Colonial spring 

A bout of warm, springlike weather toward the end of winter; the opposite of an Indian summer.

I think we ought to dub this period of unseasonably warm weather a 'Colonial spring' because:

1) It wasn't invited and it's not supposed to be here,
2) It seems like a blessing at first, but you suspect a global calamity may be lurking,
3) Its 'gifts' are often laden with nasty surprises like illness and flash flooding, and
4) Things could turn ugly again really quickly, without warning, despite assurances to the contrary.

Colonial spring by BornBlitzed February 24, 2017