Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0759 AM CDT Wed Jun 17 2026
Valid 171300Z - 181200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS ILLINOIS
AND INDIANA...
...SUMMARY...
An outbreak of severe thunderstorms is expected today into tonight,
with the peak threat centered over Illinois and Indiana. Several
intense tornadoes, swaths of damaging gusts over 75 mph and damaging
wind-driven large hail will all be possible.
...Midwest including Illinois/Indiana/Missouri/Ohio...
A very active/potentially dangerous day is expected today into
tonight, although some sub-regional forecast details remain a bit
uncertain. This is largely attributable to an upscale-growing
intense MCS which has been evolving in the predawn hours across
central/eastern Iowa, moving south of the I-80 corridor as of 730am
CDT. Measured significant wind gusts in excess of 80 mph have been
observed before sunrise, including a measured 94 mph measured wind
gust in Marshall County, Iowa.
This intense and increasingly well-organized MCS will continue to be
influenced by an extremely strong southwesterly jet of 70 kt and
robust warm advection/moisture transport, and likely continue
southeastward from southeast Iowa into north-central/west-central
Illinois. This will likely include an increasing potential for
surface-based storms and an appreciable uptick in damaging
wind/tornado potential early today, especially on the
south-southwest flank of the MCS in closer proximity to the
northeastward-shifting warm front.
Ahead of this early activity, a warm front will push
north-northeastward across Indiana/Ohio. New severe storm
development, perhaps MCV-influenced and transitioning out of the
remnant activity and/or forming near the warm front, is possible
across Indiana into Ohio. Shear profiles will be excessive, with
tornado risk only conditional on minimal instability being present.
The result may be a isolated tornadic supercells. The warm frontal
position will need to be monitored northward toward the
Indiana/Michigan border vicinity. Even if instability is elevated
into Michigan, extreme shear and lift may still yield damaging winds
and even a tornado risk.
To the west, rapid air mass recovery is expected on the
west-southwestern flank of the early day MCS from Missouri into
central/possibly parts of northern Illinois, even where appreciable
early day MCS impacts occur early. This recovery will be fueled by
mid-June insolation/moisture content and robust advection, again
attributable to atypically robust (50-65 kt) low/mid-tropospheric
west-southwesterly flow by mid-June/diurnal standards. In the
presence of 65-70 F surface dewpoints, this will likely result in a
corridor of 2000-3500 J/kg MLCAPE.
This will likely set the stage for the development of a broken line
of intense supercells, potentially favoring prior outflow (or
outflow-augmented warm front) and post-MCS recovery zone of
differential heating across Illinois and perhaps eventually into
western Indiana. Given the clearly supercellular shear profiles and
ample venting aloft, linear storm mode is unlikely for most of the
event. Long-tracked supercells producing tornadoes and damaging
large hail are likely. Some strong to potentially intense tornadoes
are plausible given the magnitude of the low/mid-tropospheric flow,
and again potentially heightened in vicinity of the prior
outflow/warm front. Some damaging wind threat will likely also
increase by evening as storms continue across Indiana into Ohio and
potentially southward toward the Ohio River.
...Upper Texas Coast/Middle Gulf Coast...
The NHC forecast shows Potential Tropical Cyclone One moving into
southwest Louisiana by this evening. Wind fields associated with
this system will strengthen out of the south ahead of it, resulting
in areas of strong low-level shear from the upper Texas coast across
southern portions of Louisiana and eventually into southern
Mississippi. Mid to upper 70 F dewpoints will contribute to modest
CAPE values, supporting embedded stronger cells with tornadic
potential.
..Guyer/Broyles.. 06/17/2026
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