(CNN) A significant winter storm will take shape beginning Friday over the Southern Plains and Texas, then spread this weekend through the Southeast and Mid-Atlantic.

How far south freezing air will reach remains an open question, making it difficult to precisely forecast ice and snow accumulation.

Winter storm watches are already in place Thursday for 6 million people in northern Texas, Oklahoma and parts of Arkansas and Missouri.

As the storm develops, forecast models will more accurately predict the freezing line and thus the type of precipitation places will experience, CNN meteorologist Brandon Miller said.

Here is what CNN meteorologists expect, where and when:

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