High pressure anchored across the Midwest is forecast shift east of the area by Monday. Southwest winds are then forecast to persist much of the upcoming work week. This pattern change is expected to result in a gradual, but sustained influx of tropical air.
Temperatures are forecast to stair-step upward to around 90 degrees by midweek. In addition, dew point temperatures, a measure of water vapor present in the air, are expected to rise from the mid 50s early in the week, to the mid and upper 60s by Thursday. This increase in moisture will act to destabilize the atmosphere, elevating the potential for thunderstorm development. It has been dry of late. Portions of the Chicago area have received as little 25 percent of normal rainfall since July 1st. Thunderstorm chances are forecast to peak Thursday afternoon and evening.