A thick plume of Saharan dust is headed toward Florida

Washington Post

2 days ago

A plume of thick dust suspended in the atmosphere is moving westward across the Caribbean Sea and is expected to reach Florida on Wednesday. Areas in its path can expect haze, locally poor air quality and the potential for vibrant morning and evening skies.
The plume has meandered thousands of miles across the Atlantic Ocean from its source on the continent of Africa, already reducing visibility in Puerto Rico and nearby locations.
Technically called the Saharan Air Layer (SAL), this dust is a relatively common feature overall. Carried westward by the trade winds that kick up hurricane season, it is also somewhat unusual to see SAL survive the journey across the ocean in this way.
The dust will be thickest through Thursday, but the SAL eventually runs into an area of lighter winds aloft that will allow it to sit and dissipate around Florida through the rest of the week. There may be multiple days of fiery sunrises and sunsets as a result.
As the name implies, the SAL is born in the desert of Africa. The dust is formed by light, dry sands and other minerals. They require a jet stream positioned correctly to send them westward. At other times, SAL might shoot northward toward Europe.
This batch can be seen stirring up over Africa, particularly in and near Chad, during the latter part of May.
The same winds that drive tropical disturbances westward off the African continent pick up the dust and move it along as well. Most years have some, but excessive levels of dust can augment storm development since it stifles cloud development and can lower sea surface temperatures if it lingers.
Similar to dense wildfire smoke, fine particles contained in Saharan dust can be harmful if people are exposed persistently or in large quantity, particularly for anyone with respiratory issues.
In the Caribbean and United States, by the time Saharan dust makes the 4,000-plus mile trek, it is typically mostly aloft with minimal impact on surface air quality. This minimizes risks to population but creates the appearance of thin clouds in midday skies.
The current batch of SAL was passing Puerto Rico and surrounding locations Monday and will continue to do so Tuesday. San Juan weather observations showed periodic reports of haze and visibility as low as 6 miles over recent days.
'The SAL will result in degraded air quality and hazy skies across the islands,' the National Weather Service wrote in a technical discussion. '[It] will also contribute to warmer-than-normal temperatures, especially during nighttime hours, reducing radiational cooling and contributing to hotter minimum temperatures each morning.'
High pressure to the north of the Caribbean and lower pressure in the eastern Gulf of Mexico will help draw the dust toward the Bahamas and Florida through Wednesday. Code yellow 'moderate' air quality conditions are forecast in South Florida given the likelihood of increased particulates in the air.
The dust plume should eventually dissipate in and around the Sunshine State but some of it may reach other portions of the Gulf Coast and Southeast U.S. in lesser quantities.
Although the SAL tends to squash precipitation chances versus what they would be without it, the two can intermix. In locations that occurs, some so-called dirty rain may fall.
The dust particles floating in the atmospheric column where air rises and condenses into storms gets picked up, nucleates into raindrops then falls to earth.
It's probably not so noticeable during rainfall, but when the storm has ended and the raindrops have dried, dusty residue is left behind. This is particularly true on metal or dark objects.
It's difficult to make much of this and recent bouts of dusty air off the Sahara. Most seasons do have some and it is most common early in the season.
Sea surface temperatures across the Atlantic basin are running above average for the date, and the same can be said in the main development region where tropical waves tend to form. Thus far, impact from dust has been minimal.
Forecasts call for an active season, a common theme of the past decade in the Atlantic Ocean and across much of the globe.