"Not a ton" of benefit is derived from rainfalls like that, according to Corey Enus of the Southern Nevada Water Authority, who points out that Las Vegas is "one of the cities with the lowest amount of rainfall in the United States." According the Las Vegas Review-Journal, our annual average is 4.19 inches of precipitation.
As for the drought, Enus says that Lake Mead and the Colorado River are replenished mainly by rainfall and snowfall in the mountains, so the principal benefit of a rainy day is that there is simply less domestic, outdoor water use in Las Vegas: "We can let Mother Nature take care of that for us."
