Originally (several years ago), SW+ Alerts was sold as a one-year subscription service. However, after several months, we changed to a "one-time purchase" lifetime subscription model. If you purchased SW+ Alerts as an annual subscription, your subscription has been automatically converted to a lifetime subscription. You will pay no annual subscription fees and may continue to use the service for as long as you wish. You will not need to do anything to convert your subscription. it is already done!
NOTE: With the introduction of StormWatch+ 4.0, SW+ Alerts is considered a subscription service, requiring either a monthly or (discounted) annual subscription. Your subscription auto-renews at the end of the purchased subscription period unless you cancel prior to renewal.
More information: http://www.stormwatchplus.com/sw-blog/note-on-sw-alerts-pricing