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What is the Arizona Monsoon? We see the same thing happen every year. From spring to early summer there are blue skies and great weather and then things start to get dry, really dry. Suddenly, at the end of June or the beginning of July, everything changes. We are starting to see clouds building up in northern and central Arizona skies and monsoon storms are continuing dropping much needed moisture.
So why do we always see this dry period before the rains come every year? And what is the monsoon?
Dr. Curtis James, Professor of Meteorology at Embry-Riddle Aeronautical University, shares with us what the monsoon is, why we see the same patterns, and some weather safety tips.
What is the monsoon?
The monsoon, more technically called the North American monsoon, is a seasonal change in winds from a prevailing westerly wind to a prevailing southerly or southeasterly wind during the summer months, drawing moisture primarily from the Gulf of California. This change which usually takes place from late June to September has a dramatic impact on our climate here.
Why do we see similar weather patterns every year?
The weather we see each year comes from the wind directions. Typically in June we see the wind coming from the southwest through Arizona and that wind tends to be drier, in part because the eastern Pacific is much colder ocean water than the Gulf of Mexico or Gulf of California. This cooler air tends to have much less humidity than the air in the gulfs.
Additionally, this air must pass through the California and Baja Coast Ranges to get here, which removes some of the existing moisture in the air, making the air drier and warmer. This is often the reason why we see dry, hot weather in June and it will often be the peak of fire season in Arizona.
In July, things start to move. Often the high pressure builds up in the four corner region, creating a clockwise rotation. This rotation brings moisture from the south or southeast and into Arizona. The high pressure will also have an effect on how the winds are structured with height and this is important for the organization of thunderstorms.
Arizona thunderstorms are different
The thunderstorms that form in Arizona are usually smaller in magnitude than what you’ll see in the Great Plains. They are usually only a few kilometers in diameter and tend to ignite on higher ground first due to thermal circulations moving upslope and converging at higher peaks or around higher ground. For Arizona, thunderstorms tend to start over the San Francisco Peaks and the Mogollon Rim. Around Prescott, storms are building over the Black Hills to the east and the Bradshaws to the south.
Ultimately, wherever there is an upward movement in the winds, or an updraft, that is where a thunderstorm is likely to form.
Typical Weather Events
Part of monsoon storms are common weather events like lightning, hail, rain, flash floods, microbursts, and sometimes even tornadoes. It’s important to stay aware and be safe during the Arizona monsoon.
Flash floods are a common hazard in Arizona. A flash flood is an intense flow of water through a particular watershed (stream, river bed, etc.). Be aware that rain does not have to be falling overhead for an area to be under watch or flash flood warning. Precipitation could occur upstream and water can rush through these areas. The intensity and frequency of flash floods will depend on factors such as how wet the soil was when it started raining, if there have been previous forest fires or burnt soils, which tend to run off much faster.
Stay out of low areas and do not drive on roads covered in runoff. These waters are powerful, a single cubic foot of water weighs 62 pounds, and a rushing stream can certainly lift a vehicle. Also, it’s hard to tell how deep the water is in the roadway and if any of it has washed away and isn’t visible. Follow the common advice we hear every year: Turn around, don’t drown.
Lightning is another common hazard in Arizona. Lightning is a discharge of static electric fields that build up between the thunderstorm and the ground or within the thunderstorm itself. These discharges, lightning, tend to occur where there are more conductive objects, especially larger objects.
If you are outdoors, stay away from tall objects and power lines and leave swimming areas. As soon as you can get indoors, the safest place you can be during lightning is inside an enclosed building or vehicle.
When you are indoors, avoid plumbing, landline telephones and plugged-in electrical appliances. Avoid doing the dishes or taking a shower during a thunderstorm.
Even if a thunderstorm is not directly overhead, lightning strikes can occur. If you can hear thunder, lightning can strike.
*Technical tip: Unplug your electronic devices before a thunderstorm to protect them from lightning strikes. A surge protector is no match for the jolt of a direct lightning strike.*
A microburst is a rapidly falling surge of rain-cooled air from a thunderstorm. Microburst winds are straight ahead and can exceed 60 mph. When air from a microburst hits the ground, it spreads outward and becomes very turbulent with the potential to cause damage and also cause intense dust storms in deserts.
Microbursts are very common at the start of the monsoon when it is dry and hot. The thunderstorms we see with huge highs provide a long path for rain to fall through the hot, dry storm air and evaporate. Cool air is much heavier than warm air and as evaporation occurs this air cools rapidly and with only warm air below the storm this cooler air falls producing this microburst.
Hail is also an event that we can see with monsoons. Hail is formed when an updraft of thunderstorm lifts a droplet of water above freezing level in the atmosphere. “The hailstones then grow by colliding with drops of liquid water which freeze on the surface of the hailstones” (nssl.noaa.gov). The process will continue until the storm’s updraft can no longer support the weight of hailstone caused by the stone cutting or the updraft within the storm weakens.
If a thunderstorm has a particularly dark base or is colored, bluish, or purple in appearance, it may contain larger hailstones. The light diffuses a little differently with the hailstones. Any dark cloud base will indicate that there is a strong updraft which also allows larger hailstones to form.
Although tornadoes are a bit less common in Arizona, they do happen. Tornadoes occur when the thunderstorm is able to begin to turn. Sometimes the airflow around the terrain can cause rotation and if a thunderstorm forms it amplifies that rotation and creates a rotating updraft. This rotation is necessary for a funnel cloud, or tornado, to occur.
Stay informed and stay safe
Regardless of the weather scenario, the safest place during a thunderstorm is inside a fully enclosed building.
Flash: stay away from tall objects when outdoors and get indoors as soon as possible. Indoors, avoid objects connected to the outside such as plumbing, electrical appliances and landline telephones.
Flood: Avoid flooded roads and avoid spending time in dry riverbeds during the monsoon. Have a source of information handy like a weather radio or your cell phone to stay informed.
Tornadoes: Don’t stop to open the windows, just move to an indoor room and stay put. Have a source of information handy like a weather radio or your cell phone to stay informed.
The best safety measure is to stay tuned to National Weather Service forecasts, watches, and warnings (https://www.weather.gov/fgz/).
For weekly informal updates specific to the Quad City area, viewers can email me at [email protected] to add to our MetMail mailing list.