June 24th Afternoon Update
A severe thunderstorm and isolated tornado threat exists across portions of the middle Mississippi River Valley late this afternoon into the early evening hours. While severe thunderstorms with large hail and damaging winds will be the primary threats, there is a conditional risk for a few tornadoes. The elevated focus area is across southeastern Iowa into far northeastern Missouri and portions of west-central Illinois. Other severe thunderstorms are expected surrounding the conditional threat zone with a tornado threat in the larger surrounding yellow shading as well.
Convection was ongoing across much of Iowa at 2 p.m. CDT. A quasi-stationary front was draped from near the Nebraska/Iowa/Missouri tri-state border, southeastward into northern Missouri and west-central Illinois. To the southwest of the front, strong instability in excess of 3000 J/kg MLCAPE was observed via mesoanalysis across much of Missouri, eastern Kansas and far southeastern Nebraska. This warm sector should gradually spread northeast with time. Near the frontal boundary, 20-30 knots of 0-1km shear was noted, with 50+ knots of bulk shear and 0-1km SRH of 150-250 m2/s2.
This is a complex setup, but the region on the southern flank of ongoing convection and to the immediate south is where the greatest tornado threat exists. The area near the Missouri/Iowa border should remain capped for most of the afternoon, but there is a conditional threat for supercells to develop late this afternoon into the early evening hours across south-central to southeastern Iowa. Any storms that remain discrete could take advantage of an inflow environment characterized with moderate to strong instability and sufficient low level shear to potentially form a few tornadoes. High resolution convection-allowing models have showed little consistency in timing, storm mode and coverage of such convection. Nonetheless, the threat exists and there is a small window for a strong tornado, if such a storm were to develop and remain isolated. Look for this potential activity to spread east-southeast to southeast into the evening.
The ongoing convection may begin to strengthen somewhat over the eastern half of Iowa and eventually northwestern Illinois. This will pose a threat for mainly large hail. There is a greater probability that more widespread convection develops into the evening hours as the low level jet increases and the environment, at least elevated, becomes more unstable with northeast extent. The evening threat looks to be predominantly strong winds, although some large hail will be possible. This should develop near to just north of I-80 in eastern Iowa and spread into northwestern Illinois. Upscale growth may result in a significant MCS forming from this activity as it spreads into northern and central Illinois. This MCS threat should remain south of Chicago, but may spread as far east as portions of Indiana overnight.
