(Anecdote alert)
I used to use a tarp-like cover on my air conditioner unit in winter. In 2009, I didn't even have to uncover the A/C until late June, and I only ran it just before the July 4 holiday just to make sure it was working before the inevitable hot weather arrived.
BWI temp departures May thru Oct: -0.7, -2.1, -3.7, +0.4, -2.6, -2.1 (Only Aug was positive)
BWI didn't hit 90f until July 12. Hit 94 a few days later, and that was the highest temp of the summer.
Remarkable summer, by our standards.
This year has been rather cool, but we've already had highs in the upper 90s. Not really comparable IMO.