I can't imagine they would even try to match RDR. But if they were going for realism, here are some pointers (I'm kinda "into" weather):
* Be sure to include concepts such as relatively steady rain from nimbostratus (sheet) clouds, as well as rain showers. Rain showers are often more heavy, particularly if the clouds develop fully into a thundercell.
* Most games does the mistake of having bad weather be the worst weather imaginable. Whatever happened to light rain?
* Let thunderstorms vary in intensity and duration. I've witnessed once a thunderstorm with 50 strikes per minute in the same town area, which was hefty. But only once, and I'm 40. The thunderstorms I see in my area are extremely weak, and extremely rare. Maybe 50 strikes total during the hour the event lasts, and maybe only three or four times a year. I'm not as frequent and intense as *all* games try to make it. Stay reasonable.
* Why does no games have hail during thunderstorms? Hail can *only* be formed in the thundercloud (although it doesn't have to produce thunder). Even summer thunderstorms bring hail. Let there be hailstones
I've seen cars get really smashed up by them, although at this size we're usually talking systems that can produce tornadoes.
* Signs of weather would be nice. From looming clouds in the horizon coming in in relation to a front, to towering cumulus clouds forming above your head in relation to typical summer thunderstorms, before they go cumulunimbus (thunder cell) on you.
* The obvious thing that we can have that RDR (probably) didn't have, are blizzards. Make them cause full whiteouts, where cloud and ground becomes one not many meters in front of you.
* A trick that I've seen in Arma2: It's unlikely that softer shadows are easily done due to overcast, so just fade away the amount of shadow the character casts. At maximum overcast, there should be pretty much no shadows at all, as it is the whole "sky" that it casting light rather than the small size of the sun.
* Separate wind from rest of weather. There can be plenty of wind without a cloud on the sky, and there can be a thunderstorm without any wind at all (especially before the rain gust hits).
* If we have to manage cold (hypothermia, and I hope we do as part of "needs"), wind and rain should act as multipliers of cold due to temperatures. Furthermore, if we have seasons, model weather (incl temperatures) according to seasons. Thunderstorms are typically more frequent during summer months than winter months (disregarding special places like Tornado valley for now).
Again, I just *BEG* you, reduce the frequency and randomize the intensity of thunderstorms and precipitation if you want it to look real. As cool as they are, they also grow old fairly quickly, so there is no need to overdo it. Which is THE most common problem in most games featuring some kind of weather.