Your Provincial healthcare coverage may be good for up to six months after you leave Canada , depending on your Province of residence (some terminate it immediately).  There are several problems in continuing your Provincial coverage in the US that make it of virtually no use to you.  First, your Provincial healthcare will only pay "table" rates.  For example, the table rate for a skin graft in Canada may be C$400 . . . that is all that will be paid.  If you need a skin graft while resident in the US, and it costs U$650, your Provincial healthcare will pay C$400 and you will be billed by the US hospital for the balance.  Second, there are limits on the daily rate that will be paid for hospitalization that provide a very small portion of what is required (B.C. pays C$75 per day) and is inclusive of everything required for that hospital stay including the room, bandages, food, medicines, etc.  In the US , however, there is one fee for the room, another for the doctor and each bandage, pill and needle is tracked separately.  Needless to say, this leaves you with the balance to pay personally. Third, you will have to prove to the provincial health authorities that your intent was to remain in Canada and not move to the US . The other risk you face is being subject to double taxation from Canada because you have not severed your ties, and from the US , because you are resident there.

Travel insurance can assist you on a temporary basis (usually less than six months) for any balance owing but if you have a "pre-existing" condition, you may not be able to get insurance coverage for it. Likewise, the insurance company will attempt to attribute any illness to the pre-existing condition so they don't have to pay benefits.  Therefore, you should apply for health insurance before you leave the safety of Canada 's healthcare system in the event you are "uninsurable."