075
FXUS63 KLOT 251147
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
647 AM CDT Fri Jul 25 2025

.KEY MESSAGES...

- Additional rounds of showers and storms into Friday morning
  with a locally heavy rain, and isolated gusty wind threat.

- A period of slow-moving showers and thunderstorms Friday night
  through Saturday may bring another round of torrential
  rainfall and the potential for flooding across portions of the
  area.

- Dangerous heat will build back across the area on Monday
  (possibly as early as Sunday), with periods of thunderstorms
  over or just north of the area Monday through Wednesday.

&&

.DISCUSSION...
Issued at 119 AM CDT Fri Jul 25 2025

Through Saturday:

Complex warm season pattern is in place early this morning, with
continued concerns for locally heavy rainfall and gusty
thunderstorms being the main near term concerns. Challenges will
center on timing individual shortwaves/boundaries that will serve as
focus for thunderstorms.

The low level atmosphere, and really the entire atmospheric column,
has not been scoured out ahead the afternoon storms, and you can
make out an outflow boundary along a line from Peru to Gary.
Dewpoints north of the boundary are still holding in the lower 70s,
but mid to even upper 70s are in place across central Illinois. THus
from an instability perspective, the atmospheric is still fairly
primed.  Similarly, an assessment of PWATs show an enhanced area of
2"+ PWATs along the boundary.  Aloft, water vapor imagery depicts
numerous MCVs/compact vort maxes from central IL extending back to
the Texas Panhandle. 850-700 mb flow from the SW ahead of the first
wave shows some uptick as well.

It appears the focus tonight into early Friday will be locally heavy
rainfall with some embedded thunderstorms. Most guidance suggests
that the environment may be a bit more conducive along and
south/east the aforementioned boundary. Higher DCAPE values will be
in place ahead of this first wave, and with a shallow stable layer,
we do still need to monitor for gusty winds south of Chicago into NW
Indiana which should peak before early-mid morning. Outflow is
surging ahead of the storms, thus at least for the time being gusts
should hold under 40 mph.

The challenge will be that there is not that much of gap
between each upstream wave, but it looks like coverage may ease
a bit through the morning into early afternoon. The next wave(s)
will be moving into northeast IL/northwest IN during the
afternoon. With extensive cloud cover, instability will be a bit
more muted. Deep layer shear will also be lower. Therefore
coverage of showers and storms this afternoon will likely be
focused on any inland surging lake boundary, and would be
scattered in nature. Locally heavy rainfall again would be the
main hazard with tall/skinny CAPE profiles and very little wind
in the steering layer.

Another nocturnal uptick in low level southwest winds will shift the
moist axis back toward more of the area overnight Friday night into
early Saturday. The shower and storm chances never really end Friday
evening, however better chances will come in this latter time frame
as a more organized trough axis aloft will come overhead in the
conjunction with the low level jet response and commensurate uptick
in 2" + PWATs areawide (versus further south on Friday). This will
be another period to watch for heavy rainfall. And with the approach
of the trough/some enhanced shear will again bring some concern for
damaging winds. Much depends on the arrival time of the wave.

KMD

Saturday Night through Thursday:

Saturday night the low level flow turns more westerly and it looks
like the more organized shortwaves will pass east of the area. There
will still be subtle waves in westerly flow moving in, but rising
heights aloft should limit storm coverage on Sunday.

Concern will then shift to a return to heat and humidity. Dewpoints
will again be on the rise on Sunday and a portion of the thermal
ridge across the plains will look to shift back overhead early next
week, especially into Monday as an expansive high will setup smack
dab in the middle of the country right in the heart of the
climatologically favored time of year.  Dewpoints also look to be
oppressively high, upper 70s to near 80, which would place heat
indices close to 110. Beyond that, we will see if the ridge holds
into Tuesday, it certainly may, which would continue the heat and
humidity at least for some areas, and away from the lakeshore.

It appears that a longwave trough across Canada will look to squash
the ridge axis south, opening the door first to heavy rainfall
producing storms. Severe weather would also be on the table with an
upper jet nearby, southward surging cold front, and west-northwest
flow in place. A much cooler and drier airmass toward the latter half
of the week.

KMD

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 646 AM CDT Fri Jul 25 2025

Forecast concerns for the terminals...

- 20-30% chance for scattered showers this afternoon mainly
  south of I-80 and along the lake breeze.

- Waves of showers and thunderstorms late tonight into Saturday
  morning.

- Period of MVFR ceilings, possibly IFR, tonight through
  Saturday morning.


Several disturbances continue to traverse across eastern KS, MO,
IL, and IN this morning which is supporting the development of
showers and thunderstorms in the region. The leading disturbance
has exited northern IL and northwest IN and no longer poses a
concern for the terminals, however, the second disturbance is
moving across northern MO and will move into north-central IL
this afternoon. Given that this disturbance is forecast to be
weakening as it arrives and the modest instability forecast to
develop this afternoon, it appears the coverage of showers and
thunderstorms will be very hit and miss and mainly confined to
areas south of I-80. That said, a lake breeze is expected to
move through the Chicago area terminals this afternoon which may
kick off some scattered showers and perhaps an isolated storm
between 23z and 04z this evening. With the shower/storm coverage
remaining uncertain, have opted to maintain the PROB30s in the
TAFs for showers.

A better coverage of showers and thunderstorms is expected to
develop with the disturbance currently moving out of eastern KS
which is forecast to arrive in northern IL and northwest IN after
midnight. While confidence on showers/storms is fairly high
tonight, there is some uncertainty as to where the greatest
coverage will be and the exact timing. Therefore, have decided
to introduce PROB30s in the TAFs to attempt to highlight the
most likely arrival times at each terminal. Regardless, storms
tonight will once again have the capability to produce heavy
rainfall which will result in reduced visibility and perhaps
some instances of flooding. Additionally, the strongest storms
may also contain gusty winds in excess of 30 kts.

Outside of the rain, VFR conditions are expected to prevail
through this afternoon. Though, some MVFR clouds may drift
overhead at times this morning until diurnal mixing takes full
effect. Winds will start off north-northwest this morning but
will become easterly behind the aforementioned lake breeze this
afternoon with speeds increasing into the 8-10 kt range.
Directions will become more southeast this evening before
turning southwesterly Saturday morning. As the third
disturbance arrives tonight, ceilings are expected to lower into
the MVFR range with a 30% chance for some IFR ceilings to mix
in as well. These lower ceilings are expected to persist through
Saturday morning before improving.

Yack

&&

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

&&

$$

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