Saharan air drifts in, slashing rain chances

Abnormally dry week expected; less rain in August helping Lake O.

A modest drift of Saharan air will bring abnormally dry conditions to South Florida this week with rain chances in some areas plummeting below 10 percent — a summertime rarity.

Larry Kelly, a meteorologist with the National Weather Service in Miami, said normal rain chances for this time in the wet

season are generally at least 40 percent, but this week water vapor levels in the air will be

near record dry.

“It will definitely be less moist, our dew points will come down to the low to mid-70s, but the main thing with the drier air is the rain chances drop quite a bit,” he said.

August has already averaged below normal rainfall for most areas from Lake Okeechobee to the Keys, including Palm Beach County where coastal areas received an average of 2.31 inches through Sunday, according to the South Florida Water Management District.

That’s about 80 percent of what’s normal nearing the middle of August. In the 16-county area managed by the district, average rainfall was 1.92 inches through Sunday, nearly an inch below normal.

The lower rainfall amounts are good for Lake Okeechobee, which stood at 14.52 feet above sea level on Monday.

After record May rains, the Army Corps of Engineers began discharging water from the lake into the Caloosahatchee River and St. Lucie Estuary. Both waterways developed harmful blue-green algae blooms that have tested positive for toxins in 84 percent of samples taken, according to district officials. While the majority of toxin levels were below what the World Health Organization considers harmful to humans, about 25 percent were at harmful levels.

As of Friday, satellite images show about 38 per- cent of Lake Okeechobee was infected by blue-green algae with the majority concentrated in the northeast section of the lake. While algae from the lake does leak into the estuaries when released, the fresh lake water also further reduces salinity levels in the brackish waterways, which encourages algae growth.

The most recent Lake O algae image from the National Oceanic and Atmospheric Administration is a reduction from a bloom that covered 90 percent of the lake in July, but has fluctuated to as low as 10 percent.

Richard Stumpf, a scientist with the National Centers for Coastal Ocean Science, said his forecast for future algae levels is “vague.”

“We don’t expect it to get stronger,” he said.

But it could continue at the current level into September.

“We don’t have sufficient data on conditions surrounding past blooms to predict which is likely,” Stumpf said.

Without May’s record rainfall, Lake Okeechobee would be roughly a foot below its current level, said John Campbell, a spokesman for the Army Corps of Engineers, which controls the water releases.

“It’s not the 14.5 feet that’s concerning, it’s that we are coming into the peak of hurricane season and where do we end up after a storm if we’re starting from there?” Campbell said. “The system is being operated for flood risk management in which none of the options we have are particularly attractive.”

The Corps prefers to keep lake levels between 12.5 and 15.5 feet above sea level to maintain the integrity of the Herbert Hoover Dike.

The drier air should remain over South Florida into Friday. After that, there are no signs of additional Saharan dust outbreaks in the near future, said Jason Dunion, a meteorologist with the University of Miami.

But Saharan air had a good run this summer with June and July “much dustier than usual,” Dunion said.

A Saharan air layer is made up of sand and mineral particles that are swept up from 3.5 million square miles of desert and carried by air currents 4,000 miles west across the Atlantic. The largest plumes can be the size of the continental U.S.

It’s not clear why this summer might have experienced more Saharan dust, Dunion said. But some things he’s considering are the strength and position of the Bermuda High over the Atlantic and recent drought trends in Africa.

And, just because some Saharan air may trek overhead this week, South Florida’s summer thunderstorms still have a chance of forming with daytime temperatures expected to reach into the low 90s.

“We’ve had some very intense dust outbreaks and we still get spotty rain,” said Joseph Prospero, professor emeritus at the University of Miami’s Rosenstiel School of Marine and Atmospheric Sciences. “South Florida is a good place to generate deep, intensive cumulus clouds that can penetrate the dust.”