With the air mass overhead gradually becoming more unstable and an upper-air impulse buried in the southwest flow aloft expected to ripple across our area during the early morning hours Saturday, the probability of thunderstorms increases here after midnight tonight and in the hours before sunrise Saturday.
Note the thunderstorm probability maps below show a 10 percent chance of thunderstorms within 12 miles of a given location over most of Illinois and Indiana between 7 and 11PM CDT this evening (brown-shaded area on the first map below). Then probability of thunderstorms increases to 40 percent between 11PM and 5AM CDT over a good portion of Northeast Illinois into northwest Indiana (light-blue-shaded area on the second map below). A closer analysis of storm-triggering impulses indicates the best chance of thunderstorms occurring here may be during the pre-dawn hours of 3 to 6AM.
The Slight risk of severe storms continues overnight (yellow-shaded area on the third map below) with the greatest risk being large hail. Tennis-ball sized hail was observed with strong storms in SW Missouri late Friday afternoon and this instability area will be shifting NE.