Erica's Answer:
That is because we are positioned under the jet stream more often! The jet stream is the primary storm track, so the moisture tends to follow the jet stream. In the southwestern US, the jet stream is usually to the north of their area. It's a self-perpetuating process, too… the Delaware Valley has a lot more vegetation, which not only absorbs moisture but also gives off moisture. So, we stay humid in the Northeast, and the Desert Southwest remains a desert!- Erica
