Las Vegas rain by month, NOAA normals and the monsoon

The twelve month table

Why February outranks August on paper

NOAA's monthly normals put February at 0.82 inches and August at 0.22. Winter rain tends to come as longer, lighter storms. Summer rain comes from monsoon thunderstorms. The National Weather Service says those storms typically form in the mountains around the valley, then move in, and the flash floods they cause often sweep down normally dry washes or pour into low lying areas.

How often it rains at all

Caliche and runoff

The NWS says caliche is almost impervious, so when rain falls in the valley almost all of it runs off. That is why a storm that would barely wet the ground elsewhere can fill a street here. It is also why the valley has a regional system of channels and detention basins. FEMA's case study says the Nevada Legislature created the Clark County Regional Flood Control District in 1986, funded by a quarter percent sales tax.

July 8, 1999

The NWS calls it severe, unprecedented flash flooding across the Las Vegas Valley. Most of the rain fell in 90 minutes, between 10:30 am and noon. It caused widespread street flooding and record flows in normally dry washes. Two people died, a motorist and a transient caught in Flamingo Wash. The event fell between the 50 and 100 year flood criteria along parts of Las Vegas Wash, and the city was declared a disaster area on July 19.

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