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243
FXUS63 KMKX 210314
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1014 PM CDT Thu Jun 20 2024

.KEY MESSAGES...

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- 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.
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&&

.UPDATE...
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Issued 1010 PM CDT Thu Jun 20 2024

The effective front created by northeasterlies off Lake Michigan
currently resides over northern IL, with a few showers and
thunderstorms along and north of it. As these storms continue to
drift northward (deeper into the stable marine airmass) they
will weaken and dissipate. Low pressure developing over Colorado
on Friday will enable southerly synoptic winds to resume south
of the boundary and lift it northward as an effective warm
front. As mentioned in the prior discussion, it is uncertain
how far north it will lift. The latest (00z) model guidance
continues to show the warm front lifting to the northern tier of
our CWA by the mid to late afternoon Friday, with scattered to
numerous showers and thunderstorms triggering along it and
overspreading the region. Slow corfidi-upshear vectors and
modeled convective modes suggest that slow moving storm clusters
are likely, hence heavy rainfall will be the main concern
Friday afternoon and evening. Forecast soundings show TEI of 25
or greater (western areas) and PWAT of 1.5 to 2 inches (all
areas), suggesting the potential for some wet microbursts and
damaging wind gusts. The highest instability resides further
west in our CWA on Friday, with CAPE exceeding 2000 joules.
Hence, our CWA remains under a marginal (level 1 out of 5) risk
for severe storms on Friday, with a slight (level 2 out of 5)
risk for far northwestern areas.

Sheppard
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&&

.SHORT TERM...
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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
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&&

.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...
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Issued 1010 PM CDT Thu Jun 20 2024

Light winds and slight chances for showers and weak
thunderstorms continue tonight into Friday morning. The cool /
humidified Lake Michigan airmass overspreading the region has
resulted in intermittent periods of IFR / LIFR cloud ceilings
intermixed with higher VFR ceilings. Areas west of a Janesville
to Fond Du Lac line are most likely to observe these low
ceilings through the overnight hours into early Friday AM.
Ceilings gradually improve Friday, then descend Friday night,
this time with the lowest cloud ceilings north of a Port
Washington to Wisconsin Dells line (along and north of the
effective warm front).

Only slight chances for showers and weak thunderstorms
overnight into Friday AM (PROB30 groups in TAFs where
applicable). Showers / thunderstorms are likely to become
stronger and more numerous Friday afternoon and evening. Storms
could produce heavy rain and gusty winds, with some severe wind
gusts possible.

Sheppard
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&&

.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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