Do we know how the weather in the Reach compares to weather in the North during Westerosi wintertime? Presumably, southron regions are still relatively warmer than northern regions, but I don’t believe GRRM specifically addresses the issue.

Here’s what GRRM has to say:

And, by the way, does it snow in the South during the winter?

Yes, some times, in some places. The Mountains of the Moon get quite a lot of snow, the Vale and the riverlands and the west rather less, but some. King’s Landing gets snow infrequently, the Storm Lands and the Reach rarely, Oldtown and Dorne almost never.

So the Reach rarely has snow even during the winter.