109
FXUS63 KLOT 260825
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
325 AM CDT Thu Jun 26 2025

.KEY MESSAGES...

- Isolated to widely scattered showers and storms are possible
  again this afternoon and evening. The strongest storms will
  be capable of strong to locally damaging wind gusts.

- Hot and humid conditions will continue through the weekend,
  especially this afternoon and again on Sunday when heat
  indices could exceed 100 degrees.

- Periodic shower and storm chances exist through early next
  week with many dry hours in between rounds.

- A bit of relief is anticipated early to middle of next week
  with dry conditions and seasonal temperatures.

&&

.DISCUSSION...
Issued at 325 AM CDT Thu Jun 26 2025

Through Friday:

Today will be similar to yesterday when it comes to shower and
storm potential. With the better mid-level forcing remaining
northwest of the area, any storm development will likely be due
to free convection within a hot and humid environment,
potentially focused along any subtle convergence
axes/boundaries. Hi-res guidance has not been all that great in
resolving storms the past couple of days so am a bit leery of
some of the drier solutions and expect at least isolated to
widely scattered pulse storms by mid afternoon into early
evening. Ample instability, high PWATs, and weak mid-level lapse
rates will once again be supportive of precip loaded downburst
winds that could be strong to locally damaging - consistent with
SPC`s Level 1 of 5 severe threat for the entire area. Given the
hit and miss nature of the storms many areas will stay dry
through the daytime hours. If outdoors be sure to head indoors
if you hear thunder.

Elected not to issue a Heat Advisory for the local area today
with this update. The reasoning for this is due to concerns
that there will be just enough mixing to hold peak afternoon
heat indices in the 98-102 degree range, which is solidly below
our 105 degree criteria. Admittedly this makes us stand out a
bit on the map of regional headlines. If it appears that dew
points are struggling to mix out by late morning, a short-fused
heat advisory issuance may be warranted for portions of the
area. With all of this said, it is still going to be hot and
humid out there today so be sure to take proper precautions to
avoid heat related illnesses, especially those sensitive to
heat. A late afternoon lake breeze may help cool things off
slightly along the immediate Illinois shore before the rest of
the area begins to cool toward sunset.

Heading into this evening and overnight, upstream convection
across IA/MN/WI is expected to move into northwest IL this
evening. As it approaches the general expectation is for it to
be in a gradually weakening phase with the loss of daylight.
However, it may hold together enough to produce strong to
locally damaging outflow winds near the Rockford area. Expect
further weakening as it enters the Chicago metro, though a gusty
northwest wind shift and frequent lightning may persist into
the early overnight hours before eventually dissipating.

The shower and thunderstorm trends on Friday are less certain
and may be tied to a few different features, the first of which
being a remnant outflow boundary from the overnight storms that
stalls out somewhere near/south of I-80 and could serve as a
source of lift for isolated storm development. Additionally,
some of the hi-res guidance is attempting to resolve a remnant
MCV out of convection originating out of the KS/NE/MO border
that tracks across the southeast CWA during the afternoon
(generally southeast of I-55/80). If this were to occur,
couldn`t rule out a sneaky severe storm threat (though not
currently outlooked by SPC at this time). Lastly, the trailing
cold front will begin moving across the area by late afternoon
which could serve as a focus for additional storm development
into the evening (more on this in the long term discussion
below).

Temperatures on Friday will be cooler than today, with highs in
the lower to upper 80s, warmest southeast of I-55 where
afternoon heat indices will still peak in the low to mid 90s.

Petr


Friday Night through Wednesday:

Late Friday, we should find a cold front pushing south across
our CWA while the associated surface low moves across the Great
Lakes. Showers and likely embedded thunderstorms are expected to
push along and south of the front during the evening and
overnight and are expected to clear out from northwest to
southeast behind the boundary. The front will become more
zonally oriented and weaken substantially as it gets stretched
across northern IL and gains distance from the departing parent
low Friday night into Saturday. The deep moisture initially tied
to the front will also get sheared off to the east away from
the surface for Saturday. The diffuse front will be parked in
our south CWA during the day. Ample low level moisture will
remain pooled near and south of the front to maybe stir up some
showers and storms south of I-80, but chances look slim farther
north on Saturday. The front doesn`t even look to result in any
obvious temperature gradient on Saturday with highs expected to
be in the middle and upper 80s around the area, cooler around
the lake with onshore winds.

Low level flow will turn southerly late Saturday ahead of
another approaching wave. This should pull the whole area back
into the lower 90s for Sunday and low level moisture will fill
back in over the area. Heat indices during the afternoon are
expected to be in the upper 90s to lower 100s. Scattered storm
chances will increase into the afternoon and evening as the
airmass becomes highly unstable. A real lack of shear should
hinder an organized severe storm threat, but we`ll have to keep
an eye on a potential for deep moist convection, which should
wane into the night as instability does. Another cold front will
then move across the CWA Sunday night into early Monday. Lesser
instability and still a lack of shear should keep severe probs
low near the front itself, but deep layer moisture ahead of the
front could present a heavy rain threat.

A slow-moving upper ridge and relatively high pressure will
overspread the central CONUS keeping conditions quieter for a
couple of days during the middle of next week. Mid-summer heat
will continue through next week, generally near to a few degrees
above normal with highs forecast in the middle and upper 80s.

Doom

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1236 AM CDT Thu Jun 26 2025

Key messages for the 06Z TAF period include:

* A period of showers and thunderstorms possible Thursday
  evening, capable of producing gusty winds and MVFR vsbys.

Isolated showers and thunderstorms around the region tonight
should remain outside of the VCY of the TAF sites through the
first half of Thursday. Late Thursday afternoon into early
evening, a couple of isolated showers and maybe a thunderstorm
look to pop up around the region before a much better chance
arrives mid-evening. This expected line of showers and possibly
embedded thunderstorms should last on the order of a couple to a
few hours at any given site, and could feature gusty winds out
of variable directions and a period of reduced vsbys. The rain
will move away overnight and we should begin Friday rain-free.

Expect winds out of a SW direction throughout the period, below
10 kt tonight then gusting to around 20 kt during the day on
Thursday. With the exception of higher gusts around storms,
winds should then subside to near or below 10 kt for Thursday
night.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 4 AM CDT early this morning for the
     IL nearshore waters.

&&

$$

Visit us at weather.gov/chicago