246
FXUS63 KLOT 231939
AFDLOT

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

.KEY MESSAGES...

- Late Monday night through Tuesday night may feature a couple
  rounds of thunderstorms including the threat for severe
  weather and flash flooding.

- Tuesday may be hot and humid with heat indices above 100,
  depending on the coverage and timing of thunderstorms.

- Chance of thunderstorms late Thursday night through Saturday
  morning.

&&

.DISCUSSION...
Issued at 239 PM CDT Sun Jun 23 2024

Through Monday Night:

In sharp contrast to Saturday`s weather, conditions are much
cooler today following last nights cold frontal passage. This
cooler weather is also be aided by filtered sunshine complements
of a persistent broken deck of strato-cu, which continues to
stream southeastward across the area in advance of the next
approaching short wave impulse. These conditions are expected to
persist the remainder of the day, with inland temperatures
holding in the upper 70s to low 80s. Temperatures will be a few
degrees cooler near the lakeshore.

Aside for a small chance of couple of light showers or sprinkles
this evening across far northeastern sections of IL associated
with a quick moving impulse swinging southeastward across
southern Lake Michigan, the weather will remain dry tonight.
Decreasingly cloud cover and light winds under an area of high
pressure will allow temperatures to drop back into the lower to
mid 60s for overnight lows. These conditions may foster some
patchy ground fog towards daybreak Monday morning, especially
over some of the areas that received heavy rainfall last night.
However, significant areas of fog are not currently anticipated.

Monday will be another decently pleasant weather day as a mid-
level short wave ridge shifts overnight. Conditions will be
turning warmer again, however, as southerly winds return
following the eastward departure of an area of high pressure.
Accordingly, inland temperatures are expected to top out in the
mid to upper 80s under mostly sunny skies. However, another
afternoon lake breeze will keep temperatures along the southern
Lake Michigan shore several degrees cooler.

Locally, our weather is expected to remain quiet through Monday
evening. However, later Monday night into Tuesday morning we will
have to keep an eye on the potential for a developing and
organizing MCS across the Upper Midwest to dive southeastward
into our area along the eastern periphery of an MLCAPE gradient.
While questions remain on the evolution and timing of this
potential MCS shifting into our area, there continues to be a
signal for a period of stormy weather near our area into early
Tuesday. Gusty winds, potentially near severe levels (58 mph),
and locally heavy rainfall would be possible with any organized
MCS that dives southeastward into the area through early Tuesday
morning.

KJB

Tuesday through Sunday:

Primary forecast concern and challenge is the potential for
thunderstorms Tuesday and Tuesday night and their impact on high
temps/heat index values Tuesday afternoon. There remains quite
a bit of uncertainty with the potential for ongoing convection
Tuesday morning. Just about all of the models show convection
developing to our north Monday night, but then quite a spread in
how it evolves and where it moves by Tuesday morning, with some
consensus it could be over southern Lake Michigan into
southwest lower MI and northern IN. Even lower confidence for
the potential for strong/severe weather during this period. But
if an organized line can reach the area, some damaging wind
potential will be possible, along with heavy rainfall.

If storms do not materialize Tuesday morning, high temps will
likely reach the lower 90s and possibly mid 90s. Coupled with
dewpoints in the lower 70s, heat index values in the 100 to 105
range will be possible. If storms do reach parts of the area
Tuesday morning, they will likely weaken/dissipate through the
morning, allowing for the potential for temps to recover to the
lower 90s. From this distance, however, confidence is low for
high temps, dewpoints and associated heat index values. Winds on
Tuesday also have the potential to gust into the 30-35 mph
range, if there are several hours of precip free-warming.

By afternoon, a cold front will be approaching and thunderstorm
chances will be increasing. Already outlooked for a level 2 of 5
for Tuesday and wherever storms develop, they will likely be
severe. Though morning convection could have impacts on where
convection forms later in the day and thus low confidence for
location. In addition to the severe potential, pooling dewpoints
in the lower/mid 70s will help push precipitable water values
into the 2+ inch range with heavy rainfall also possible. Given
the heavy rains that fell across parts of northern IL last
night, flash flooding could be more of concern over a wider
area. There may be some lingering showers or thunderstorms
Wednesday morning across the south, but a cold front will move
south of the area by Wednesday morning, ending the chance for
precipitation.

Cooler and drier air will spread across the region Wednesday
night into Thursday. Low temps by Thursday morning may be in the
50s across most areas. Northerly winds will increase across Lake
Michigan Wednesday into Wednesday night. A little early to tell
exactly how strong the winds will be, but the potential for high
waves and dangerous swimming conditions at Lake Michigan beaches
will need to be monitored with later forecasts.

The pattern quickly shifts back to southerly flow Thursday night
and there will be increasing chances for showers and
thunderstorms by Friday morning, likely in a decaying phase from
earlier activity further west of the local area. A cold front
will then move across the area Friday night with a period of
showers and thunderstorms possible. High temps on Friday also
tricky, depending on the amount of cloud cover/precip earlier in
the day and the amount of recovery before additional convection
later in the day.

This cold front currently looks to be fairly progressive and
possibly southeast of the area by Saturday morning. Blended pops
on Saturday are likely too high and with high pressure moving
across the region, next weekend may end up being dry both days
with highs near normal and less humid conditions. cms

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1229 PM CDT Sun Jun 23 2024

Clouds will continue to linger around the area as a weak upper
level wave moves past. While there might be the occasional
FEW/SCT cig that slips below 3000 feet, the expectation is for
VFR to remain through the rest of the TAF period. Winds will
primarily be out of the west northwest with occasional gusts up
to 20 knots through the afternoon. The exception being that a
weak lake breeze is still expected to develop later this
afternoon. The expectation is for it to reach KGYY and
potentially KMDW, but not farther inland though it will be
monitored closely. Winds will diminish after sunset. There
remains the slight chance for some light sprinkles or even some
shallow ground fog Monday morning but was kept out of the TAF
currently. VFR conditions are expected through the end of the
TAF as winds turn to the east-southeast around 10 knots or
less.

DK

&&

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

&&

$$

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