A typhoon is looming. However, hopefully we are leaving the city.

You need a comma after however: it's an adverbial that makes a meaning connection between sentences and cuases a short pause. Using a comma after hopefully is optional: "hopefully the road should be finished by next year" Oxford Dictionaries.

A typhoon is looming. However, hopefully we are leaving the city.

You need a comma after however: it's an adverbial that makes a meaning connection between sentences and cuases a short pause. Using a comma after hopefully is less common:

hopefully the road should be finished by next year

Source: Oxford Dictionaries.

Also here's a search in Google Books. As you see in most cases hopefully isn't followed by a comma.

A typhoon is looming. However, hopefully we are leaving the city.

A typhoon is looming. However, hopefully we are leaving the city.

You need a comma after however: it's an adverbial that makes a meaning connection between sentences and cuases a short pause. Using a comma after hopefully is optional: "hopefully the road should be finished by next year" Oxford Dictionaries.