180
FXUS63 KLOT 260819
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
319 AM 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 /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1252 AM CDT Wed Jun 26 2024

Key Aviation Messages:
- 10% chance of isolated TS early this morning
- Lake breeze expected at MDW and possibly at ORD this
  afternoon, followed by a NE wind shift areawide early this
  evening.

VFR conditions are expected through the period. While dry
conditions are generally favored across northern IL and
northwest IN through tonight, spotty -SHRA cannot be ruled out
through early afternoon. There is also a very low chance (10%)
that isolated elevated TS develop prior to and around sunrise
(08-11Z) over the Chicago metro as ongoing subtle saturation in
the 4-6kft layer may allow the realization of higher elevated
instability currently over the area. If these TS do develop,
they would be capable of +RA and potentially small GR.

W/NW winds under 10 knots will settle NNW around 10 knots by
late morning. A lake breeze is expected to develop, then stall,
in the vicinity of ORD/MDW early this afternoon. The current
forecast depicts a NE wind shift at MDW early/mid afternoon. It
remains about a toss-up on whether the lake breeze reaches ORD
during the afternoon, with a 60/40% chance in favor of no
passage. Either way, a reinforcing backdoor cold front will
ultimately shift winds NE at all sites early this evening.

Kluber

&&

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

IN...Beach Hazards Statement from 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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