746
FXUS63 KLOT 170531
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1231 AM CDT Tue Jun 17 2025

.KEY MESSAGES...

- Low chance (10-15%) for a few isolated showers and
  thunderstorms west and south of the Chicago Metro through this
  afternoon.

- Rain and thunderstorm chances increase late Tuesday, with
  multiple rounds of storms possible through Wednesday evening.
  Wednesday some storms may be severe and have heavy rainfall.

- Hot and humid conditions expected to arrive this weekend.

&&

.DISCUSSION...
Issued at 326 PM CDT Mon Jun 16 2025

Through Tuesday Night:

Under the influence of an exiting sfc high pressure, mostly dry
conditions have remained across the forecast area. This sfc high
will continue to pull northeast into the New England State through
this afternoon. As this high exits, winds have turned southeast to
south. Winds should remain light behind the exiting high and under
the slow moving upper low. Southerly winds will issue in some WAA
which will increase temperatures and humidities through the evening.
Temperatures have already climbed into the low to mid 80s across
northern Illinois. Immediate lake shore areas a likely to be
cooler in the mid to upper 70s as cooler air is brought onshore.


The small chances for rain and a few storms remain for areas west
and south of the Chicago Metro through the afternoon. These chances
equate to roughly 10 to 15% or less for many areas. Southerly winds
have brought up dewpoints across northern and northeastern Illinois,
with the far southern and western portions of the forecast area
bringing in some of the higher dewpoint from the south. Dewpoints in
the low 60s are expected to be advected in which will create a
moderately unstable environment for a few showers/storms to
develop during peak heating. Coverage will be isolated as the
wind profiles and lapse rates are pretty weak. As we loose
diurnal heating this evening rain and storm chances will
decrease and any ongoing convection will dissipate. Quiet
conditions are expected across the forecast area through the
overnight hours.

The next chance for rain come Tuesday morning along and ahead of an
approaching cold front from the northwest. Any showers/storms that
develop Tuesday morning will be along an outflow boundary that
surges  ahead of the main front. A few CAMs continue to hint at a
few showers or storms developing to the north and west then moving
into northern and northwestern Illinois. Confidence on this is
low as many models have struggled with rain/storm timing and
location over the last few runs which is why POPs have been
capped at 20 % or less. Did lower the chances a bit from the
previous run as the outflow is likely to be the main driver of
low/mid level convergence.

The potential for rain and storms will increase heading into Tuesday
evening and overnight. The high chance POPs (which top out around
80% for northern Illinois), will slowly expand southward heading
into Wednesday. The driving force for more widespread rain and
storms will be a shortwave trough moving east overtop of the warm
and humid airmass at the sfc. By the time this shortwave moves into
the area, diurnal heating will have brought up temps and made the
environment that much more favorable for storms. Shear again looks
to be the limiting factor here with values around 20 kts. Can`t rule
out a few stronger storms with the good CAPE and lapse rates, but
main concerns with any stronger storms will be brief gusty winds and
some hail. Storms will have to be pretty well organized to develop
any severe hail and winds. So while the chances for stronger storms
are low its non-zero. Will be keeping an eye on the mesoscale
environment as this gets closer since a few runs of guidance
try to develop an upstream MCS like structure. Should an MCS
develop this would provide some better large scale structure to
any storms that roll through.

Patterson

Wednesday through Monday:

Surface low pressure is expected to pivot through northern
Illinois/southern Wisconsin throughout the day Wednesday. With
much of northern Illinois and Indiana remaining in the warm
sector of the low, thunderstorm chances remain elevated going
into Wednesday morning. Confidence in storm mode remains low at
this time, although MUCAPE of approximately 2000 J/kg and deep
layer shear of 45 to 55 kt lends credence to strong to severe
storm potential along a cold frontal feature progressing
northwest to southeast by the afternoon hours. Timing and
upstream convection will be key in determining the level of
severity possible, with earlier overnight Tuesday storms
yielding the potential of exhausting the environmental
instability prior to the cold front. However, if the timing of
cold front slows down and allows for more return flow, the
atmosphere may reinvigorate and produce stronger storms,
especially as the front nears eastern Illinois and northern
Indiana. PWATs also remain high (1.7 to 2 inches) throughout
Wednesday, leading to potential for localized flooding within
training cells regardless of severity.

Wrap-around precipitation continues across northern and central
Illinois into Wednesday night, but severe threat is expected to
diminish quickly after the cold front propagates eastward out of
the area. Expecting temperatures to recover Thursday with
sunshine and light westerly winds.

Strong longwave pattern ridging then builds into the Upper
Midwest going into Saturday, bringing high temperatures in the
low to even mid-90s and dewpoints above 70 degrees as winds kick
southwesterly and bring Gulf moisture into northern
Illinois/Indiana. Confidence is increasing in increasing heat
risk through the weekend into early next week, with heat indices
in the afternoon exceeding 100 degrees each day. Concern remains
that with the sufficient moisture, scattered diurnal
thunderstorms or even overnight clusters may prevent sunshine
from creating the heating necessary for the aforementioned heat
indices. Regardless, trends are toward a very warm weekend.

MH

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1231 AM CDT Tue Jun 17 2025

Key Aviation Messages for the current TAF period:

- Chances for thunderstorms Tuesday afternoon and evening.

VFR conditions are expected through this morning as light
southwest winds become westerly. A system will move through the
region from west to east providing the next chance for showers
and thunderstorms at area terminals in the afternoon and evening.
There is moderate confidence in the timing in the PROB30 group
listed in the TAFs, though it should be noted that recent model
runs are trending slightly later with arrival times. There is a
nonzero chance for showers and storms to arrive as early as 19Z,
but it was decided to leave that out of the TAF and handle that
tactically should they materialize. Current expectations are
for VFR cigs with these storms. Any reduction in flight rules to
MVFR levels would be due to reduced vis from heavier downpours.
Nevertheless, if there is continuity in the next iteration of
models, it would not be surprising to update the PROB30 to a
TEMPO for the 12Z forecast.

Lower confidence on the end time for showers/storms at
terminals, but it is looking like there could be break at area
terminals behind the storms and into Wednesday morning. While
conditions will remain VFR and winds are expected to be light
through the overnight, there is lower confidence in wind
directions and they could very well be light enough to be
variable.

DK

&&

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

&&

$$

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