A cloud of Sahara dust blankets the city of San Juan, Puerto Rico. An expansive plume of dust from the Sahara is traveling westward across the Atlantic Ocean. (Ricardo Arduengo/AFP/Getty Images)

(This article, first published on Monday afternoon, was updated Tuesday midday.)

A nearly 4,000-mile-long plume of thick dust from the Sahara Desert that arrived in Puerto Rico, Hispaniola and Cuba is forecast to move into the Gulf of Mexico, inland from Texas to the lower Mississippi River Valley Thursday and Friday, before reaching Washington this weekend.

The plume is part of a phenomena that develops every year off the coast of Africa, known as the Saharan Air Layer (SAL), but the current one is unusually intense and is setting records, scientists say.

The SAL typically forms over the Sahara Desert from the late spring into the early fall, and it moves in pulses out into the tropical North Atlantic every few days.

Hurricane forecasters closely track these outbreaks, because such air masses can prevent incipient storms from forming and choke existing storms, starving them of moisture, until they meet an early demise.

It is common for dust to be blown from the Sahara Desert in northern Africa and reach the Lesser Antilles and Gulf Coast, where it can cause captivating sunsets. But it is less common for such events to dramatically lower visibility and lead to air quality problems, which have occurred with this event.

Studies have found microbial life can hitch a ride on the SAL and land on the shores of Florida and waters of the Gulf of Mexico after a turbulent journey above the sea. In fact, research even suggests harmful algal blooms may follow such events due to the dust’s deposition of iron into the sea.

Where the dust has arrived


Close-up view of dust plume over Caribbean Tuesday morning. (NOAA)

Strong winds blowing out from thunderstorms over the Sahel initially kicked up dust to an altitude of around 20,000 feet above the desert before it slipped westward, out to sea on June 14, according to Jason Dunion, a hurricane researcher at the National Oceanic and Atmospheric Administration.

On satellite imagery, the dust resembles a long appendage extending from the western edge of northern Africa, clear across the tropical Atlantic Ocean. The dust is divided into batches connected by thinner wisps.

The leading edge of the approaching Saharan dust cloud had made it to Antigua, part of the Lesser Antilles on Sunday, where its arrival was heralded by a milky translucent veil draped overhead.

The high concentrations of fine particle pollution caused air quality levels to tank into the unhealthy range in many parts of the Lesser Antilles, where they have remained through Tuesday.

The first significant area of dust concentrations reached Puerto Rico and the U.S. Virgin Islands on Monday.

Visibility dropped to three miles at the St. Croix airport in the U.S. Virgin Islands and five miles in San Juan as dust caught up closer to the surface. That is unusual, since dust associated with SAL plumes typically stay higher in the atmosphere, where it does not interfere with surface visibility as much.

“You usually have 10 miles-plus [of visibility] unless we’re under a heavy rain, so this tells you how significant this Saharan dust episode is,” said Gabriel Lojero, a meteorologist at the National Weather Service in San Juan. “The sky is actually a white appearance. It’s milky.”

“We have many models that indicate the highest dust concentration is from around 5,000 feet all the way down to the surface. It’s pretty extreme,” Lojero said. He said weather models were overestimating high temperatures, unable to resolve the thickness of dust, which is acting to reflect incoming solar radiation and cool the surface.

By Tuesday morning, impressive concentrations of dust had spread as far east as Cuba and the Yucatán Peninsula.

Where the dust is going


NASA model projection for movement of dust plumes over the tropical Atlantic toward North America over next 10 days. (WeatherBell)

The dust plume is forecast to push northwestward, spilling over into the western Gulf from eastern Mexico along the Texas coast and perhaps into coastal Louisiana by late Wednesday

Computer models indicate the area of dust may consolidate and affect areas from East Texas into Louisiana, Arkansas and perhaps Mississippi between Thursday and Friday, with a dusty haze potentially even making it into areas from the Lower Mississippi River Valley to the Florida Panhandle.

Around the same time, a second wave of dust will drift westward across the Atlantic, once again eyeing the Leeward Islands and Lesser Antilles, and eventually the U.S. mainland. And behind that will be still more dust.

Into the weekend, there are growing indications a significant swath of dust could become caught up ahead of a cold front and ride northeastward, over the southeastern United States, even reaching the Mid-Atlantic, as far north as the Washington region.

While the dust can sometimes bring spectacular sunsets when in low quantities at higher altitudes, this unique event may manifest differently, particularly in areas that see it arrive earlier.

Clouds of dust may be thick enough to expel the delicate amber washes of sunrise and sunset, a dank gray hue replacing the pastel spectrum as if a wildfire was burning nearby.

There is a chance that, if enough dust makes it into the lower atmosphere, air quality concerns could arise. Since the dust cloud will be several thousand miles away from its origins late this week, much of the larger sand will have already fallen into the ocean, with only smaller, infinitesimal granules left in the sky.

How unusual is this dust episode?

Scientists are trying to quantify how rare this dust event is.

Several experts who track these types of events say it is memorable for its magnitude and aerial coverage, and it is bringing unusual impacts. One approach to putting this event into historical context would be to use satellite-derived, as well as surface-measured, aerosol optical depths, which measure how much light is scattered by the dust, said Paquita Zuidema, a professor of atmospheric sciences at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, in an email.

Joe Prospero, a professor emeritus at the University of Miami, said the dust raised the aerosol optical depth to about 1.5 in Barbados on Saturday, which was a monthly record for that location. Guadeloupe, about 250 miles to the northwest, had a reading showing even more dust in the air, with an aerosol optical depth of about 2 on Monday.

In Puerto Rico on Monday, the aerosol optical depth was measured as high as 2, which is a level not seen there during June in 15 years of record-keeping, said Olga Mayol-Bracero, an atmospheric chemist at the University of Puerto Rico, via email.

According to Zuidema, “An optical depth of 1.5 reduces the direct sunlight reaching the surface to almost 20 percent of what it would be without the dust.” She said scientists will be ready for the dust when it makes it to Miami, with a lidar unit ready to take detailed measurements that could help them rank this event against others.

Thomas Gill, a professor of geological sciences at the University of Texas at El Paso specializing in wind interactions with the landscape, says this is among the most impressive such events observed and is especially noteworthy for its likely impacts on North America.

“This one is, by all of the data and reports I’m seeing, one of the biggest and possibly the biggest and most expansive ever,” Gill said in a phone interview on Monday. “It’s not unusual that the Saharan dust cloud is coming, but this one is a whopper.”