730
FXUS63 KLOT 230600
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
100 AM CDT Sun Jun 23 2024

.KEY MESSAGES...

- A few strong storms with gusty winds and localized flooding
  early this morning across east-central Illinois and northwest
  Indiana.

- Breaks in heat and humidity are expected Sunday and Monday,
  and again Wednesday through the end of the workweek.

- Tuesday may feature dangerous heat and/or severe weather in
  the general region.

&&

.DISCUSSION...
Issued at 309 PM CDT Sat Jun 22 2024

Through Tonight:

Forecast attention this afternoon continues to focus on the
expected development of severe thunderstorms to our west-northwest
across northeastern sections of IA. Satellite imagery continues
to depict breaks in the cloud cover across this area within the
vicinity of a west-to-east oriented stationary frontal boundary.
Heating of a very high theta-e airmass along and south of this
boundary is resulting in rapid destabilization. This was noted in
the recent 18z RAOB from DVN. Continued heating of this airmass
over the next hour or two will likely continue to erode what
little cap still exists, thus allowing for the initiation of
thunderstorm development.

As noted in previous discussions, the kinematic environment
across the area will be favorable for the development of organized
severe convection, with supercells likely becoming the initial
mode across northeastern IA late this afternoon. While a few
tornadoes and instances of hail will be possible with this
supercellular activity for a couple hours late this afternoon,
amalgamating outflows are expected to transition the predominant
storm mode into east-southeastward moving organized linear
clusters across northern IL this evening. Rather poor mid-level
lapse rates with eastward extent over northern IL in combination
with a very high PWAT airmass (up to 2.25") suggests the threat of
hail will remain rather low outside of any well developed
supercell structures.

Aside from the damaging wind threat, the near record high PWATs
will also support instances of extremely heavy rainfall with the
storms, with rain rates of 1 to 2"+ per hour likely. For this
reason, even a brief period of training storms could result in
some localized 2 to 3"+ amounts before the storms sag southward
through the evening. While we still cannot rule out the need for a
short fused flash flood watch for western parts of the area, we
will continue to run with the current ESF (hydrological outlook)
for areas generally along and north of I-80 in IL.

The heavy rain and severe weather threat will gradually end from
northwest to southeast later this evening into the overnight as
the clusters of storms sag southeast out of the area.

KJB


Sunday through Saturday:

Forecast focus for the extended is on two time periods, Tuesday
and Friday.

Before then, lingering showers will be possible Sunday morning
as the cold front departs the area and isolated showers will be
possible Sunday afternoon into Sunday evening, but confidence is
very low and maintained a dry forecast for now. Cooler and
slightly drier air will spread across the area on Sunday.
Lowered high temps to the lower/mid 80s but these are still
above some of the cooler guidance. Winds will turn northerly
over the lake, which will keep temps cooler near the Lake
Michigan shore.

Low temps by early Monday morning likely to be in the lower/mid
60s for much of the area. Dry and mostly sunny for Monday as
highs likely to be back into the mid to possibly upper 80s. A
lake breeze will turn the lake shore areas cooler for the
afternoon.

Much of Monday night will likely be dry but by early Tuesday
morning, there is considerable disagreement/spread among the
models and their ensembles. There is one solution of morning
convection either over or near the cwa which continues through
midday Tuesday, as it moves south. The other solution keeps the
local area dry through early/mid afternoon Tuesday and then as a
cold front approaches, convection develops along/ahead of the
front and continues into Tuesday evening. While this time period
has consistently been shown in the models for convection for
the local area, either solution seems plausible and there is
little to support one over the other at this time. Strong to
severe storms along with heavy rain would be possible in either.

When precipitation develops, assuming it does, will have a big
impact on temps. Morning convection will keep temps cooler and
while they`ll still likely recover into the afternoon, unlikely
to get past lower 90s. If Tuesday morning is dry, southwest
winds will likely tap into the stronger winds aloft, with gusts
into the 30 mph range, pushing highs into the mid 90s, perhaps a
few locations reaching upper 90s. The strong winds and deeper
mixing will likely keep dewpoints in the upper 60s/near 70. This
would result in heat indices in the upper 90s to around 100.

There is the potential for thunderstorms to linger or continue
into Wednesday morning, though if they do, it would likely be
south of I-80 and departing through mid morning. Dry conditions
are then expected through at least Thursday evening. Cooler and
less humid conditions are expected Wednesday and especially on
Thursday, along with cooler conditions near Lake Michigan with
northeast winds.

Friday has been another time period the models have been showing
the potential for thunderstorms, again with little confidence
for timing. Another cold front will moving across the area
during this time period. Similar to Tuesday, there is the
potential for both morning and afternoon/evening. cms

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1258 AM CDT Sun Jun 23 2024

TSRA is currently shifting south of the Chicago terminals early
this morning, with remaining TS ending in the next hour or two.
Otherwise, beside widely isolated -SHRA this evening/early
Monday morning, dry conditions are expected through the
remainder of the period.

SW winds around 10 knots will veer NW 10 to 15 knots behind a
cold front prior to sunrise. A lake breeze will reach GYY and
likely MDW mid-afternoon, but may stall just east of ORD. Have
not included a NE wind shift at ORD, but if the lake breeze does
reach the terminal, it would occur in the 21-23Z window.

MVFR ceilings will expand early this morning along and behind
the cold front, with a few brief periods of IFR ceilings
possible prior to sunrise. Ceilings should gradually lift and
SCT by late morning, with VFR conditions through the remainder
of the period.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None

&&

$$

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