<P> Late spring through summer is the wettest time of the year for the region . Weather systems in the westerlies that cause precipitation move along jet stream, which migrates north into the region by summer . This also increases the likelihood for severe weather to develop due to stronger upper - level divergence in its vicinity . Mesoscale convective complexes move into the region from the Plains from May through August, with June the peak month for Michigan . These systems contribute about 2% of the annual precipitation for the region . Also, remnants of tropical cyclones occasionally move northward into the region, though their overall contribution to precipitation across the region is minimal . From the spring through the summer, areas near the shores of the relatively cooler Great Lakes develop sea breezes, which lowers rainfall amounts near the immediate coastline . </P> <P> Average precipitation across the region show maxima along the coastal plain and along the mountains of the Appalachians . Between 28 inches (710 mm) and 62 inches (1,600 mm) of precipitation falls annually across the area . Seasonally, there are slight changes to precipitation distribution through the year . For example, Burlington, Vermont has a summer maximum and a winter minimum . In contrast, Portland, Maine has a fall and winter maximum, with a summer minimum in precipitation . Temporally, a maximum in precipitation is seen around three peak times: 3 a.m., 10 a.m., and 6 p.m. During the summer, the 6 p.m. peak is most pronounced . </P> <P> Coastal extratropical cyclones, known as nor'easters, bring a bulk of the wintry precipitation to the region during the cold season as they track parallel to the coastline, forming along the natural temperature gradient of the Gulf stream before moving up the coastline . The Appalachian Mountains largely shield New York City and Philadelphia from picking up any lake - effect snow, though ocean - effect snows are possible near Cape Cod . The Finger Lakes of New York are long enough for lake - effect precipitation . Lake - effect snow from the Finger Lakes occurs in upstate New York until those lakes freeze over . Bay - effect snows fall downwind of Delaware Bay, Chesapeake Bay, and Massachusetts Bay when the basic criteria are met . Ocean effect snows are possible downwind of the Gulf Stream across the Southeast . </P> <P> During the summer and early fall, mesoscale convective systems can move into the area from Canada and the Great Lakes . Tropical cyclones and their remains occasionally move into the region from the south and southwest . Recently, the region has experienced a couple heavy rainfall events that exceeded the 50 - year return period, during October 1996 and October 1998, which suggest an increase in heavy rainfall along the coast . </P>

What air mass brings the most precipitation to the eastern united states