There are a few freeware utilities to download and inject the real weather in P3D or FSX.
For example, FSRealWX might be a good start
However, the current "best" weather engine is Active Sky.
The previous version was Active Sky Next.
The current version is Active Sky 2016.
Both versions have a very interesting feature: the historical mode. It will download the weather corresponding to the exact date and time you set for your flight.
So, if you want to fly somewhere in summer time, then Active Sky will download the live weather from that place at that day/time.
Other weather engines have only "live" weather. So even if, today, you create a flight set in July, you will get some November weather on it.
Active Sky 2016 is quite expensive for the moment.
However, Active Sky Next (which is the previous version... isn't that confusing?
), is frequently on sales and thus gets a very reasonnable price