101
FXUS63 KLOT 111319
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
819 AM CDT Tue Aug 11 2026

.KEY MESSAGES...

- Scattered thunderstorms will affect far northern IL/northwest
  IN early this morning, with isolated instances of large hail
  and damaging wind as well as localized flooding are possible.

- Possible line/complex of severe thunderstorms to affect the
  area by mid/late morning into early this afternoon. Severe
  wind gusts to 80 mph possible, along with heavy rain and
  localized flooding.

- Pattern remains active with additional thunderstorm chances
  through the end of the week/weekend.

&&

.MESOSCALE...
Issued at 819 AM CDT Tue Aug 11 2026

While it took quite some time overnight, the convection over IA
has evolved into a more well organized MCS since 11z. DVN WSR-88D
imagery shows an impressive rear inflow jet has developed with
strengthening cold already resulting in an acceleration of in
forward speed, now moving at around 55 kt.

Morning soundings from ILX and DVN depict a very unstable air
mass in advance of this MCS with steep lapse rates and very
moist boundary layer. While there is still CINH with surface
based parcels, some weakening of this CINH will take place this
morning as sfc warms and dewpoints creep up this morning. Even
with some sfc based CINH, strongly forced ascent on leading edge
of this MCS should have no problem forcing parcels up to the
LFC.

KDMX 88D VWP shows a nice 50kt mid level speed max feeding into
the backside of this MCS, providing ample deep layer shear for
continued mesoscale organization. SPC mesoanalysis depicts an
axis of >3000 J/kg of MLCAPE from northwest IL east-southeast to
west central Indiana. This is in close proximity to a diffuse
surface boundary that remains draped across a very similar area.

Expectation this morning is MCS should continue maintain its
intensity and probably begin to intensify by mid-late morning as
it encounters weaker CINH and diurnally destabilizing boundary
layer. Environment appears primed for a higher-end bow echo and
possible derecho to evolve from this MCS as it rips across our
CWA this morning. Given the orientation of the diffuse sfc
boundary and instability axis, some southward propagation to
this line is likely.

Primary hazard will obviously be damaging winds, potentially
swaths in excess of 80 mph as this complex races across our CWA.
Presence of the weak surface boundary running perpendicular
this QLCS could provide a source of low level vorticity for
mesovortices and potential QLCS tornadoes as well. This threat
is likely to increase as the activity gets into our eastern CWA
as boundary layer destabilizes further.

- Izzi

&&

.DISCUSSION...
Issued at 253 AM CDT Tue Aug 11 2026

Forecast remains complex today, with ongoing storms early this
morning and the potential evolution of a more significant
widespread severe weather/damaging wind threat later this
morning and early afternoon.

Early morning surface analysis depicts a rather diffuse warm
frontal zone stretching from north-central IA east-southeast
across northern IL and northwest IN, roughly from north of the
Quad Cities to LOT and VPZ. KLOT VAD wind profile and SPC/RAP
mesoanalysis indicates a ~25 kt west-southwesterly low level jet
impinging on the frontal zone, with associated isentropic
ascent atop the shallow frontal slope resulting in isolated
thunderstorms from near ARX over to the Lake Michigan shore near
Michigan City IN at this hour. Low-level jet forcing may
increase a bit during the predawn hours as a subtle mid-level
short wave tracks across the area. There has been a signal in
CAM guidance for potential additional development along the
front prior to sunrise, and we continue to monitor for that.
MUCAPE near 3000 J/kg and ~40 kts of effective shear per SPC
mesoanalysis would support storm organization and an associated
severe (primarily wind/hail) threat as evidenced by the
scattered supercell storms currently over LM/IWX`s cwa.

