Recording technique, particular for 5.1, is undoubtedly a more complex topic than can be covered in a forum such as this one, as was mentioned earlier.
However, recording a thunderstorm
monaurally - one microphone and some cheap / free software - is very easy. My question would be why the desire to accomplish this in 5.1? If the desire is for a soothing recording for playback on a small stereo, mono is just fine. An inexpensive stereo microphone can also be considered, and is available from just about anywhere (music store, Radio Shack, even Walmart might have one). Simply record to a computer using built-in software (if available) or freeware that records to WAV format.
Further, if there is a desire to use all the loudspeakers in a system, a stereo recording of a rainstorm played back on a surround system using the Dolby ProLogic setting should be just fine. Especially if all that is desired is some soothing ambient background noise.
The only other thing I will mention is cautionary: Beware that when listening to a "live" storm versus listening to a playback of a recorded storm, there are two different psychoacoustical mechanisms at play. When sitting on the front porch, listening to a storm, the ear-brain focus is on the sounds of the storm. The result is (for you) a soothing experience. When the storm is recorded, the ear-brain is not able to focus as well (or at all). Further,
everything is recorded, not just the storm sounds. Thus, insect and bird noise, air conditioner noise, traffic noise, airplane noise, dogs barking, and so on will all be present in the recording. These are noises that are present in the "live" experience as well, but are psychoacoustically "filtered out." Because of the way the nervous system is wired, these noises will not be "filtered out" when listening to a playback of the recorded storm. Dissatisfaction with the end product (after all the time involved to produce it) could result.
That's a long-winded way of saying that it may be best to drive several miles outside of Calgary on a little-used country road to make the recording. (Bearing in mind that there are few places free of insects and birds, even during a storm.)
Alternatively, I'm betting commercial recordings (done professionally - perhaps even in 5.1) of rainstorms are readily available for sale on the Internet. Foley houses, sound effects companies, etc., usually offer such things. Unless there is something particularly unique about how rain, thunder, and wind sound in Calgary, this could be the best choice.