To the best of my knowledge, we do not admit students mid-year. Admission at Berkeley is highly competitive. We are eager to accept the best, most intellectually exciting students. If this is you, I highly encourage you to apply. I highly recommend stressing this in your application.
Most faculty receive a large amount of e-mail regarding admissions.
An e-mail contact will not persuade a faculty member to pursue an application. For masters students, the situation depends on the circumstances of the student, and Berkeley will discuss this with you after you have been accepted. I hope this simple FAQ answers most of your questions. As I receive further questions, I may extend this list.