Monsoon Season Hits Arid Rim Country


After months of relentlessly dry heat, the parched Rim country may finally be seeing the start of the monsoon season.

With humidity and dew points on the rise, and thunderheads building in the afternoon heat, the atmosphere is signaling a change.


Payson resident Maryanne Montellano pushes her daughters Dominique and Zoey on the rope swing at Green Valley Park Friday afternoon. Montellano, who is eight-and-a-half months pregnant with her third child, wanted to enjoy the cooler monsoon weather that hit Payson over the weekend.

Mike Staudenmaier, the science and operations officer at the National Weather Service in Flagstaff, spoke to the Roundup about monsoon season.

"What happens in monsoon season is that high pressure develops over the Four Corners region and circulation around a high is clockwise," Staudenmaier said. "So if we are on the west side of that high, it means we get southerly or southeasterly wind flow and that's different than the westerly flow that we normally have."

"When the wind shift comes from the south, it usually brings up moisture from Mexico and that's usually when our thunderstorms start to pop after a dry June," Staudenmaier said.

The official start of monsoon season is typically declared by Phoenix, he said.

"Their criteria is that their dew point at the surface of the airport is 55 degrees or greater for three days in a row," Staudenmaier said.

It started sometime last week, officials said.

Staudenmaier explained the difference between humidity and dew point.

"Humidity is a ratio of how much water the atmosphere currently has to how much it could hold at that temperature," Staudenmaier said. "Dew point is the actual measure of how much moisture is in the atmosphere at that point -- the higher the dew point, the more moisture."

What is a thunderhead?

The scientific name is cumulonimbus, defined as an exceptionally dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lightning, thunder and sometimes hail.

A super cell thunderstorm, as Staudenmaier explained, is a type of storm that can cause severe weather, even tornadoes.

"We get them every once in awhile," Staudenmaier said. "With Doppler radar, we can tell a super cell thunderstorm by seeing the velocity of wind inside a storm and it tells us there is rotation. That's what a super cell is -- an entire storm is actually rotating and that keeps the storm more organized and able to produce more severe weather because it isn't ripped apart by terrain and other factors that limit out severe weather potential."

How is hail created?

Hailstones are pieces of transparent or opaque ice ranging in size from peas to golf balls or larger. The largest hailstone recorded fell on a town in Kansas and had a diameter of 5.5 inches and weighed 1.67 pounds.

Hail is produced in cumulonimbus clouds as large, frozen rain drops, or any particles act as embryos and grow by accretion.

The violent upsurging air currents within the cloud, carry the objects high above the freezing level, where they drop, only to be lifted again by other up-drafts. During each up and down cycle, another layer of ice is formed around the object. A hailstone can get large if it is trapped in continuous up and down cycles within the cloud's air currents.

"I've seen hail the size of a tea cup saucer," Staudenmaier said. "We do get a lot of hail in Rim country because we are at a higher elevation and it does not have a chance to melt before hitting the ground."

What causes lightning?

Lightning is a discharge of electricity, a giant spark, which occurs in mature thunderstorms. Lightning may take place within a cloud, from one cloud to another, from a cloud to surrounding air, or from a cloud to the ground.

The extreme heat causes the air to expand explosively, initiating a shock wave that becomes a booming sound wave called thunder.

When a lightning stroke is very close, thunder sounds like a clap or a crack followed immediately by a loud bang, When it's farther away, it is a rumbling sound.

Lightning safety

An average of 100 people are killed by lightening every year in the United States. Many are struck while in open places, riding on farm equipment, golfing, or boating.

According to Staudenmaier, this is one of the top lightning areas in the country.

"I believe we are in the top third of the nation for lightning strikes," Staudenmaier said. "It's because this is the warm southwest and it generates thunderstorms day after day."

If caught outside in a lightning storm, avoid high places, and don't take cover under an isolated tree. It is best to crouch down as low as possible and minimize contact with the ground.

If your hair begins to stand on end, or your skin tingles and you hear clicking sounds, lightening is about to strike. If you are standing upright, you are acting as a lightening rod.


Tornadoes are rare in Arizona, especially in Rim country, however, strong winds associated with thunderstorms can cause damage. These are known as micro-bursts. A micro-burst is a strong, localized down-draft, or down-burst that occurs beneath a thunderstorm. A downburst more than four kilometers wide is called a macro-burst. A strong microburst can do heavy damage similar to a tornado, but the path of debris will appear different than that of a tornado.

"We get micro-bursts more often early in monsoon season when we get high-based thunderstorms," Staudenmaier said. "Rain falls from a cloud and evaporates before hitting the ground and it cools the air. You get this huge blob of cool air that hits the ground and rushes out in all directions."

Severe Thunderstorm Watch

A severe thunderstorm watch means that atmospheric conditions are favorable for the formation of severe storms and those in the area should be keep an eye to the sky.

Severe Thunderstorm Warning

Unlike a watch, a warning means that either Doppler radar or a weather spotter has identified a storm. Those in the path of the storm should seek shelter immediately. If the storm contains frequent lightning strikes, stay off the phone (except a cell phone), turn off electrical appliances and stay out of the shower.

"Monsoon season usually lasts 90 days," Staudenmaier said. "We issue warnings on about 65 of those days."

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