339
FXUS63 KLOT 251138
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
638 AM CDT Tue Jun 25 2024

.KEY MESSAGES...

- Two potential rounds of severe thunderstorms capable of
  producing damaging winds and flash flooding today. The first
  window mid-morning across northern Illinois and the second
  window late afternoon into mid-evening across much of the
  forecast area.

- Another period of active weather with strong to severe
  thunderstorms and flash flooding is possible Friday into
  Saturday.

&&

.DISCUSSION...
Issued at 413 AM CDT Tue Jun 25 2024

Through Tonight:

A substantial pool of high theta-e air in eastern Iowa owing to low
to mid 70s dew points will gradually advect eastward into the
forecast area this morning below a growing EML plume. The result is
an impressive CAPE profile primed for explosive thunderstorm at any
point through this evening. The major caveat is the corresponding
cap that will also be advecting into the area this morning. At this
time, we are focused on two areas of potential severe convection in
our area:

1) A mature MCS across central/east-central Wisconsin has started to
turn more south along the existing MUCAPE gradient as the veering
low-level jet impinges on the western flank of the line. Recent
elevated convection developing over southwest Wisconsin indicates
that the incoming EML has not capped the environment east of the
Mississippi River. So, there is growing concern that the gap between
this convection and the MCS begins to fill in over the next hour or
two and grow upscale into northern Illinois in the 6-10am window.
Will be closely watching this activity through sunrise as the
downstream thermodynamic environment supports a damaging wind threat
with any convection this morning.

2) Conditional on the absence of any substantial lingering cold pool
across northern Illinois through mid-morning, recent CAM guidance
suggests convective temps in the mid to upper 90s may be reached by
mid afternoon. With a mid-level wave over eastern NE/SD and
resultant convective enhancement of a decaying MCS early this
morning, this feature may provide the needed forcing to allow
convection to rapidly grow roughly around the I-80/88 corridors by 3-
5pm. Though deep-layer shear will be quite modest, MLCAPE values
over 4,000 J/kg and DCAPE values surpassing 1,500 J/kg will support
initial discrete cells/clusters with damaging hail/winds congealing
into a slow-moving line with damaging winds and flooding from
torrential rainfall. Ultimately, the convective line should drift
south through the remainder of the CWA through this evening,
continuing to produce a notable flood threat as low-level flow
becomes only slightly oblique to the convection (and backward
propagation vectors begin to turn to the southwest).

Kluber


Wednesday through Monday:

A cold front will press across the region on Wednesday. Depending
on how things evolve Tuesday evening and overnight, some
convection may be ongoing across the area Wednesday morning,
mainly south of I-80, but the trend should be towards a drier one
through the day. Some guidance does show some fairly deep boundary
layer mixing developing through the afternoon, perhaps enough to
pop a few high-based showers, but this doesn`t seem like a
significant enough signal to justify PoPs at this point. A brief
period of dry and quiet conditions will then prevail through
Thursday as a transient region of high pressure drifts overhead.

By Thursday night into Friday morning, the surface high will be
departing and shuttling across the northeast, yielding a return of
moist southerly trajectories in the region. A series of low-
amplitude/subtle shortwaves emanating from a robust upper low
pinwheeling across Montana and into Saskatchewan will likely get
swept up within freshening mid and upper level west-southwesterly
flow. This in turn will lead a commensurate increase in low-level
warm advection across Iowa, which will eventually spill east of
the Mississippi River during the day on Friday.

Latest model guidance suggests that warm advection/isentropic
upglide-induced convection will probably develop late Thursday
night into Friday morning across Iowa within the plume of
returning elevated instability. However, latest indications are
that this activity may outpace the slowly-returning elevated mixed
layer leading to a gradual weakening trend through the day.
However, Friday night continues to look like a period we`ll have
to watch closely, as another area of low pressure and associated
cold front will be impinging on the region from the north. Ahead
of this, intensifying warm advection acting on any remnant outflow
from morning activity will also finally be able to impinge on the
returning warm/moist sector as the kinematic parameter space also
becomes more supportive of storm organization. Still pretty far
out with plenty of spread and uncertainties, but this is a period
we`ll continue to closely eye for a severe and also flash flood
potential with PWATs forecast to surge into the 2 to 2.5 inch
range.

We`ll get into another brief quiet period over the weekend as high
pressure returns. From a synoptic perspective, winds may surge a
bit down the lake as the high arrives which could yield a building
wave/rip current potential over the weekend.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 638 AM CDT Tue Jun 25 2024

Key Aviation Messages:
- Line of TS with strong winds potentially in excess of 40
  knots over the Chicago terminals through 14-15Z.

- Conditional chance of another line of TS with strong winds
  late afternoon into early evening.

A line of strong TS bearing down on the Chicago terminals at
TAF issuance should shift east in the 14-15Z window. IFR to
briefly LIFR visibility and wind gusts of 40 knots or higher can
be expected with this line of storms. Winds may be quite VRB
during and shortly after the TS.

Behind the line of TS, the low-level environment will begin to
recover on W/WSW winds gusting around 25 knots by early
afternoon. Another line of TS is expected to develop late
afternoon into early evening, but confidence of the exact
placement (e.g. over or just south of the terminals) of the TS
supports maintaining only a PROB30 from 21-01Z at this time.

Behind this potential second round of TS, winds will settle
west around 10 knots late evening and overnight before
potentially shifting N or NE at near the end of the TAF period
(18Z Wednesday).

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 1 PM CDT this afternoon for the IL
     and IN nearshore waters.

&&

$$

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