A320 pilot reported severe turbulence while attempting to land at original destination. Flight was diverted to a local airport.

Narrative

Encountered severe and then moderate turbulence on arrival into ZZZ1 in vicinity of ZZZZZ intersection on ZZZZZ1 arrival. Notified ATC. Unable to hold altitude initially and aircraft descended below assigned altitude with autopilot on. Autopilot disconnected so I hand flew aircraft. Requested lower altitude. Turned away from arrival northbound. After situation stabilized; turned Autopilot back on and was advised ZZZ1 no longer taking arrivals. Diverted to ZZZ2 out of caution to aircraft and passengers. Spoke with flight attendants who had been advised to have cabin ready for arrival and were seated 15 minutes prior to turbulence event. Advised no injuries to passengers aside from report of some passengers vomiting. Landed ZZZ2 without incident. Entered severe turbulence encounter in aircraft logbook. All passengers sent by ground transportation to ZZZ1. No passenger injuries. Aircraft subsequently underwent severe turbulence inspection and released back to service. Re-positioned aircraft to ZZZ1 and arrived around XA:30 am local. Cause - Large weather system that spawned convective and tornado activity in multiple states. Dynamic weather on arrival without many PIREP's - last ride report we obtained from ATC was light chop on arrival. Weather radar returns indicated light to moderate rainfall with aircraft staying on arrival ahead of us. Communication with ATC challenging with numerous aircraft on frequency. Suggestion - Update/retrofit of older weather radar sets/displays with higher resolution and return/refresh rates on older fleet would be a great improvement to safety on our fleet.