I like to use Wunderground Wunderground Las Vegas This site has tons of info, not just the current forecast. it even has an Almanac feature where you can check the weather for past dates if you want to get an idea of what the weather is historically like during a certain time period - great for planning visits!
Those are both good, because they both have 10 day forecasts. One says 40% rain on Sunday, and one says 10%, so at least I know they're not both using the same data.
I use wunderground. Lots of features and other links. If theres a special weather statement they put the NOAA text at the bottom of the page. They have a very good iPhone app too.
weather.com is likely best site on a nationwide basis.
You can also use the LVA Weather Watch Center: Just e-mail odysseus55, he'll go look out the backdoor, watch the weather and maybe report back to you in a couple days.
