Well, I grew up hearing them referred to as the Sierras, and las sierras is the correct plural in Spanish for Snowy mountain rangeS.
And if you want some historical reference thanks to Google and project Gutenberg from 1915...
THE LAKE OF THE SKY - LAKE TAHOE:
IN THE HIGH SIERRAS OF CALIFORNIA AND NEVADA
And you can be all bitchy about it when people start pronouncing other Spanish words in Nevada correctly. and this would be more of a spelling/semantics issue than grammar anyway.