867
FXUS63 KLOT 120829
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
329 AM CDT Wed Aug 12 2026

.KEY MESSAGES...

- Isolated strong storms possible early this morning.

- Another fast-moving line of strong/severe thunderstorms is
  expected later this morning with a damaging wind and heavy
  rainfall threat.

- Our active, stormy weather pattern is expected to persist into
  the weekend with periodic chances of severe weather and heavy
  rainfall which could lead to flooding.

- A period of dry weather may finally arrive early next week.

&&

.DISCUSSION...
Issued at 329 AM CDT Wed Aug 12 2026

Forecast remains complex within our persistent pattern of active
weather. Main concern in the near term is the potential for
another fast-moving line of wind-producing storms across at
least a portion of the area later this morning/midday.

A nearly stationary frontal boundary remains in place across
the Midwest early this morning, stretching roughly from near
Omaha to north of Des Moines to Milwaukee. Complicating the
surface pattern however, is the remnant outflow boundary from
Tuesday`s derecho, which intersects the front near Waterloo, IA
and extends southeast and bisects IL along a SQI-BMI-MTO-EVV
line. The west/northwest periphery of the cold pool appears to
be fairly shallow however, per 00Z DVN and ILX soundings (as
compared to ILM farther to the east/southeast). Aloft, a subtle
mid-level short wave has been evident in GOES vapor imagery
transiting northern IA and entering northwest IL, inducing a
modest west-southwest 25-30 kt low level jet from northern MO
into northern/central IL/IN. With an axis of 3500-4000 J/kg
MUCAPE along/north of the front/outflow boundary across the
region, there`s been concern since last evening for the
potential to develop elevated convection within a northwest-
southeast arc across the area (along/northeast of the remnant
outflow boundary). So far however, only isolated updraft
attempts have been noted and successive CAM runs have backed off
considerably in convective coverage across the forecast area
through daybreak. That said, we`ll have to continue to monitor
for any deeper updraft development, as the environment would
support the potential for strong elevated storms with a hail
threat if they develop.

Of greater concern this morning, is an MCS which has been
developing southeast from western South Dakota into northern
Nebraska overnight in association with an attendant mid-level
short wave and MCV. There is notable CAM support indicating this
MCS becoming more organized across eastern NE/western IA toward
daybreak, then propagating quickly eastward along the
aforementioned surface front/outflow boundaries into our IL cwa
after 14/15Z (9-10 AM) this morning. While not quite as robust
as yesterday`s derecho, high-res guidance does explicitly
indicate 60-70 mph winds across parts of our forecast area with
this MCS. CAMs indicate an easterly propagation of this system
across the cwa, though with the remnant outflow likely mixing
back north across the southern half of our cwa that may support
more of an east-southeast or southeast track as we saw
yesterday. SPC has pulled their new Day 1 slight (level 2 of 5)
severe risk up to about a Dixon-Renssalear line. It`s possible
that could be adjusted further if the evolution of the MCS
supports it later this morning. For now, will be advertising the
severe threat (primarily wind) even a little north of that for
this morning/midday. Thunderstorm threat should then diminish
markedly from the west into this afternoon in the wake of the
MCS and parent short wave.

There may be somewhat of a lull in convection
coverage/potential tonight into early Thursday morning,
depending on the footprint of the anticipated MCS cold pool, and
with some weak mid-level height rises indicated in model
guidance. Have trimmed pops during this period especially up
north along the IL/WI border and Lake Michigan. Another mid-
level wave (currently over NV/UT) is then progged move into the
region on Thursday which look to increase shower and
thunderstorm chances again during the day and with additional
rounds through at least Thursday night. These continued rounds
of storms will maintain at least a marginal severe threat and
a threat of flash flooding if storms train over the same areas.

Unfortunately, medium-range guidance continues to depict our
active pattern remaining in place across the region into the
weekend. There does appear to be decent agreement in the global
models/ensembles that high pressure eventually settles across
the western Lakes region by Monday-Tuesday of next week, pushing
our stalled frontal zone to the south and providing a period of
dry weather at that time.

Ratzer

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 145 AM CDT Wed Aug 12 2026

Key Messages:

- There is a low (~20-25%) chance for storms to affect the
  terminals overnight through early this morning.

- There is an increasing likelihood of storms affecting the
  terminals between mid-morning and mid-afternoon today. Strong
  to severe wind gusts will be possible with these storms.

- Lowered ceilings and possibly lowered visibilities are
  expected overnight through early this morning. LIFR/IFR
  conditions will be possible, especially at RFD, DPA, and GYY.


Isentropic ascent atop a surface boundary may attempt to
initiate showers and storms near this boundary through the
night. Thus far, convection has struggled to develop in
northern Illinois. There may be a better chance for something to
develop later tonight, but confidence in that is fairly low. It
also appears that the surface boundary may remain far enough to
the south and southwest of the terminals for any convection
that develops near it to remain south of the terminals, so the
probability of showers/storms affecting the terminals later
tonight appears to be low enough (20-25%) to not carry any
formal SHRA or TSRA mentions in the TAFs at this time. However,
observational trends will need to be monitored closely, and it
can`t be ruled out that such a mention will need to be
reintroduced into the TAFs at some point.

It appears that the best opportunity for storms to affect the
terminals today will come between mid-morning and mid-afternoon
as a convectively-enhanced disturbance associated with an
organized thunderstorm complex in northern Nebraska approaches
the area. Similar to yesterday, an organized bow or line of
storms may develop and track through the area, producing swaths
of severe wind gusts in excess of 50 kts. There is still some
uncertainty in just how far north the northern terminus of this
thunderstorm complex will end up, but there has been enough
consistency in the most recent high-resolution model solutions
to warrant the introduction of TEMPO groups for TSRA at the
Chicago metro terminals and a PROB30 group for TSRA at RFD. That
said, if several storms end up lighting up along the
aforementioned surface boundary tonight, then that may increase
the chances that this midday thunderstorm complex will end up
passing south of the terminals later today.

Lastly, patchy MVFR ceilings are currently being observed in the
area at press time, and it is possible that lower ceilings
(possibly IFR or LIFR) could be observed later tonight through
the early-mid morning. Fog/mist may develop in some areas as
well and could also result in IFR or LIFR conditions being
observed, especially at RFD, DPA, and GYY. Another episode of
low ceilings and visibilities could occur early Thursday
morning, and that potential will need to be assessed in greater
detail for forthcoming TAF issuances.

Ogorek

&&

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

&&

$$

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