|Las Vegas Valley Heavy Rain Event: Amount of Rain Needed for Significant Flash Floods
Nevada is located along the lee side of the Sierra Nevada Mountain Range.
Las Vegas is situated in a north-south valley in southern Nevada 43 miles north of the California
state line and 25 miles west of the Hoover Dam and the Arizona State line.
The north-south mountain ranges are between 50 and 100 miles in length.
Las Vegas is also located 2200 feet above sea level.
The Spring Mountains 25 miles west of the valley, average about 6000 feet above sea level.. The range
includes two peaks that are significantly higher: Mount Charleston at 11,910 feet and Mount Potosi at 8,800 feet.
At the north end of the valley are the Sheep Range with Hayford Peak standing at 9,912 feet.
East and South of Las Vegas there are isolated mountain ranges averaging 4,500 to 6000 feet.
Outside and east of the Las Vegas valley the topography changes. It begins to slope rapidly down to the
Colorado River Basin.
Communities located in the Las Vegas Valley include North Las Vegas, Summerlin, Henderson and Boulder City
According to the U.S Soil Conservation Service between 1905 and 1975 there have been 184 different
flooding events. Between 1960 and 1973. twenty-one separate flash floods took the lives of 31 people.
In 1985, the Nevada State Legislature approved the creation of a flood control district. It's agenda
to develop, create, co-ordinate a flood plan to alleviate flooding risks in and around the area.
Since then there has been $836 million dollars spent on flood control improvements including 62 completed
detention basins and 343 miles of channels and storm drains.
The current population of the Las Vegas Valley is 1.8 million people. With growth running at nearly 7
thousand new residents a month the Flood Control District continues to spend millions more each year, upgrading
and building flood control measures in newly created subdivisions across the valley with special emphasis
on northwestern areas.
Deserts surround Las Vegas Mojave and Sonoran Deserts. Along with the combination of the mountain ranges
and the area has a very dry climate. There are more than 300 days a year of sunshine and nearly 100 days
a year with temperatures at or above 100 degrees Fahrenheit. In winter the average daytime winter
temperature is in the 50's with evening temperatures averaging in the low 30's.
Strong surface heating is common in southern Nevada, but because this is also a desert climate there is
rapid cooling of temperatures at night even during the summer months. Prevailing winds are from the
south. While often laden with moisture as they ascend the western slopes of the Sierra range, the air
cools causing condensation and eventually rain on the windward side of the mountain ranges. As the
winds descend on the leeward side the air compresses and heats up.
While rain can and does fall anytime the year the primary rainy seasons are from January to March caused
by Pacific Storms that move either across the mountain ranges or down south from the Pacific
Northwest. The annual rainfall according to NWS climatologic data is just
less than 4.5 inches per year.
The other rainy season occurs from mid-July into mid October known as the Monsoon season. At this time
of the year much of the rainfall occurs from systems moving up from the Sea of Cortez and from a wrap
around effect of storms in the Gulf of Mexico.
The monsoon season also produces several weeks of higher humidity for the valley.
Moist upper airflow helps support development of irregular desert thunderstorms and strong shear
but often very little or no rainfalls due to the mountain shadow effect.
Storms moving north from the Sea of Cortez and into southern Nevada from eastern lower slopes of the
ranges in California have the best chance of producing rain for Las Vegas. The precipitation from these
storms often stays south but if the upper air winds are strong enough they tend to blow the systems
into the Las Vegas Valley. These are storms are more frequent during the winter months and thus
because of the colder temperatures most of the rain falls before arriving in the Valley.
The wrap around from the Gulf of Mexico is often a catalyst for stormy heavy rainfall events during the summer
and early fall. As the flow from the gulf moves east climbing the terrain it hits the drier desert
air the two masses blend often producing dry thunderstorms. If the humidity is high enough and the
desert temperature starts to cool it results in downpours that cause
flooding and occasionally flash floods.
One of the most severe flash floods to hit the area occurred in July 2003. An intense yet brief thunderstorm
dumped less than three inches of rain in the northwest area of the valley. According to National Weather
Service rain gauges two waves of rain fall exceeded the 100-year event criteria and overwhelmed flood
drainage facilities and washes.. Some had water flows as high as 3 to 5 feet and were near
capacity. Several basins measured considerably more runoff but were well below maximum depth. Lone Mountain
Detention Basin water depth was 12 feet. The maximum depth of water it could handle is 32 feet. The Gowan
South Detention Basin recorded 18 feet of water with its maximum depth capacity of 24 feet.
Backed-up storm drains and overwhelmed washes turned hundreds of streets into flowing rivers several inches
deep. While there were no serious injuries there were numerous helicopter and swift water rescues as
commuters were trapped by floodwaters. Total public property damage exceeded 1.5 million dollars. Private
property damage estimates have still not been released by the insurance industry.
While this was considered a 100-year event, the third in twenty-five years, the uniqueness of the valley
topography can produce flooding with even a minor rainfall event.
Another occurred in July 1999 when 3 inches of rain flooded hundreds of homes and businesses and left
two people dead. Some roadways remained closed for up to a week after that event. It was considered
so severe at the time that then President Clinton declared Clark County a disaster area providing
relief for those impacted by the flash floods.
On August 10, 1983, four inches of rain causes flash flooding killing a child who was actually carried
off by soil that had collapsed under him.
Less rain events also cause flooding and flash flood events. On August 23, 1995 1.5 inches of rain in ten
minutes and a 75-mile per hour wind killed one man who was found pinned to a tree at a watershed near a golf course.
While some of these events are among the most severe since the 1960's rainfall as little as a quarter of
an inch of rain, can cause serious street flooding and ponding.
At a half inch of rain very serious and dangerous floods often filling basins with rushing water and it
drains into Lake Mead 25 miles east of Las Vegas.
Officials at The Clark County Flood Control District admit even with their best engineered plans they
are at the mercy of Mother Nature and cannot prevent flooding.
What they do hope to accomplish is to reduce the damages and effects of lesser storms in the coming
years, with expansion, upgrades and new watersheds and basins.