307
FXUS63 KMKX 202123
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
423 PM CDT Thu Jun 20 2024

.KEY MESSAGES...

- Isolated to scattered showers with a thunderstorm or two
  possible along the WI/IL border this afternoon/evening, but
  low chances for stronger storm due to northeasterly flow off
  of Lake Michigan.

- Additional thunderstorm chances for Friday and Saturday
  afternoon/evening. Could see a stronger storm or two Friday
  with the better potential further west. However, Saturday`s
  setup is lending way to a better risk (2 out 5) to see a few
  stronger to even severe thunderstorms. With any stronger
  thunderstorms, main concern will be damaging winds and large
  hail.

- Heat indices will climb back into the upper 80s to mid 90s
  Friday and again Saturday, especially for inland areas along
  and south of I- 94/HWY-18.

&&

.SHORT TERM...
Issued 421 PM CDT Thu Jun 20 2024

Tonight through Friday night:

The frontal boundary looks to have pushed south of the WI/IL
border this afternoon with northeasterly winds off Lake Michigan
overtaking the region. This has brought a less favorable/more
stable airmass which may be the limiting factor to afternoon
shower development despite the ongoing development just south of
Cheddar Curtain in northern IL. While the current obs favor the
convection to remain further south where the frontal boundary
and thermodynamics are located, it would not be out of the
question to see a shower or two develop along or just north
later this afternoon and evening, especially for southwestern
WI. However coverage is likely to be isolated to spotty and most
locations with the increased precip chances will likely stay
dry through the evening.

Overnight will be cooler compared to earlier in the week with
lows in 60s with lingering cloud cover. With the high pressure
meandering around Lake Superior, expect lighter variable winds
overnight. Also cannot rule out some patchy fog overnight in
low-lying spots given the ample surface moisture and weak winds.


The frontal boundary stalled south of the WI/IL is then progged
to lift back northward turn more warm front through Friday
afternoon. The main question is how far north will it make it.
Generally the deterministic models have it at leas reaching the
northern tier of our CWA, but the onshore flow ahead of it may
limit its northward progress or pinch it to our inland counties.
Nevertheless this will be the focus for the afternoon
thunderstorm potential, but also south of the boundary for where
our higher heat indices will be.

For the thunderstorm potential, there is not much in the way of
upper-level dynamics to support sustained storm development
other than the surface front lending way to more brief pulse
type mode if anything develops. However, given the building
instability with SBCAPE nearing 1000 J/kg, ample moisture with
PWAT 1.5-2.0 inches, and meager 20 knots of deep layer shear
(0-6km) cannot rule out seeing a stronger storm or two to
develop in this environment along the boundary, especially if
there is any mesoscale/locally driven forcing/boundaries.
Overall the strong thunderstorm potential is there, but coverage
will be spotty/scattered and exact locations is still up in the
air due to uncertainty on where the effective surface warm
front sets up. Will likely see this activity along the front
linger overnight and even into Saturday morning, but likely to
transition more to a heavy rainfall threat overnight, especially
if it trains over the same area for extended period of time.

Otherwise, areas south of the warm front will see temps warm
back into mid to upper 80s with heat indices in the low to mid
90s. However, expect it to be cooler north of wherever the front
sets up and for areas along the lakeshore.

Wagner

&&

.LONG TERM...
Issued 421 PM CDT Thu Jun 20 2024

Saturday through Thursday:

Saturday looks to present a better more upper-level dynamic
driven setup for stronger thunderstorms. Southern WI will be in
an area where a mid-level shortwave trough passage aligns with
upper-level diffluence paired between the left exit and right
entrance of a 250mb jet. Combined with 700/850mb trough and
surface low tracking across northwestern WI and developing LLJ
through the evening, it is leaning toward favorable synoptic
forcing for convective development Saturday. Again it will be
hot and humid and instability looks to build to +1000 J/kg and
deep layer shears looks to be a bit better around 30-40 knots.
Thus, the ingredients to support stronger to severe development
in the warm sector ahead of a cold front continue to improve and
will monitor to see if the trends continue with later model
run. One limiting factor to this will be if the overnight
showers and thunderstorms linger through the morning,
stabilizing the environment. So will continue to monitor trends.

After a cold frontal passage overnight Saturday, dry conditions
under high pressure are expected to start Sunday and linger
through early week. Temperatures will slowly increase to near 90
degrees once again by Tuesday. An additional developing system
may bring shower chances Tuesday into Wednesday (30-50 percent
chance). Timing is less certain on this impulse, so kept NBM
output for the time being.

Wagner

&&

.AVIATION...
Issued 421 PM CDT Thu Jun 20 2024

A swath of MVFR/IFR continue across portions of southern WI
this afternoon and evening while areas along and just north of
the WI/IL are siting under mainly VFR conditions. There will be
additional spotty/scattered shower and a thunderstorm or two
possible this afternoon as a stalled frontal lingers along the
WI/IL. JVL and MSN continue to see the better chances of seeing
impacts from this afternoon activity, but cannot rule out a
chance for ENW, especially with ongoing activity in northeast
IL. Otherwise, lighter northeast to easterly winds off Lake
Michigan will keep conditions less favorable for shower/storm
develop this afternoon. Overnight expect high pressure over Lake
Superior to influence the pattern with light winds, but cannot
rule out some patchy fog and low ceilings to develop given the
increased surface moisture, especially for areas closer to the
shores of Lake Michigan.

Wagner

&&

.MARINE...
Issued 421 PM CDT Thu Jun 20 2024

Patchy dense fog lingers across southwest portions of Lake
Michigan this evening and will continue overnight into early
Friday morning. Expect light northeasterly winds over the Lake
Michigan through tonight as high pressure meanders over Lake
Superior with a stalled frontal boundary sitting across northern
IL. Expecting lighter winds to persist through Friday, but will
increase and shift more southerly overnight Friday into
Saturday as low pressure develops across the Plains and lifts
across the Upper Midwest through the day Saturday. Then
expecting a cold front to push across the Lake later Saturday
as the low lifts to the northeast and winds turn more west-
northwesterly for the end of the weekend.Wagner

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

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