What it means
A flash flood in Scottsdale does not look like a river rising. It looks like nothing, and then it looks like a brown wall. Desert soil is crusted and rocky, and when a monsoon cell parks over the McDowells it sheds the rain almost entirely. That runoff gathers in the washes, the dry sandy channels that braid across north Scottsdale, and arrives downstream all at once. The sky over your head can be clear. The first warning is often a roar.
Locals use the phrase with a particular tone, the one reserved for things that kill people who think they know better. Every summer someone drives into a flooded dip on a road they cross daily, and every summer a helicopter or a swiftwater crew pulls them off the roof of the car. The slogan the National Weather Service uses everywhere fits Arizona best: Turn Around, Don’t Drown.
The city’s own answer to flooding is one of its proudest pieces of infrastructure, the Indian Bend Wash Greenbelt, a floodway disguised as parks. The storm driving rules are in the monsoon field manual.
Where you’ll meet it
Flash Flood Warnings on your phone during the monsoon, barricades and depth gauges at road dips in the north, closed Greenbelt underpasses the morning after a storm, and trail signs telling you not to linger in wash bottoms.
Where the word came from
“Flash flood” is ordinary English and Weather Service vocabulary, and Arizona did not coin it. What Scottsdale contributed is two of the floods that taught a growing city what the phrase means.
The first came over Labor Day weekend 1970. The remnants of Tropical Storm Norma stalled over central Arizona and killed 23 people statewide, the deadliest storm in the state’s recorded history; 14 died in flash flooding on Tonto Creek near Kohl’s Ranch below the Mogollon Rim. In Scottsdale, Indian Bend Wash ran as a mile-wide river through subdivisions built across it, and the city was split in two for days. The site’s day page for September 5, 1970 tells that story.
The second came on June 22, 1972, when a thunderstorm dropped three to five inches of rain over the north half of the Phoenix metro in a month whose normal rainfall is about an eighth of an inch. Floodwater reached hundreds of homes in Phoenix and Scottsdale, and Indian Bend Wash tore through south Scottsdale again. Ten months later voters approved the bond that built the Greenbelt, a grass-and-lakes floodway instead of the concrete channel the Army Corps of Engineers had proposed. The June 22, 1972 page has the numbers.
The law followed the culture. Arizona’s ARS 28-910, known to everyone as the stupid motorist law, makes a driver who goes around barricades onto a flooded road liable for the rescue, up to $2,000.
What it is not
A flash flood is not the slow, river-rising kind of flood people from the Midwest picture, and it does not need rain where you are standing. It is also not the same storm hazard as a haboob, though one monsoon cell often delivers both: dust first, then water.
The fine print
The physics are unforgiving. The National Weather Service’s numbers: six inches of fast-moving water can knock over an adult, about 12 inches can carry away most cars, and two feet can carry SUVs and trucks. More than half of flood drownings happen when someone drives into hazardous water. Road dips are the trap, because the pavement under the water may have washed out and there is no way to tell from the driver’s seat.
In Arizona the toll ranks second among weather hazards. A study of deaths from 1955 to 2011 by Weather Service meteorologists and a University of Arizona researcher put extreme temperatures first, flash floods second and dust storms third.
Scottsdale’s geography concentrates the risk in two places. In the south, Indian Bend Wash is now contained by the Greenbelt, which still floods its golf courses and soccer fields a few days a year so that the neighborhoods beside it stay dry; the underpasses that make it a great bike route are also its low points and close after storms. In the north, much of the desert is still laced with natural washes, and many roads cross them at grade rather than on bridges. Where a crossing has a depth marker, it is there for a reason.
The rule for hikers is the same as for drivers, translated. Do not shelter in a wash bottom or a slot canyon during monsoon afternoons, and if water starts moving, climb to the side.