941
FXUS63 KLOT 261731
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1231 PM CDT Wed Jun 26 2024

.KEY MESSAGES...

- Isolated to scattered thunderstorms may develop this
  afternoon, mainly along and southeast of I-55 (40-60% chance)

- Strong to severe thunderstorms and flash flooding are possible
  Friday night and potentially into Saturday.

&&

.DISCUSSION...
Issued at 319 AM CDT Wed Jun 26 2024

Through Thursday:

A recent hand surface analysis revealed a surface low pressure
system centered in southern Ontario and an associated surface
pressure trough extending westward across the Lower Great Lakes. A
wide reservoir of instability remains in place along and south of
the surface pressure trough, which continues to feed several forward-
propagating clusters of thunderstorms from Topeka, Kansas to
Clarksville, Tennessee. A distinct MCV (convectively-generated
low pressure system) is evident swirling northeastward along the
Ohio River between clusters of thunderstorms. Meanwhile, upper-
level water vapor imagery depicts a wound-up shortwave
propagating southeastward into central Minnesota, and surface
observations across the Upper Great Lakes depict a southward-
moving cold front extending from near Isle Royale to Mackinaw,
Michigan. When put altogether, our area is in a somewhat benign
region within the broad and unstable zone of low-level
confluence characterizing the surface pressure trough, but far
from the zones of active thunderstorms to our south, the cold
front well to our north, and the incoming upper-level shortwave
to our northwest. As a result, it`s somewhat stagnant outside
with relatively light winds and relatively high humidity levels
thanks to surface dew points in the upper 60s to lower 70s.

With limited to no signs of low-level forcing to activate the
instability plume in our region this morning, prospects for
showers and thunderstorms appear low (only a 15-20% chance) and
may be limited to nebulous micro-scale objects of forcing, such
as a weak gravity wave feature that kicked off a few short-lived
thunderstorms earlier this morning near Chicago. Later today
however, forcing for ascent by means of low-level lift
along/ahead of the approaching cold front from the north and
DCVA provided by upper-level shortwave from the northwest (which
may actually link up with the MCV lifting into central
Indiana), should provide enough "oomph" to kick off isolated to
scattered thunderstorms. Timing out the approach of all
features as well as the expected southward "ooze" of the
instability axis, such a threat should materialize near and
southeast of I-55 this afternoon (40 to 60% PoPs). Meanwhile,
behind the cold front, we can`t rule out a few sprinkles beneath
the deepest stratocumulus clouds (only a 10 to 15% chance).

This evening, winds over Lake Michigan will become breezy out of the
north to northeasterly direction causing waves to chop up to 3 to 6
feet along all shoreline beaches. For this reason, will go ahead and
issue a Beach Hazards Statement, valid from 10 PM to 10 AM.

Tomorrow looks relatively quiet beneath a passing surface high
pressure system. Highs in the mid to upper 70s (mid to upper 60s
lakeside), plenty of sunshine, and light winds should be the
norm.

Borchardt


Thursday Night through Tuesday:

High pressure over the Great Lakes region on Thursday will
drift eastward through Friday as a mid-level wave lifts ENE from
the Great Plains into a flattening low-amplitude ridge. Initial
convection over the central Great Plains Thursday evening will
quickly shift into an airmass characteristic of poor mid-level
lapse rates and a weakening kinematic field around and east of
the Mississippi River Valley. Expectations are for an area of
decaying showers with perhaps some embedded storms to shift
across the forecast area through the day Friday.

Strong low-level moisture transport below a plume of modest
mid-level lapse rates across the mid and Upper-Mississippi
River Valley will then advect toward the area Friday night.
Conditional on an available impetus (most likely low-level
WAA/isentropic ascent) for convection Friday night, effective
shear profiles and PWATs rising over 2" will foster strong to
possibly severe storms capable of producing hail, gusty winds,
and flash flooding. An approaching cold front late Friday night
will provide an added focus for regenerating convection and a
continued potential for flash flooding well into Saturday.
Spread of the placement of potential flooding concerns remains
quite broad, with guidance ranging from far southern WI through
the south half of Illinois.

We transition from June to July with a couple dry and
seasonably cool days Sunday and Monday before another signal for
active weather appears Tuesday into Wednesday.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1226 PM CDT Wed Jun 26 2024

A cold front has been advancing southeast through the area this
morning bringing scattered showers to all of the TAF sites. It
is currently just south of KORD, however, some scattered
showers are still hanging around on the backside of the
boundary. Thus have opted for a prevailing VCSH at KORD through
19Z, KMDW through 20Z, and KGYY through 21Z. Clouds are
expected to clear out behind this boundary with VFR conditions
expected at all five TAF sites through the end of the TAF
period.

Northwest winds will shift to northeast this afternoon at KORD,
KMDW, and KGYY as a lake breeze pushes inland. A shift to
northwesterly winds will also occur at KDPA and KRFD later in
the evening. Winds will become stronger with a few gusts
possible with the northeasterly shift. Winds will remain
northeasterly through the end of the TAF period though the
strongest winds will be confined to areas near the shore of Lake
Michigan.

Carothers

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 10 PM CDT this evening through
     Thursday morning for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from 10 PM CDT this evening through
     Thursday morning for INZ001-INZ002.

LM...Small Craft Advisory from 10 PM this evening to 10 AM CDT
     Thursday for the IL and IN nearshore waters.

&&

$$

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