650
FXUS63 KARX 180822
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
322 AM CDT Tue Jun 18 2024

.KEY MESSAGES...

- Warm and muggy today with heat indices around 95 to 100
  degrees.

- Next round of thunderstorms moves through tonight. Severe
  weather threat looks low, but locally heavy rain of 1-2"
  expected.

- Periods of showers and storms through Saturday night.
  Multiple rounds of locally heavy rain Thursday through
  Saturday may increase flooding concerns.

- Cooler and drier early next week.

&&

.DISCUSSION...
Issued at 320 AM CDT Tue Jun 18 2024

Today: Warm and Muggy

The surface boundary that lingered over the Driftless region
yesterday and served as the instigator for multiple waves of
storms has started to wash out in response to a 50-60-kt low-
level jet developing west of I-35 ahead of a 990-mb low pressure
cell lifting through the eastern Dakotas early this morning. A
+11C 700-mb cap has likewise expanded over the region with this
deepening warm air advection, shunting any overnight convective
threat further north into central and northern MN/WI.

While today is shaping up to be the warmest of the seven day
period, it is by no means a slam dunk temperature forecast.
Periods of mid level clouds may develop by sunrise, followed by
diurnal cumulus areawide and thicker afternoon mid-level clouds
towards the I-35 corridor. All told, these variables have the
potential to take the edge off our our high temperatures, but
even the coolest of the NBM inputs are only a few degrees below
our forecast highs. Therefore, no matter how you slice the
forecast, it looks to be warm and muggy. Given the clouds and
heat indices staying below advisory criteria (or briefly
touching 100 degrees for an hour), have refrained from any
headline issuance.

Tonight: Rainfall/Storm Details

The surface low continues lifting northeastward ahead of a PV
lobe noted in water vapor imagery across the northern Front
Range that reaches western Ontario by Wednesday morning. A
surface cold front slides southeastward in the wake of the
departing low, serving as the focus for convective initiation
around mid to late afternoon in western and central Minnesota. A
more favorable kinematic and thermodynamic environment at
initiation will favor some storms reaching severe levels west of
the forecast area, but with the storm motion along the boundary
given the line-parallel steering flow, the aggregate outflow
boundary/front is expected to steadily undercut the convection
through the night as it steadily works through the Driftless
region from northwest to southeast. If anything, any severe
weather threat in the evening may come in the form of localized
damaging winds as emphasized by the last few runs of the HRRR
NCAR Neural Network output.

While the convective evolution does not favor a severe weather
threat lasting long past sunset (and staying mainly to the
west), the environment will still be characterized by
anomalously high precipitable water values of 1.75 to 2 inches,
above 97-99% all other PWAT values observed around this time of
year. The slower storm motion, despite the likely fragmented
convective structure, will still lead to localized pockets of
1-2 inches of rain as depicted by the 18.00Z HREF max QPF progs.

It is worth noting that some HRRR/RAP forecast soundings go
nearly uncapped in the warm sector ahead of the main line
during the afternoon today (especially when adjusted for the
virtual temperature correction). At this stage, it is looking
more likely that the inhibition will be strong enough withstand
CI attempts, but the environment ahead of the front will be
favorable for organized supercell updrafts given the 2500 J/kg
of MLCAPE and veering low-level hodographs--IF a storm can
initiate.

Wednesday: Cooler, Rainfall Threat Briefly Slides South

The forward progression of the front steadily slows as its
upper level support wanes--its forward progression by Wednesday
morning driven in part by the strength of the cold pool from the
previous night`s storms. Most global and convective-allowing
models bring the boundary to along a line from south-central
Iowa to south-central Wisconsin for the daytime hours on
Wednesday. Subtle upper level ridging slides through during the
day with only weak 300K isentropic ascent over the now quasi-
stationary boundary, greatly limiting any forcing for stronger
storms. There still look to be periods of light to moderate
showers along the surface to 850-mb front driven the moist
atmosphere during the day, but any heavy rain threat looks muted
and south of the region. Expect more clouds than sun during the
day in the wake of the front, tempering highs in the low to
mid- 70s.

Thursday - Saturday: Warmer, Repeated Rounds of Heavy Rain

Stout upstream ridging along the Eastern Seaboard retrogrades
westward for Thursday and Friday and in doing so allows the
aforementioned surface front to advance back northward into
Minnesota and Wisconsin by Thursday afternoon. A series of weak
undulations in the zonal flow will instigate repeated rounds of
storms along the frontal surface Thursday through Saturday,
increasing the flooding risk with each passing wave. The details
of exactly where the heavier rain sets up and the timing of the
storms remains in flux, but it is safe to say that some areas
may see another 2-4 inches of rain before all is said and done
on Saturday.

With the strengthening ridge, warmer temperatures in the 80s
become increasingly likely for the latter part of the week.
However, there exists a 10 degree spread in the NBM
inter-quartile guidance, indicative of the continued convective
contamination that has plagued the temperature forecast for the
past few days. A stronger mid-latitude trough passes through on
Saturday night, once again shunting the baroclinic zone
southeastward as a cold front.

Sunday - Monday: Cooler and Drier

A shot of Canadian air arrives in the wake of the cold front
for Sunday and Monday, dropping dewpoints below 60 degrees and
highs to around 75 to 80 degrees. The potential for diurnal
showers in the post-frontal cyclonic flow necessitated lower
PoPs on Sunday, but otherwise this period features the best
potential for drier conditions with surface high pressure
sliding through.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1010 PM CDT Mon Jun 17 2024

CIGS: with warm front moving northward overnight, SKC/SCT
conditions favored through Tue afternoon. However, a few short
term models produce a period of MVFR stratus Tue morning -
converting the moist airmass advancing northward into cloud.
Think this is overly ambitious and will keep it VFR...but will
keep a close eye on satellite trends and update if the low
clouds do come to pass.

Come Tue evening, a line of SHRA/TS set to push in from the
west and a drop into MVFR looking likely with the pcpn.

WX/vsby: Next round of shra/ts looks tied to a cold front and
line of shra/ts moving in from the west. Timing for thunder
threat during the evening, trending southeast overnight and
decreasing in intensity (moreso showers). Locally heavy rains
will bring MVFR vsby, perhaps brief IFR periods.

WINDS: becoming strong and gusty for Tue with tightening
pressure gradient and favorable mixing promote sustained 15-20
and gusts into the lower 30 kts.

&&

.HYDROLOGY...
Issued at 320 AM CDT Tue Jun 18 2024

Multiple rounds of storms capable of heavy rain remain in the
forecast for tonight and then from Thursday through Saturday.
Localized rainfall amounts of 1-2+" are possible with each
round of storms. There exists a 50 percent chance that any given
location in southeast Minnesota and west-central Wisconsin will
see storm total rainfall amounts exceed 3 inches by Saturday
and a 20 percent chance of those totals exceeding 5 inches. Each
successive round of rain will increase the risk for flash
flooding and river flooding. River flooding concerns, especially
along the Mississippi River and its Minnesota tributaries will
need to be monitored closely. 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...Rieck
HYDROLOGY...Skow