The National Weather Service’s Chicago office warns that a prolonged spell of dangerous heat will grip northern Illinois and northwest Indiana from Saturday through at least Tuesday. Afternoon heat indices are forecast to range between 100°F and 110°F, while actual high temperatures hover in the low 90s and overnight lows stay in the mid-70s, offering limited relief. The oppressive heat will coincide with several rounds of thunderstorms. Forecasters say the strongest cells could arrive on Saturday morning, bringing damaging wind gusts and torrential downpours, with additional storms possible near the Wisconsin state line later in the period. High moisture levels will keep conditions sultry even during breaks between storms. Residents are advised to limit strenuous outdoor activity, stay hydrated and monitor updated forecasts. A cooler, less humid air mass is expected to push into the region by the middle of next week, ending the heat wave and reducing storm chances.