Fenómenal-fenómeno
 

A Spanish-speaking friend told me that it generally means freakish/freak, but sportscasters use it to mean exactly as it is used in English. Dictionaries give me a mixed reading. Are these two words generally negative terms?

The first is an adjective (no accent mark).
The second is both a noun and an adjective (fenómeno/a).

The adjective (both of them) means remarkable, exceptional. Synonyms are estupendo/a, admirable, espectacular.

The noun has several meanings. The one we know -phenomenon- and genius, ace, freak, curiosity, wonder, unprecedented event.

Apparently only the noun can be used in both a positive and a negative manner.

Fenomenal means terrific or brilliant, the same way bárbaro also does in some countries.

Fenómeno is a tiny bit ambiguous. It is widely used meaning someone is noteworthy, outstanding, a star in his or her activity, or a genious. But it still means phenomenon, including all the unusual facts or occurrences, both good and bad. That's why there's the set expression fenómeno de feria (the bearded lady; an extremely flexible skinny and funny looking contortionist, etc.) hence the term "freak" is replaced by fenómeno in media dubbed in Mexico and other places as the correct expression bicho raro is too long and troubles lipsynch, while raro could be used but it remains ambiguous as it also means weird or gay in a politically incorrect fashion.