
Kelly P. answered 07/07/20
Linguist with 10+ Yrs Teaching & Writing Experience (All Ages)
Sorry about the mix-up Bobby! (Relationships are tough; so is language). As a linguist I feel obligated to tell you that there is no 'wrong' answer. What matters more is how people actually use the language ('descriptive' grammar vs. 'prescriptive' grammar). So let's see how they use it.
Based on a very quick corpus search and some Googling, it looks like nearly every instance I found uses "deceptively cold" in the way that you've used it, including in a couple weather reports claiming it is "deceptively cold," so you better bundle up. So, you were right!
That said, if we look at a different usage of "deceptively" such as "deceptively simple" we find that there are more instances (at a glance) of the meaning that it looks simple, but it in fact is not. The usage of "deceptively" requires a contextual understanding of 'seemingness,' it seems, which changes depending on the use and context.
Of course, this makes language usage deceptively simple. I've heard that 90% of language is idiomatic, meaning any exact combination of words has its own specific meaning. (Then again I've also heard 50% of statistics are made up). So, sorry prescriptivist grammarians, the answer isn't that easy :)