Fir Tussock Moth

My second (educated) guess would be the white-marked tussock moth, which looks somewhat similar. This is what Bug Guide had to say about it: See Also Orgyia detrita (fir tussock) caterpillar is similar in coloration to Orgyia leucostigma (white-marked tussock), but "the sides of the body are gray and supraspiracular warts are orange; the black middorsal stripe is flanked by yellow spots on A4-A7

I searched online for quite a while before finding a tussock moth caterpillar that resembled yours. I believe that this is a Fir Tussock Moth (Orgyia detrita), but I am not certain. The range is a bit off for your area. http://bugguide.net/node/view/50667