‘GARGANTUAN’ HAIL STRIKES DESERT NATION

In a place that barely sees 10 inches of rain per year, Libya, a largely desert country in North Africa, isn’t exactly a hotspot for severe weather. But atmospheric scientists are investigating an episode of “gargantuan” hail that pummeled Libya’s capital city last month, with individual hailstones nearly 7 inches in diameter.

The city of Tripoli, home to roughly 3 million people, averages less than 11 inches of rainfall per year. Most of Libya is part of the Sahara Desert. The atmosphere didn’t seem to care the evening of Oct. 27 though, when a rotating supercell thunderstorm formed over the southern Mediterranean Sea, battering Tripoli with hail the size of bowling balls as well as strong winds and torrential downpours.

Some researchers believe the enormous chunks of ice may establish a record for Africa, and be among the largest hailstones ever photographed worldwide.

Libya typically sees primarily dry summers, but rainfall begins to increase late in September and through October as the changing seasons precede a slightly wetter winter. Still, it is exceptionally rare for high-end severe thunderstorms to develop, particularly those rivaling some of the fiercest storms of Tornado Alley in the United States.

The recent storm formed ahead of a shortwave trough, or dip in the jet stream accompanying a strip of high-altitude cold air. The presence of chilly air aloft allowed surface air to rise, while jet stream winds provided the necessary dynamics to sculpt thunderstorms into rotating cells.

Weather model simulations from Oct. 27 revealed impressive instability, or the tendency for air to rise. That produced buoyant updraft plumes of rising air that could carry hailstones high in the sky. At those heights, temperatures were well below freezing, allowing layers of ice to continually accrete.

Because the storm was rotating, the powerful updraft suspending hailstones was able to persist for an extended period of time without being choked out by downward-crashing surges of cold air.

Winds from the east at the surface and south at the low levels also helped to provide an uninterrupted supply of warm, humid air to fuel the potent storm. It helped that the storm was what storm chasers refer to as “tail end Charlie,” or the southernmost storm in a line. That way, its warm “inflow” wasn’t interrupted by other storms.