Why the southern US is prone to December tornadoes

La Niña, such as the one now occurring, is considered to be conducive to wind shear, a known factor in tornado conditions – as mentioned below.
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On the night of Dec. 10–11, 2021, an outbreak of powerful tornadoes tore through parts of Arkansas, Mississippi, Missouri, Kentucky, Tennessee and Illinois, killing dozens of people and leaving wreckage over hundreds of miles, says The Conversation (via Phys.org).

Hazard climatologists Alisa Hass and Kelsey Ellis explain the conditions that generated this event—including what may be the first “quad-state tornado” in the U.S.—and why the Southeast is vulnerable to these disasters year-round, especially at night.

What factors came together to cause such a huge outbreak?

On Dec. 10, a powerful storm system approached the central U.S. from the west. While the system brought heavy snow and slick conditions to the colder West and northern Midwest, the South was enjoying near-record breaking warmth, courtesy of warm, moist air flowing north from the Gulf of Mexico.

The storm system ushered in cold, dense air to the region, which interacted with the warm air, creating unstable atmospheric conditions.

When warm and cold air masses collide, less dense warm air rises upward into cooler levels of the atmosphere. As this warm air cools, the moisture that it contains condenses into clouds and can form storms.

When this instability combines with significant wind shear—winds shifting in direction and speed at different heights in the atmosphere—it can create an ideal setup for strong rotating storms to occur.

On a tornado ranking scale, how intense was this event?

At least 38 tornadoes have been reported in six states during this outbreak, causing widespread power outages, damage and fatalities.

The National Weather Service rates tornadoes based on the intensity of damage using 28 damage indicators from the Enhanced Fujita, or EF, scale. Storm assessments and tornado ratings can take several days or longer to complete.

As of Dec. 12, at least four EF-3 and five EF-2 tornadoes have been confirmed. EF-2 and EF-3 tornadoes are considered strong, with wind speeds of 113–157 mph and 158–206 mph respectively.

Continued here.

via Tallbloke’s Talkshop


December 13, 2021 at 11:06AM

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