Rumors were all there for September but if you acted on your plan last month, you would probably miss the severance package this month (if there is any). Even today we are not sure about October layoff. So you have limited control over your new job hunting timeline and absolutely no control over Juniper layoff timeline. With such little control over the process, consider yourself lucky if you are able to grab a severance check on your way out, but that is not something you can engineer.
IMHO, getting a new job that makes you want to go to work in the morning is the most important thing. If you bump into something like that, say yes to hiring manage and start as soon as possible. Forget about severance check as it is not something in your control.
That said, I think the way Juniper handles layoff is seriously wrong. Keeping it a big secret and letting rumors float around damages morale/productivity everyday. With or without layoff, this is gonna cause another round of low quality.