680
FXUS63 KLOT 122337
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
637 PM CDT Thu Jun 12 2025

.KEY MESSAGES...

- Scattered showers and possibly a couple of thunderstorms
  Friday into Saturday, especially south of I-80.

- Very warm and humid with occasional chances for showers and
  thunderstorms early to mid next week.

&&

.DISCUSSION...
Issued at 255 PM CDT Thu Jun 12 2025

Through Friday night:

Weakening surface boundary is draped across northern IL, but
outside of areas influenced by the marine layer, there is very
little theta-e difference across the boundary. We are seeing
a modest, albeit rather mushy, cumulus field developing near and
south of this weak boundary beneath the cirrus canopy. Aided by
little more than some weak low level convergence, the chances
for any showers developing late this afternoon appears low.
Opted to hang onto slight chance pops late this afternoon until
around sunset on the off chance a couple of showers manage to
develop.

Upper low over the southern Plains is still progged to devolve
into an open wave as it moves slowly northeast toward the mid
Mississippi Valley. Scattered showers could encroach on our
far southern late tonight with the shower chances spreading
slowly northward Friday morning. Still appears the best chances
of rain will be south of I-80, though some showers can`t be
ruled out farther north.

Any filtered sunshine through the cirrus shield Friday
afternoon could theoretically allow for some weak boundary
layer destabilization. Nearly moist adiabatic mid level lapse
rates will further limit destabilization, but cannot rule out
some isolated thunderstorms Friday afternoon, with chances
increasing the farther south. Some scattered showers could
continue into Friday night as the slow moving system continues
to meander ever so slowly east-northeastward across the region.

- Izzi

Saturday through Thursday:

On Saturday, the open wave (remnant closed low) to our south and
its associated weak surface low pressure reflection will track
toward the central Ohio Valley. With this farther south low path
and stabilizing influence of high pressure over the Great Lakes
region, shower chances have trended markedly southward over the
past few model cycles. There should still be some shower activity
south of I-80, but probably more isolated to widely scattered in
nature, with highest coverage (55-60% PoPs) generally south of
US-24. Better mid-level moisture being focused down near the
surface low (and 700-500 mb RH will be trending drier PM hours)
and already sub-marginal (5.5C/km or less) mid-level lapse rates,
there`s only a slight chance for a few isolated embedded
thunderstorms where measurable precip may occur. High
temperatures inland of Lake Michigan will range from the upper
70s to mid 80s inland, warmest west/northwest due to increasing
sunshine and "coolest" southeast due to thicker clouds. Dew points
generally in the 60s will translate to moderate humidity levels,
common for mid June.

Following a quiet and seasonable Saturday night into Sunday
morning, a convectively modified impulse (possible MCV) may move
toward the mid MS Valley by early evening. This could conceivably
bring some threat for showers and isolated thunderstorms to
far interior sections in the afternoon, particularly near/west of
I-39 in north central/NW Illinois. With a sharp west-northwest to
east-southeast instability gradient resulting in limited
instability locally, suspect that the 12z ECMWF/EPS (ECMWF
Ensemble) cycle was too bullish with the convectively driven QPF
footprint. The 20-30% PoPs well away from the lake, highest
northwest, in our official forecast, appear appropriate for now in
light of the rest of the guidance being less bullish/farther west
with their QPF output than the EC. Expecting high temperatures a
couple degrees warmer on Sunday vs. Saturday, in the mid to upper
80s away from the lake, along with muggy conditions, especially
south of I-80 (near 70F dew points). Temps may be able to climb
a couple degrees lakeside as well, but still notably cooler.

Looking ahead to next work week, we`ll enter into a classic
summer-time pattern with very warm (aside from any lake cooling)
and humid conditions and occasional shower and thunder chances.
Modest forcing and weak flow aloft Monday-Tuesday should translate
toward largely diurnally driven convection (peak coverage in the
afternoon-evening) and the main thunderstorm threats being
lightning, downpours, and localized gusty winds. Despite the day 6
lead time, there`s a decent signal in the ensemble guidance for a
stronger synoptic system and associated cold frontal approach
into the region in the Wednesday-Wednesday night timeframe. This
may be a period to watch for organized convection and a threat for
strong to severe thunderstorms in the region. If the Wednesday-
Wednesday night timing comes to fruition, then Thursday would
likely end up mainly dry and slightly cooler/less humid in the
wake of the cold front passage.

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 636 PM CDT Thu Jun 12 2025

Key Messages:

- Spotty shower or storm possible near RFD early this evening
  (15% chance)

- Showers lift into portions of the area on Friday, with the
  best potential N of I-80 mainly in the afternoon (30% chance)

Dry conditions are forecast through tonight at area terminals.
Will have to keep an eye on RFD early this evening, however, as
spotty showers/storms are beginning to develop across far
northwest Illinois. If this activity expands further east
along the stalled surface boundary, a VCSH/VCTS mention would
need to be added to the TAF.

Confidence in shower trends during the day on Friday north of
I-80 remain rather low and opted to not make any changes to the
inherited PROB30s at ORD/MDW during the afternoon and added
them to the other sites for consistency. Coverage would be
isolated to widely scattered and could persist into the
evening as a slow moving disturbance lifts across the region.
An eventual VCSH or TEMPO for -SHRA mention may be needed with
later updates.

Winds will remain east to northeasterly through this evening,
becoming light and variable overnight. Winds then settle back
into a mainly southeast direction during the day on Friday.
There is some hi-res guidance that attempts to turn winds
briefly SSW late morning into early afternoon at ORD/MDW.

VFR conditions are forecast through the period, though some MVFR
stratus may lift toward the area late Friday evening.

Petr

&&

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

&&

$$

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