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Updrafts. Downdrafts. Mammatus. Funnels. Land spouts. Lightning. At some point during a two-hour online severe weather spotter training class April 14, National Weather Service …
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At some point during a two-hour online severe weather spotter training class April 14, National Weather Service meteorologist Scott Entrekin explained these weather terms and types of clouds to 235 people interested in becoming severe weather spotters.
It’s one in a series of seminars the weather service offers in advance of severe weather season.
The explanations included the various types of thunderstorms common (single-cell, multi-cell and supercell, the type more likely to cause tornadoes) in this area. All require air to flow into the clouds.
The amount of wind determines the type of storm.
“In a single cell storm, the winds are weak, and you can’t really tell the updraft and the downdraft,” Entrekin said. “After the heating of the day, there’s usually good updrafting. You can see well-defined edges to the clouds” like the shape of a cauliflower.
The multi-cell storms have a series of up and downdrafts. Entrekin said storms can last about 30 minutes, but the complex can last for several hours. Supercell storms contain rotation in the clouds. Another key ingredient for these types of storms is wind shear, a rapid change in either velocity or direction.
“If you stand under a hose with no wind, you’re probably going to get wet,” Entrekin said. “But if you stand under a hose with strong winds, those winds take the water away from you. Wind can cause a storm to tilt.”
Entrekin highlighted two other storm features. One is a wall cloud, which tends to form at the back edge of a storm and in an area free of rain.
“When you see a wall cloud, there is a lot of in-flow,” Entrekin said. “A shelf cloud is linear and slopes away from the rain. A wall cloud moves with the rain.”
Outflow winds, winds that come from the bottom of the clouds, can lead to storms in other areas. In some cases, they can cause strong winds up to 2½ miles away. In some cases, those winds can cover larger distances.
The front edge of a thunderstorm is a shelf cloud, which tends to create a lot of wind and rain.
Entrekin also discussed last year’s derecho, the wind event that started in Utah, went through the Rockies and on up into the Dakotas. The weather service received more than 200 reports of winds that were more than 58 mph.
One thing Entrekin stressed to the spotters was safety.
“When thunder roars, go indoors,” he said. “Victims of lightning strikes are not charged, so it’s OK to touch them. Wait 30 minutes after you see the last lightning bolt before you go back outside.”
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