So, after looking at the section on the Penumbra in the Umbra sourcebook, I found that what was represented in the Penumbra really comes down to one factor, importance. Things that have a spiritual reflection, or any reflection at all, need to be important or significant to the area.
In the context of the weather, if it say, snows in Boston in winter, it is unlikely to have as large an impact on the Penumbra, since snow at that time of year is an expected thing, and not as important to area. On the other hand, 2 feet of snow in Virginia would be a very big deal and important, and probably reflected in the Penumbra. In a general sense, I could see representations for the general weather for a season or natural disaster, but not day to day weather.
All of this is, of course, subject to ST ruling.