Is 3.5 lbs a safe amount to lose weekly?

I'm starting to try to lose weight, and I WILL get to a doctor as soon as I can, but in the meantime I'm trying to figure out some basics to get started. I currently weigh 429. I'm 6'1, female, age 28. The site I used placed my BMI at 56.6 and I know that is dangerously high. I want to cut down so I can cut my health risks and get into shape. I'd like to get down to 180lbs, and I know that will take quite awhile. So when setting my goals, is 3.5lbs a week too much to lose safely?