"Here" is used in this case to say that San Francisco is where that person is. When I comment on discussions that ask about differences between countries, I usually start with "Here in the US..." By saying that, I'm telling people that I live in the US, and I will be talking about what it's like in the US. In your example, if the person who sent the message wanted to leave something out, they would have left out "in San Francisco." But, if they do that, then they assume that you know they live in San Francisco. If they left out "here" instead, then it sounds a bit awkward. It sounds like they looked up the weather for San Francisco even though they live somewhere else. Like, I could say, "It was a beautiful day in San Francisco, but here in Chicago it's still like winter." I live in Chicago, not San Francisco, so I use "here" with Chicago but not San Francisco. I could also say, "It was a beautiful day in San Francisco, but here it's still like winter," but that assumes that you know what I mean by "here."