If you're looking to get a half marathon in early year in Nevada, you only have one to choose from. January and February are very quiet months for half marathons in Nevada.
Running from an Angel Half Marathon in Boulder City (near Las Vegas and Henderson) is the one and only half marathon in Nevada in January 2015. The race is quite small, as there were only 133 finishers in 2014.
There are no Nevada half marathons in February 2015. This is actually pretty shocking, as weather temps are ideal for half marathons throughout January and February, typically ranging around 60's around the Las Vegas area. With the growth of half marathons in the United States, it is surprising so few exist through these months in Nevada.