I'm in northern NJ, and for the second year in a row I have some leakage through the header of one of my kitchen windows. There are multiple things going on here, and I'm trying to sort it all out.

Upon inspection I discovered that the J-channel at the top of my vinyl windows (installed by a previous owner) was completely jammed with ice, in the same area as the leaks. I scraped out as much as I could with a screwdriver, and got it to where I think there was at least some way for melt-off to run out of the channel (other than coming inside). Is the ice build-up an indication of an improper installation, or just something that can happen?

In addition to this, the roof immediately above it has an ice dam (it's been that kind of weather lately). I've done some work to alleviate it, maybe or maybe not successful. It is possible that the ice dam led to water drippage behind the gutter which in turn found itself into the J-channel.

So for now, the leak is stopped, but I don't know exactly why. Was the frozen J channel a likely culprit? Even if not, it would seem desirable to avoid this icing. I'm getting new siding in the spring, which would be a good time to address it. what is a good solution to prevent this from happening in the future?