with it to fuel rain chances. Decent shear
with longish, straight line hodographs, but only about 250-500 J/kg
of MUCAPE. So a smattering of storms with the showers, but not
expecting anything too perky. Dry sub layer does suggest some
enhanced gustiness possible, depending on timing of any storm
(noctural sfc inversion acting as a deterrent).

* WEEKEND-EARLY NEXT WEEK: periodic rain chances, seasonable

Active, fairly zonal flow sets up for the weekend into the middle
part of the new week. EPS and GEFS continue to push various ripples
in the upper level flow across/near the region - with some expected
differences in timing/location. Nothing too organized at the moment
(locally), nor much for a tap into southernly moisture. Plenty of
saturation though to fuel occasional rain chances. For now, the
model blend will paint low end chances (20-40%), which are
reasonable given the time frame/strength of the shortwaves/model
variances. Enough instability to spark some storms too, but
currently no day is holding much for a severe threat (all subject to
change of course).

For temps, the flow points to a return to normal to a few degrees
above. GEFS and EPS have trended this way, although the GEFS has
remained the cooler of the two with 75% of its members generally at
or below the early May normals. The grand ensemble of members
suggests a 30-40% for 70+ highs for next week, but driven up by a
warm Canadian solution. Meanwhile, the NBM tops the suite of model
guidance (5 to 10 degrees above normal), riding some of the top 10%
of the EPS and GEFS which push temps into the 80s. No anomalous
fetch of warm air on the horizon per the NAEFS and EPS. The NBM
isn`t "out to lunch" if we have mostly sunny days and keep any rain
chances outside of the afternoons. For now, will ride with the
NBM...but not completely sold on its output yet.

&&

.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...Rieck
AVIATION...Naylor