Monsoon Season in Phoenix

© Judy Hedding

Arizona, as well as other regions of the world, including India and Thailand, experience a monsoonal season where high temperatures, high winds, and high rainfall can result in potentially disastrous conditions. The term "monsoon" comes from the Arabic word "mausim" meaning "season" or "wind shift." And this is exactly what happens in Phoenix when the monsoonal winds blow in, bringing with them heavy rain, hail, thunderstorms, flash floods, and even debilitating dust storms.

Phoenix's Monsoon Season

Up until 2008, the arrival of Arizona's monsoon varied from year to year in both start date and duration. Technically, the Arizona monsoon begins after the third consecutive day of dew points above 55 F. On average, this occurs around July 7 and continues for the next two months. However, the National Weather Service decided to take the guesswork out of the start and end dates and officially deemed June 15 as the first day of the monsoon, lasting until September 30. During this time period, authorities urge residents and travelers to the state to focus on safety by being aware of unexpected, and sometimes extreme, changes in weather conditions.

Monsoonal Weather Hazards

Monsoonal storms range from minor dust storms to violent thunderstorms. And while very rare, they can even spawn tornadoes. Typically, when a monsoon hits the Phoenix region, it starts with heavy winds, sometimes resulting in a visible wall of dust hundreds of feet high that appears to be moving across the Valley. These dust storms can be accompanied by frequent thunder and lightning, often leading to heavy downpours. Monsoonal storms drop an average of 2 1/2 inches of rain, which amounts to one-third of Phoenix's annual rainfall.

Damage Potential

Serious damage to homes, cars, and people can occur from high winds or from blowing debris. It is not unusual to see downed trees, damaged power lines, and home and roof destruction after a storm. In fact, manufactured and structurally inferior homes are more susceptible to wind damage, and especially to tornadoes that can wipe out whole communities. Relatively short power outages are also common, resulting from downed lines and trees.

Road Travel During Monsoon Season

When a high volume of rain descends upon the Valley of the Sun, the water table in the ground swells and the streets flood. And due to low annual precipitation rates, most roads in the Phoenix area are not built to efficiently drain water, as area officials opt to forgo the extra costs involved in constructing elaborate drainage systems. Quite often, the rain will pool on the streets for a few hours following monsoon storms and this can cause dangerous driving conditions, especially on the highway. Expect topographical washes (small gullies where heavy rains drained off the land long ago) to have the worst flooding.

It may seem strange to have "flood warning" signs in the middle of the desert, but they do serve a practical purpose during the monsoon. Head this warning carefully because, even if the rushing water looks like it's only an inch or two deep, it may actually be deep enough to swallow your car. Firefighters and other rescue workers regularly rescue motorists stuck in washes before their vehicles are completely covered.

Additionally, trapped drivers can also be charged (quite heavily) should they become stuck in a flooded wash. Arizona's "Stupid Motorist Law" states that municipalities and rescue agencies can charge people for the cost of being rescued if they fail to observe posted warnings.

Monsoon Safety: Dos and Don'ts

Watching an Arizona monsoon from the safety of your home can be an awe-inspiring experience. But if you're caught outdoors, head these safety tips:

  1. Take any sign that says "Do Not Cross When Flooded," seriously. Still, if you end up getting caught in a wash, climb out the window and onto the roof of your vehicle and wait for help. Use your cell phone, if available, to call 911.
  2. Slow down when driving during a rainstorm. Remember, the beginning of the storms is the most dangerous time to drive. Oil and other automotive fluids wash onto the road at the start of storms causing unusually slick conditions.
  3. Reduce your speed when driving if your visibility becomes impeded by heavy rain or blowing dust. Keep driving straight and don't change lanes unless it's absolutely necessary. Area drivers will often use their emergency blinkers (hazard lights) during storms to make their vehicle easier to see. If you prefer to leave the roadway and it is safe to do so, pull off to the side of the road as far to the right as possible, turn off your lights, and keep your foot off the brake pedal. Otherwise, drivers may come up quickly behind you and assume you're still in motion.
  4. Avoid being struck by lightning by staying away from open fields, high land, trees, poles, and standing bodies of water including swimming pools. It's also advisable not to hold a metal object, including a golf club or a lawn chair.

If you're inside your home during a Phoenix monsoon, stay safe while you enjoy the show:

  1. Turn off all unnecessary electrical appliances during a storm to decrease the draw on power. This is prime time for power outages in the area.
  2. Keep batteries, a working battery-powered radio or television, flashlights, and candles handy, should the power go out. And remember to keep lit candles out of direct drafts or away from flammable items.
  3. Stay off the phone. Even cordless and cell phones can shock you in cases of nearby lightning strikes. Use cellular phones for emergencies only.
  4. Steer clear of plumbing fixtures, including showers, bathtubs, and sinks. Lightning can travel through metal pipes.
  5. Keep away from windows during high winds. Heavy debris or a tree could break a window and injure you.
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