522
FXUS63 KLOT 091929 AAA
AFDLOT

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
229 PM CDT Sun Aug 9 2026

.KEY MESSAGES...

- Two windows for storms exist today: one from late morning
  through mid to late afternoon, and the other from the early
  to mid evening through tonight. The exact coverage of storms
  with both rounds remains somewhat uncertain.

- Any storms in our area today will have the potential to be
  severe. There is also a potential for flash flooding,
  particularly with the second round of storms.

- Additional periods of storms are expected Monday through mid-
  week. Some of these storms could be severe, especially late
  Monday afternoon or Monday night.

&&

.DISCUSSION...
Issued at 132 PM CDT Sun Aug 9 2026

We have been able to destabilize ahead of the morning line of
convection as a northward moving warm front has made steady
northward progress. 3000 J/Kg of MLCAPE is mostly uncapped at
this point, and there is a narrow ribbon of enhanced DCAPE
nosing into the Chicago area. The stronger lead shortwave is
largely moving through here over the next few hours, and this
will keep a marginal severe thunderstorm risk in place through
mid afternoon. The main concern would be these transient
downburst signatures in the more mature developed line of
storms along with a few instances of hail. The orientation these
storms are taking will also lead to varying water levels near
the shore in wake of these storms, so the sea will remain
hazardous even after the storms pass this afternoon.

As far as the rest of the day is concerned farther south, the
battle will be between the arriving mid level shortwave sliding
through the area versus some capping reinforcement from lower
level warm advection and anvil shielding from ongoing
convection. Colder temperatures aloft are focused north of
I-80. Therefore, with an uncapped environment, expect some
isolated to widely scattered storms farther south given that
forcing is more muted. The signal then continues for a bit of a
lull in activity as shortwave forcing temporarily eases, and
capping will get reinforced via an uptick in warm air advection

After that we will have to take a look and see where the surface
warm front will set up this evening. The pattern tonight has
some similarity to a Maddox flash flood setup, with a low
amplitude shortwave, upper jet to the north, and a substantial
low level jet progged. How much of mesohigh influence is a bit
more vague in the latest guidance. PMM/LPMM guidance (from 0z/
6z) had some hefty rain totals ( 4-7" in the region). The signal
is a little more muted in latest guidance, at least on the
footprint of the heavy rainfall, but the flash flood potential
this evening is still elevated. The warm front has moved into
the Chicago metro area, and it seems like that is where it will
remain into the evening hours. Forecast soundings do still
suggest, in spite of some disagreements in low level jet
strength and orientation with respect to the warm front, a
period this evening into the early overnight where slow storm
motions would be favored very strong lift (lower level moisture
convergence), even if not quite as long lived as considered
before. The most concerning time frame still looks to be in the
mid evening through early overnight across the Chicago area
before the low level jet veers more to westerly. As this occurs
the area of concern would shift to east Central IL (Kankakee
area) and into NW Indiana. There is still considerable
uncertainty here on coverage, but the signs are there to
continue this message tonight.

The forcing during the day on Monday is not as clear and
confidence on storm evolution on Monday is somewhat low. The
warm front will briefly shift south (or to southern portions of
our area) during the morning hours, however the low level flow
will turn back around to the southwest. Any weak ripples in the
flow aloft though could a trigger a storm or two during the
afternoon. Deeper forcing may not arrive until the evening with
a very warm. moist and unstable air mass in place coupled with
strong deep layer shear. This environment would support all
hazards initially and also eventually bowing/line structures.
Another flash flood threat would be in place, especially with
another uptick in the low level jet Monday evening. We will have
to watch where the warm/stationary front lays out tomorrow, with
high uncertainty on that.

Unfortunately the pattern looks fairly active with a warm/moist
airmass in place and a endless stream of shortwaves moving through
in northwest flow upon a lingering frontal boundary Tuesday and
Wednesday.  A more dynamic system will move through eastern Canada
later in the week, and this will help to move the frontal boundary
farther south. This will take the storm track with it, and also lead
to cooler onshore flow in this system`s wake Thursday and Friday.

KMD

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1226 PM CDT Sun Aug 9 2026

Aviation Forecast Concerns for 18Z TAF:

- Showers and storms currently moving into the metro terminals
  from the northwest. Another round of showers and storms is
  forecast tonight across the region.

- Varying winds throughout the period, with main focus on
  today`s southerly to westerly, then potentially southeast late
  afternoon before southerly tonight.

- Drier conditions arrive Monday morning, however unsettled
  weather pattern will continue across the Midwest through mid-
  week.

Currently tracking a line of storms from over south central Lake
Michigan, extending southwest through KVYS. Additional
development is expected off to the southwest fringe of this area
as warmer, moist, southerly air continues to feed into the
region. This line will track over the metro terminals over the
next couple of hours (18z-21z), with gusty southwest winds
(20-35KT) veering to the northwest (280-300) immediately behind
the storms. Potential for reduced visby`s in heavier rainfall,
so kept the TEMPO group in the TAF with a more narrowed window.

Winds through the afternoon become a little more unpredictable
as the westerly / northwest winds shift once again to the
southeast. Looking back over Iowa southeast winds (110-130)
prevail and should shift across northern IL into far nw IN late
afternoon into the evening. Then as the stalled out frontal
boundary continues to hold across northeast Illinois, winds
tonight are forecast to shift back southerly. Another round of
showers and storms in anticipated to develop along the boundary
and track across the region tonight, with timing a little more
uncertain, therefore left that TEMPO group as was inherited.

Excess moisture at the sfc late tonight into Monday morning
could bring some BR conditions across the area; otherwise drier
conditions should prevail throughout much of Monday. Another
wave of wet weather is possible Monday evening into the
overnight, however did not have confidence in adding at this
time.

Baker / Donofrio

&&

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

&&

$$

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