Dust usually helps keep tropics quiet in July, but not always

Published 7:00 am Saturday, June 27, 2020

By Skip Rigney

It took a little over a week for dust from Africa to drift 6000 miles, but it arrived in force above south Mississippi on Thursday. The result was a milky, hazy sky.
Yesterday the circulation around high pressure centered above Florida began pulling the heaviest concentrations of dust out of our area and spreading it eastward. Sunshine this weekend in the Sunshine State will be on the hazy side.
Another plume of Saharan dust, big enough to cover the state of Texas, will envelop many of the islands in the Caribbean Sea this weekend. Dispersion forecast models from NASA predict that the edge of this second hazy veil will skirt our area on Wednesday. However, the concentrations of this next plume are not nearly as thick as the one that passed through on Thursday.
Scientists call these Saharan Air Layers, or SALs for short. They usually extend upward just a few miles into the atmosphere. According to NOAA’s Atlantic Oceanographic and Atmospheric Laboratory in Miami, SAL outbreaks are most frequent from mid-June into early August, when they can occur every three to five days. “The warmth, dryness, and strong winds associated with the SALs have been shown to suppress tropical cyclone formation and intensification,” according to an AOML website devoted to SALs (www.aoml.noaa.gov/saharan-air-layer/).
SALs are one of several factors that usually produce a lull in tropical cyclones during July before the season ramps up in mid-August. But, as we are reminded by the events of the first week of July 1916, there can be violent exceptions to the rule.
As July began in 1916, people in south Mississippi and south Alabama were paying attention to news other than the weather. “The Great War” in Europe was in its second year. Even though the United States was still neutral, many Americans worried that the U.S. would soon be drawn into the conflict. So, it’s not surprising that on July 1st the front page of The Hattiesburg News was keeping locals abreast of the events in northern France with the headline, “British hurl 2 million men against Germans to smash their lines.” No one realized that one of the bloodiest battles in human history, the Battle of the Somme, had just begun.
At the same time, the Weather Bureau was tracking ominous developments of a different kind in the northwest Caribbean Sea. Over the next few days, a tropical cyclone formed and moved northward. It slammed into Pascagoula and Mobile as a category three hurricane on July 5, 1916. Two days later, “Storm Damage Grows” spanned the top of the front page of the Hattiesburg News, along with “17 Dead in Alabama.” By the time the storm was spent, it had spread devastating floods through Alabama, Georgia, Tennessee and the Carolinas.
Nothing like that is forecast this week. Typical warm, muggy conditions will prevail. It’s a good bet that every day, between 30 and 70 percent of south Mississippi and southeast Louisiana will have afternoon or evening showers lasting between a few minutes and an hour or two.
Exactly which 30 to 70 percent can’t be forecast more than a few hours in advance.

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