Week to feature heat, humidity and thunderstorms

Heat and humidity return with a vengeance this week, accompanied by a significantly enhanced probability of thunderstorms.
With the jet stream situated far to the north across southern Canada and weak west-east flow aloft, warm southerly surface winds will return this week, feeding low-level moisture into northern Illinois. Heat indices will rise well into the 90s Monday before crossing the 100° threshold Tuesday and Wednesday. Clouds Thursday may hold the heat index in the 90s, but the highest reading of the week may be reserved for Friday. Any three consecutive days of a 100° heat index will trigger a heat warning for Chicago.
Slow-moving upper-level troughs aloft riding over warm humid air in combination with a nearly stationary front to the north may create periods of showers and thunderstorms Thursday and again late Friday into Saturday. Before the week is up, copious rains could fall over central and northern Wisconsin.