Can't tell you the number of days I left my house in WST only 5 miles from the Ocean in shorts and tees and arrive at the beach to freeze. As soon as you hit the terminal moraine the temp can drop 20 degrees.
Ave temp Highest and Lowest temp May 20th to June 2nd since 2000. Tough to warm up a pool with these temps. Once a decade we open Memorial day and then pool is about 68.