An upper-level ridge of high pressure will build across the region over the next several days, resulting in a prolonged period of heat and humidity. While thunderstorms will be possible across north-central Illinois tonight and again Friday night, the main weather story through early next week will be the heat. Afternoon high temperatures will climb into the lower to middle 90s from Friday through Monday. Resulting heat index readings will reach 100 degrees or higher. Please exercise caution if you must spend long periods of time outdoors. Plan your work for early morning or evening when it's cooler and don't forget to drink plenty of water.