A monsoon storm can cool down a desert afternoon and create shallow puddles just right for splashing.
But if the rainwater is moving down a street or a wash, it can be destructive to property and deadly to anyone swept up in the flow.
"It only takes about a foot of water to float cars," said Ken Drozd, warning coordination meteorologist with the National Weather Service. "A foot-and-a-half or 2 feet of water will float larger vehicles like trucks or SUVs. Then they can get carried downstream into even deeper water."
In the last four years there have been 11 deaths from flash flooding in Southeastern Arizona, Drozd said. Graphic evidence of the pure power of water on a rampage is laid out - like a warning for anyone to see - along riverbeds across the Tucson area.
In the watercourse of the Rillito west of Swan Road one recent day was a mattress, several mangled grocery carts, a piece of luggage and assorted building materials - all of which were carried or damaged by the water, and all of which could knock down someone trying to escape the current.
"A flooding river or wash can be extremely powerful," Drozd said. "Flash flooding is Arizona's deadliest weather hazard during the monsoon. Overall, it's second only in weather-related deaths to extreme heat across the state."
Drivers enter a road or wash thinking they can make it through, but the swift current sweeps them away.
On Saturday, a woman was rescued after she tried to drive her minivan through a flooded intersection at Tanque Verde Road and Conestoga Avenue east of Houghton Road.
Halfway through the intersection her minivan got stuck on a sandbar hidden beneath the rushing water, said Capt. Grant Cesarek, a spokesman for Rural/Metro Fire Department. Rescue personnel walked her through the intersection - where she was greeted by a Pima County sheriff's deputy and cited for reckless driving, said Deputy Thomas Peine.
Although there were no barriers across the flooded intersection, it was obvious to the deputy that the water was fast-flowing and deep, said Peine.
"The number one thing that I try to tell everybody, you can wait it out," Cesarek said. "If you spend 15 or 20 minutes on the side of the road and wait for that water to recede, then you're not going to have to meet the sheriff's deputy and get a ticket."
Soon after Rural/Metro rescued that woman, Tucson Fire Department personnel began walking the banks of the Santa Cruz River at Congress Street looking for a man a Sun Tran driver had seen clinging to the embankment and trying to climb out, said Capt. Jeff Langejans, a TFD spokesman.
After half an hour, rescuers fished a 19-year-old man out of the river.
The National Weather Service offers advice for staying safe in the event of a flash flood:
• Never drive into a flooded roadway or around barricades.
• Be especially cautious at night, when it's difficult to recognize flooding dangers.
• Don't let children play near washes or storm drains during or after a rainfall.
• Don't camp near a stream or in the bottom of a steep-sided canyon in rainy seasons.
• When hiking or camping, be aware of escape routes to higher ground, even if it's not raining. Distant thunderstorms can result in flash floods miles away as runoff flows downstream.
On StarNet: Get the latest local weather news at azstarnet.com/news/weather
Star reporter Kimberly Matas contributed to this report. Contact reporter Doug Kreutz at email@example.com or at 573-4192.