NYC is best in September-November. But I'm totally Team Autumn so I'm biased. The temperatures are perfect and the colors are marvelous (and NY street style really comes to life in autumn, as opposed to summer when everyone is just dressing to survive).
Late November-January is not the best time to come because it's chilly and prices are high. But I have to say the holiday season is another favorite time of mine because the city is so beautifully decorated and "dressed up" so to speak. But yeah, blizzards and ice storms are not so fab. February and March are cheaper but still cold and storm-prone.
July and August are the hottest months and also coincide with tourist season (which begins in May and ends in early September). Honestly, there are times when I have really hated living in NYC during those two months. It might be different when VISITING, as opposed to living, but I still wouldn't recommend it. It's way uncomfortable and I doubt your Seattle wardrobe has enough appropriate clothing for the 90+ degree temperatures. Last summer was brutal and awful. I never like summer but it was particularly bad. It felt like living inside a sweaty crotch for 3 months (sorry if I made you choke on your bran flakes, but you know it's true).
After September-October, April-early June would be my second pick. May and June are particularly nice (the calm before the storm, I suppose). I guess it depends whether you're more of a spring person or an autumn person.
It actually seems like it rained more here over the past month or so than it did in spring, so I wouldn't worry too much about that. Rain is always a possibility here, except in winter, when it's too cold and it comes down in the form of snow or ice. Rain also tends to come in waves, unfortunately. It's rare that it rains for just one day and then passes over.
Anyway, I hope it works out Kari. I'd love to meet up if you end up in town!