I think that most rain coming from the Pacific tends to fall on the western side of the Sierra Nevada/Cascades/Rockies. The way this works is air at a higher altitude is less able to contain moisture, so as air is pushed up over mountains, rain precipitates out and falls on the windward side of mountains. You get a "rain shadow" effect on the leeward side.

So probably mostly affect California directly. I imagine that it wouldn't hurt Lake Mead, though.

searches

This says yes, though that it won't be a game-changer for Lake Mead.

https://www.newsweek.com/el-nino-forecast-predictions-winter-drought-rain-water-12310727

Colorado River Impacts: Lake Mead and Lake Powell Water Levels

Pastelok said he believes El Niño will benefit water levels in the Colorado River and Lake Mead, and a strong one could trigger a "significant jump" for the latter.

If El Niño doesn't weaken quickly, next year's monsoon season and another wet winter in 2027-28 would further improve the water levels, he also told Newsweek.

However, drought conditions in the Colorado River Basin are the product of decades of overuse, overallocation and persistent aridification, not simply a lack of precipitation, meaning one wet year is unlikely to fully reverse the region's long-term water challenges, Daniel Swain, climate scientist with the California Institute for Water Resources, told Newsweek.

“We are never going to see a year where this magically goes away," he said. "We could have the wettest year on record in the basin, and we would still have a major water deficit in the long run.”

Despite the concerning outlook, there are decent chances, especially in the Lower Basin states, of a wet winter this year, Swain said, with near- to above-average precipitation possible because of super El Niño. The climate pattern will probably “strengthen the winter-time storm track in places like Arizona, New Mexico, southern Utah, and maybe southern Colorado,” he said, meaning more rain and high-elevation snow in those states.