Attention then turns to a developing area of thunderstorms over
far southeast South Dakota, which may evolve into a more
widespread and potentially significant severe weather threat
across our forecast area later this morning and early afternoon.
CAM guidance has been consistent in this cluster of storms
eventually growing upscale into a forward-propagating MCS across
northwest IA/southwest MN over the next few hours and
eventually tracking east-southeast along the corridor strong
instability along the aforementioned frontal zone into northern
IL toward mid-late morning. The combination of unseasonably
strong westerly mid-level flow and resulting deep shear(50+ kts
at 700 mb), steep mid-level lapse rates (near 8 C/km) MUCAPE
values in excess of 3500 J/kg, the potential for significant
severe weather appears rather high. Several CAMs are depicting
pockets surface wind gusts of 80+ mph with this MCS across
portions of the forecast area. A tornado threat would also
likely exist given sufficient low-level shear and the proximity
of the previously mentioned surface frontal boundary. While the
exact evolution of this MCS remains uncertain, SPC`s Day 1
enhanced (level 3 of 5) for out forecast area appears warranted.
Currently timing of this threat looks to be from perhaps 9 AM
in our northwest cwa through around 3 PM in the far east. There
may also be some convective development along the front ahead of
the MCS, if a strong MCV develops prior to reaching the cwa
increases warm-advection ascent ahead of the main cluster as
indicated by some of the CAMs.

As if the significant severe/widespread damaging wind threat
wasn`t enough, we`ll also need to monitor the location of heavy
rainfall and flooding potential - particularly for the urbanized
Chicago metro area as well as locations which saw 3-5"+ of
rainfall Sunday and evening. Had contemplated issuing a flood
watch, though with current predawn convection slow to fill-in
(so far) and some uncertainty remaining with the evolution of
the MCS later this morning, elected to hold off at this time
pending some further clarity on location of higher rainfall
impacts.

In the wake of today`s expected MCS, a lull in convective
activity is expected from later this afternoon into tonight,
though this will likely depend on the size and magnitude of the
remnant cold pool and how far south the effective frontal zone
ends up along it`s southern periphery. The overall pattern some
uncertainty is how far south an area of surface high pressure
builds across the western Lakes in the Friday/Saturday period.
This will have significant impacts on thunderstorm trends/timing
for the local area.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 706 AM CDT Tue Aug 11 2026

Key Messages:

- An organized complex of thunderstorms is likely to impact the
  terminals between mid-morning and mid-afternoon today. Severe
  wind gusts in excess of 50 kts may occur with these storms.

- Sub-VFR ceilings and visibilities are likely to be observed
  again late tonight into Wednesday morning.


A thunderstorm complex in eastern Iowa at press time is
attempting to get organized into a solid line or bow. These
storms will impact the terminals between mid-morning and mid-
afternoon, and if it manages to become well-organized by the
time it reaches the terminals (observational trends seem to
support that occurring), then severe wind gusts in excess (and
potentially well in excess) of 50 kts may be observed as these
storms roll through. Also could see a few storms develop out
ahead of this main thunderstorm line/bow, so have PROB30 groups
for TSRA preceding the main TEMPO TSRA groups in the Chicago
metro TAFs to account for this possibility.

The persisting abundance of low-level moisture allowed for MVFR
stratus and fog/mist to develop once again last tonight, and
sub-VFR conditions are still being observed at some TAF sites as
of 1200Z. These conditions should improve by mid-morning, but
the persisting low-level moisture and light winds tonight will
likely allow for low stratus and fog to develop once again.

Lastly, there is another chance for thunderstorms to affect the
terminals tomorrow. Current thinking is that the main surface
boundary that storms will focus along tomorrow may remain south
of the terminals, but if this boundary ends up a little farther
north than presently expected, then there will likely be a need
for some sort of TSRA mention to be introduced into the TAFs.

Ogorek

&&

.MARINE...
Issued at 253 AM CDT Tue Aug 11 2026

A potential exists for a severe thunderstorm complex to produce
widespread damaging winds (in excess of 65 knots) across far
southern Lake Michigan later this morning/midday. In addition to
the wind threat itself, the line of storms are expected to
propagate in an east-southeast direction, which could
potentially lead to a seiche (water is pushed ahead of the
storms, then sloshes back and forth for several hours after the
storms pass) on the southern portion of the Lake. If this were
to occur, it could result in fluctuating lake levels along the
Illinois and Indiana shores for a time from later morning into
this afternoon.

Ratzer

&&

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

&&

$$

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