This is a fascinating report and study...about ten years ago, my spouse and I would go from Glen Burnie into Baltimore City in the evening to dine 2 or 3 times a week. In summertime, when driving home with open windows, away from the city, on the BW Parkway, the temperature would noticeably cool at the same exact spot every night! This was when passing a swampy area just past the Patapsco river and the Balto. Co./AA Co. line. It never failed and showed me beyond question how strong the "island heat effect" really is. I never thought it would have this influence on t-storms, it shows humans have a bit more to do with weather than many would like to admit.

Surely there's more at work in these parts than the heat island effect. Storm fronts persist to the north of Baltimore because they have traveled a smaller distance from the mountains. To the south they are sustained by the moisture source of the Potomac River, running southeast from the mountains. Once east of Baltimore, they are refreshed by the moisture mass of the Bay. Hence you often see a storm mass pass along the Mason-Dixon line clear to Philadelphia (by which time it has received further refreshment from Delaware Bay).