It's kind of weird for it to snow in late May, don't you think? The snow in For Whom the Bell Tolls has a bit of an aura about it. It almost seems supernatural, and if you think that, you might find it interesting that the coming of the snow is first predicted by Pilar, the character that seems to have some kind of supernatural aspects to her, and that, without seeing it, she seems to sense the snow has stopped. When the snow stops, of course, it's bad news for El Sordo, who leaves tracks in it. It then looks for a while as if the mission itself is doomed, and it would have been, if Pablo hadn't shown up at the mission. So there's almost something fated about the snow, too.
Perhaps the snow is meant to show the lack of control human beings actually have, especially in a war situation. Even the best-laid plans are totally at the mercy of circumstance, as both Robert Jordan's mission and Golz's larger attack are. And here "circumstance" = snow. Depending on whether you're Pilar or Robert Jordan, you can either see that circumstance as Fate or as Chance.
One last thing about the snow. Ever go out on a snow day (if you're lucky enough to have snow days where you live) when it's still snowing and notice how snow has a tendency to stop everybody in their tracks and quiet the bustle of everyday busy work? The same thing applies to war: while it's snowing, at least, people stop fighting. Even though it proves disastrous for El Sordo, the snow is a moment of repose in the midst of the action. It's proof that nature goes on regardless of what those nasty little human beings with their guns are doing. Even in the midst of war, there's something deeper which is left unaffected:
In a snowstorm it always seemed, for a time, as if there were no enemies. In a snowstorm the wind would blow a gale; but it blew a white cleanness and the air was full of a driving whiteness and all things were changed and when the wind stopped there would be the stillness. This was a big storm and he might as well enjoy it. It was ruining everything, but you might as well enjoy it. (14.71)