Contrary to what some people think, Las Vegas weather does have four seasons. Yes, the seasons are different than you would experience in a Northern state, but they exist nonetheless. Let's take a look at what they are like.
Normal high temperatures in Las Vegas, in the middle of Winter, are around 55 degrees farenheit. Daily normal lows hover around 35. Lows have been recorded as low as 8 degrees. Temperatures into the 80s have been recorded in March as Winter was ending. Las Vegas hasn't has a major snowstorm in years, but in January 1979 Las Vegas received 9.9 inches of snow, and in January 1949 Las Vegas was hit with 16.7 inches! January and February tend to be rainier than most months, but they still average well below an inch of rain.
The normal high for Las Vegas in Spring starts around 70 degrees, and by the time Spring is ending the average high is 100. Temperatures as high as 116 have been measured near the end of Spring, that's just a degree short of the highest temperature measured in Las Vegas in the past 80 years. Spring is particularly dry in most years, with less than half an inch, on average.falling in the months of April, May, and June combined. Humidity is also low, and bottoms out in June.
The normal high for Summer in Las Vegas is over 100, although is dips below 100 after September 1. Humidity increases in July and August, and the number of thunderstorms increases to about 3 per month for those two months. Lows have been measured in the 50s in September, but 80s is normal for July and August, dipping into the 70s in mid September.
The temperatures in Fall decrease faster than they increase in Spring. The daily average temperature on September 1 is 99, by December 1 its 60. If someone tells you they were swimming and sunbathing in Las Vegas in the Fall it was probably not late Fall. Rain levels are lower than they are in January, February, July or August.
There are two weather phenomenon that can be potentially dangerous in Las Vegas (three if you consider the rare but potentially crippling snowstorms). One is wind. Wind can happen any time of year, and gusts into the 90s have been recorded in the flat, open valley that Las Vegas is situated in. Wind is rarely life threatening, but its strong enough that you should take it into consideration when enjoying the outdoors here. The other, more serious problem, is flooding. Rain amounts that wouldn't raise an eyebrow in many locations can cause flash flooding in Las Vegas. Besides having hard ground that isn't very absorbent, Las Vegas is in a bowl-like valley, surrounded by mountains. Water runs from on high and accumulates in low lying areas. Pay attention to flash flood watches and warnings, and unless its an emergency stay off of the roads during a flooding situation.