Some users on another forum have said they've seen this quote on TWN's Facebook page:

We’re excited to let you know that beginning on Monday, The Weather Network will be available in HD for Ontario Cogeco customers and MTS customers in Manitoba!
However, I haven't been able to find it. But also, their Youtube page has videos of HD promos that were uploaded only a few days ago. So, it looks as though it will be launching in HD.