From the National Weather Service:

A hot and dry Summer thus far – but things are changing…

The hot and dry weather has prompted questions about how long it has been since Las Vegas had rain… how many days in a row Las Vegas has reached 100 degrees… and so forth. The following information is current through yesterday… July 22nd… and is valid for McCarran International Airport /the official observation site for Las Vegas/.

Consecutive days with high temperature 100f or higher

Current streak… 40 days /tied for 5th longest/
longest streak… 66 days in 1944
projection… this streak could come to an end as early as today… depending on whether or not the clouds break up this afternoon.

Consecutive days with low temperature 80f or higher

Current streak… 24 days /new record/
the old record… 21 days in 2006
projection… the best chance for the temperature to fall below 80f will be the first day that significant rain falls in the Las Vegas Valley. This could be as early as today.

Consecutive days with no measurable rain

Current streak… 97 days /23rd longest/
longest streak… 150 days in 1959
projection… this streak could also come to an end today. At 4 am… light rain was moving into the Las Vegas Valley from the east.

Consecutive days with no rain at all

Recent streak… .66 days /from may 6 through July 10/
longest streak… 101 days in 1944

The water level at Lake Mead has also suffered due to the prolonged drought and below normal snowfall in much of the western states. In 2000… the water level was over 1210 feet. It fell more than 80 feet in four years… dropping under 1130 feet in 2004. The water level briefly recovered to nearly 1150 feet in 2005 following the wet winter of 2004-05… but has since fallen to 1111.8 feet as of July 22nd. This is the lowest water level in over 40 years.