508
FXUS63 KLOT 261145
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
645 AM CDT Sat Jul 26 2025

.KEY MESSAGES...

- Torrential rain producing thunderstorms (with rates greater
  than 2 inches per hour) are likely again at times Saturday

- Strong to severe thunderstorms may develop Saturday afternoon,
  mainly near and south of I-80 (severe threat level 1 of 5).

- Dangerous heat and humidity returns Sunday and especially on
  Monday, followed by periods of thunderstorms Tuesday into
  Wednesday.

- Pattern change late week to bring seasonal and dry weather.

&&

.DISCUSSION...
Issued at 221 AM CDT Sat Jul 26 2025

Through Sunday:

The atmosphere remains primed for convection and all it takes are
subtle boundaries in conjunction with mid level vort
maxes/MCVs.The first wave is marching on through well south of
Chicago and extending into northern Indiana, with a slightly more
organized complex across east-central Illinois. Meanwhile the
slower moving area of earlier heavy rainfall producing showers
and embedded thunder will slowly meander along and north of
I-90 and into Wisconsin. Beyond this first surge there is
really no clear defined break that would easily point out as the
main trough axis and additional mid level perturbations should
easily be able to interact with any low level boundaries and
wring out additional heavy rainfall.

The next area of focus will then be the plethora of waves
upstream across Missouri into south central IA. We are starting
to feel the effects of this with scattered showers and isolated
thunder beginning to fill in from LaSalle county northeast
toward Chicago. It looks like the timing the more organized
forcing and associated uptick in convection will be into the
morning on Saturday. Still, there are numerous little
disturbances that will maintain showers and embedded storms
through daybreak.

Forecast soundings continue to feature very high freezing levels,
and near record precipitable water values. While there is some
increasing low-mid level flow to aid storm motion and organization,
the low level jet orientation still continues to suggest overall
slow storm propagations.

Admittedly instability is not super high as you get farther
north/east which will continue given the amount of cloud cover that
will remain in place through the day, thus thunder may be
initially limited north of I088. Some diurnal warning may be
all that`s needed to reinvigorate more robust showers and storms
this afternoon. This will also be able to pump up instability
just enough, especially along and south of I-80 in conjunction
with about 30 kt of deep layer shear to support a limited risk
of gusty storms. All that being said, periodic heavy rainfall
showers and thunderstorms will be the rule through much of
today, possibly even into the evening.


KMD

Saturday night through Friday:

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, and thus
storms may be favored closer to central Illinois.

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 from
the southeast and into the central US.  Sunday looks very warm and
humid, but Monday looks oppressively hot and humid.  Dewpoints will
jump back into upper 70s to near 80, which would place heat indices
close to 110, possibly as high as 115 in rural areas outside of
Chicago. The question marks will revolve around convective
chances. The pattern is aloft is the classic ring of fire with
systems mainly remaining to the north, but as the week goes on,
especially into Tuesday, we will see if the ridge holds.
Guidance is suggesting that the upper flow may turn
northwesterly and steer any upstream convective complexes toward
northern Illinois, or allow for the cap to break as early as
Monday. NBM has some low PoPs for Monday afternoon, given what
looks like extreme instability, but questions on forcing exist.

Extended guidance diverges as to how quickly any systems can
break into the ridge, as early as Tuesday morning is in the
realm of possibilities. There is better agreement that a
longwave trough across Canada will look to squash the ridge axis
south mid week, opening the door 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 SUNDAY/...
Issued at 645 AM CDT Sat Jul 26 2025

Forecast concerns for the terminals...

- Periods of showers and thunderstorms through this evening.

- A few storms could be strong to severe this afternoon with
  gusty winds and torrential rain as the main threat.

- MVFR ceilings with any showers and storms today.


The next shortwave disturbance is approaching northern IL and
will begin to move showers over the TAF sites within the next
1-2 hours. While instability ahead of the disturbance is still
somewhat modest, instability is expected to increase through the
morning which should allow for a better coverage of thunder at
the Chicago area terminals and points south by 14-15z.
Regardless of storm coverage, the humid air mass overhead will
allow any showers and/or storms to be heavy rain producers
resulting in reduced visibilities and potential even some
flooding especially in areas that saw heavy rain yesterday.

This initial wave of showers and thunderstorms should begin to
wind down towards midday before a secondary disturbance pivots
overhead and redevelops showers and storms this afternoon. Due
to the morning activity, it appears the better environment for
thunderstorms will establish south of a VYS to VPZ line.
However, sufficient instability looks to reside over the Chicago
area terminals this afternoon which should allow for some
isolated to widely scattered showers and storms to develop and
thus the PROB30s have been maintained. Similar to the morning
rain, the afternoon round will also be capable of producing
heavy rainfall in addition to locally gusty winds (in excess of
30 kts) with the strongest storms. The showers and storms are
expected to gradually get pushed south of the terminals after
00z this evening courtesy of a frontal boundary resulting in dry
conditions to close out the TAF period.

Outside of the rain, generally VFR conditions can be expected
but periods of MVFR ceilings will be possible with any showers
and thunderstorms. Winds will remain out of the southwest today
with speeds increasing to 8-10 kts this afternoon before easing
to around 5 kts this evening. Directions will turn more
northerly Sunday morning before a lake breeze arrives and turns
winds easterly at the Chicago sites late Sunday morning.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through this evening for ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-
     ILZ106-ILZ107-ILZ108.

IN...Flood Watch through this evening for INZ001.

LM...None.

&&

$$

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