252
FXUS63 KLOT 121752
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1252 PM 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 /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1252 PM CDT Wed Aug 12 2026

Key Messages:

- Threat of low CIGs and Fog late tonight into early Thursday
  morning.

- Next chance of showers and storms comes Thursday afternoon
  into Thursday night.

Aside from a small (<20% chance) for a few additional showers
this afternoon, it appears we are done with precipitation
through at least Thursday morning. The main aviation weather
focus thus shifts to the threat for low CIGs and fog tonight
into early Thursday morning. Overall, we appear to be in good
setup for fog development through the evening, and especially
overnight, particularly at RFD, DPA and GYY. The main change for
the 18Z TAFs was to hit the VSBYs and CIGs a bit harder at
these sites for a few hours into early Thursday morning. We have
also added in a mention for lower CIGs and VSBYs at ORD and MDW
late tonight. We certainly cannot rule out the possibility for
lower conditions (IFR) developing over the main Chicago
terminals overnight, though confidence with this is too low at
this time.

The low CIGs and VSBYs will improve by mid morning on Thursday.
Thereafter, we will be monitoring the evolution of the next wave
of showers and thunderstorms expected to shift eastward across
IA late tonight into early Thursday morning. The remnants of
some of this activity could attempt to shift across the area
terminals Thursday afternoon. This may thus result in our next
period of showers and possible storms. I opted to omit the
threat for TS from the TAF at this time, as the better threat
for actual storms may remain south of the terminals. Otherwise,
expect easterly surface winds through the day.

KJB

&&

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

&&

$$

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