Has anyone come across any C# code or even VB, C/++ for weather forecasting based on sensor data such as temp, humidity and pressure etc?
no, unless you could calling to an external service for the location close enough 
I thought of this myself already, but did no research so far.
But it can’t be to difficult, since every cheap little disounter weather station can do it.
Edit: I found this:
http://www.seea.org.in/special_issue/vol1/32.pdf
didn’t read it
The BME280 sensor I use is very stable and tracks quite well with a professional weather station I have here for another project. I only want to show cloudy, clear or rain type displays. Nothing needs to be accurate. The Davis station I have here does this. I could of course just fetch this from the web but thought it would be fun to do this myself.
I’ve decided to fetch and parse the weather from openweathermap.org instead. 
Found some code that Gus put on Codeshare that I can hack around to work with that site instead of Yahoo.
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My weather barometer app does weather forecasting based on pressure trends (I wanted it to have an off the grid capability like my grandfather’s barometer as that was his only weather forecast as a early homesteader) and I’ve received some pretty good reviews commenting on It’s accuracy, but I know for some folks at least it’s pretty much right out to lunch. There is a portion of weather forecasting that is depended on local factors in interpreting trends and I’m sure even those weather kits can have problems.
How much variation do you see with pressure?
Here I have seen min 999 and max 1005mB this week. Not much movement compared to what I used to get back in Scotland.