I'd cut it off and then either pull the onion now and use it or let it size up and remove the central stalk (which may get hard) and use that onion first. What's happening is that the onion thinks it's on it's second season of growth and it's making a seed head rather than a bulb that stores energy. This happens more with sets just due to the way that they are produced and the recent heat didn't trigger it.

Since onions are day length sensitive, the long day varieties that you're growing in PA will wait until they get around 14 - 16 hours of sun a day and then start to make bulbs and I'd guess that's starting in the next week or so. What you'll see next is that the onions will probably start expanding up and out of the ground so that you'll see at least the shoulders as they continue to grow. In around 3-4 weeks, the necks of the onions will start to get soft and the tops will fall over. This is the sign that they are done growing and you can harvest them at that time ... I'd guess middle to end of July for you. Once harvested, remember to hang them somewhere for a few weeks in order for them to cure and let the necks dry down so that you get the best storage out of them.

Thank you! The onion crop in general looks so nice and robust. I am really excited about it. They seem simple to grow and easy to weed.

What would I do if I wanted to produce my own sets from year to year?

Thanks, I also live here in central Pa and have some white onions. About half are tall and blooming, and the remainder have fallen over. Was wondering if I should harvest now. Looks like it is time to harvest, which is great. Will give me room in the raised bed for some other crop.. Thanks again for the advice.