The link between Saharan dust and Thursday’s severe weather in Baton Rouge area

Dr. Steve breaks down Saharan dust and your health

BATON ROUGE, La. (WAFB) - Severe storms blasted through parts of south Louisiana for the second day in a row on Thursday, June 25, leaving a path of wind damage and tens of thousands without power. Wind gusts reached 62 mph at the Baton Rouge Metropolitan Airport and 68 mph at LSU’s Tiger Stadium. Entergy was still reporting more than 4,000 customers without power as of Friday afternoon, with more than half of those in East Baton Rouge Parish.

This animation shows the aerosols in the Saharan dust plume from June 15 to 25, 2020. It was created from the Suomi NPP OMPS aerosol index. The dust plume moved from Africa’s west coast over the Atlantic Ocean into the Caribbean Sea and up through the Gulf of Mexico. The largest and thickest part of the plume is visible over the eastern and central Atlantic.
This animation shows the aerosols in the Saharan dust plume from June 15 to 25, 2020. It was created from the Suomi NPP OMPS aerosol index. The dust plume moved from Africa’s west coast over the Atlantic Ocean into the Caribbean Sea and up through the Gulf of Mexico. The largest and thickest part of the plume is visible over the eastern and central Atlantic. (Source: Colin Seftor)

Thursday’s severe weather was the continuation of a very active pattern that started locally Sunday and has delivered several rounds of storms, some severe, to the area. However, unlike the storms earlier in the week, Thursday’s activity may have gotten a boost from something known as the Saharan Air Layer (SAL).

What is the Saharan Air Layer?

The SAL is a layer of warm, dry air filled with dust that’s typically located about one to three miles above the ground. You’ve seen it if you’ve been outside and it has been clearly evident on some of WAFB’s Sky9 cameras. The SAL develops when weather disturbances moving across the Sahara Desert produce strong winds and loft large amounts of dust into the atmosphere. The same prevailing winds that often steer tropical disturbances from the west coast of Africa across the Atlantic can sometimes carry these plumes of dust thousands of miles westward through the Caribbean and toward the United States. According to NOAA, the SAL typically peaks from late June through mid-August and moves over the Atlantic every three to five days. However, only larger SAL events, such as the one experienced this week, produce noticeable impacts in the U.S.

Sky9 camera view from Our Lady of the Lake looking back toward downtown Baton Rouge on Friday, June 26, 2020. The Marriott, about three miles away, is visible, but downtown buildings about five and half miles away are difficult to see. This was the result of Saharan dust moving into Baton Rouge.
Sky9 camera view from Our Lady of the Lake looking back toward downtown Baton Rouge on Friday, June 26, 2020. The Marriott, about three miles away, is visible, but downtown buildings about five and half miles away are difficult to see. This was the result of Saharan dust moving into Baton Rouge. (Source: WAFB)

SAL Impacts

The SAL is perhaps best known for helping reduce tropical activity in the Atlantic. Warm, dry air acts as a stabilizing influence, while the burst of winds associated with the SAL can also produce wind shear that rips apart any systems in its path attempting to develop. As has been seen in the Baton Rouge area, high concentrations of dust can also result in poor air quality. More colorful sunrises and sunsets are also possible thanks to the dust in the atmosphere, but other factors influence that potential. The dry air located a little bit above the surface can also influence weather on a local level.

SAL (Possible) Link to Thursday’s Severe Weather

In the paragraph above, I outlined how the SAL is associated with some dry air and dust, factors that would seemingly work against storm development, right? In fact, I also noted it can often be a limiting factor in tropical development. So how could it have actually enhanced the severe weather Thursday? In order to answer that question, we need to do a quick dive into the meteorology of Thursday.

Meteorologists use soundings to get a picture of the environment around us from the surface to nearly the top of the atmosphere. These soundings are obtained by attaching sensors to weather balloons that are launched around the country twice per day. In the wake of Thursday’s severe storms, I went back and looked at the nearest sounding from Slidell and a possible link to the SAL was quickly evident.

The image below contains the soundings from Wednesday, Thursday, and Friday mornings of this week. There’s a lot of information on these plots, but for the purposes of this discussion, all you need to know is the red line represents a vertical profile of temperature in the atmosphere while the green line represents dew point (moisture).

Morning atmospheric soundings from Slidell, La. for Wednesday, Thursday, and Friday (June 24 through 26, 2020). Red lines represent the vertical temperature profiles, while green lines represent the dew point (moisture) profiles. Areas where there's significant separation between the red and green lines indicate the presence of dry air.
Morning atmospheric soundings from Slidell, La. for Wednesday, Thursday, and Friday (June 24 through 26, 2020). Red lines represent the vertical temperature profiles, while green lines represent the dew point (moisture) profiles. Areas where there's significant separation between the red and green lines indicate the presence of dry air. (Source: WAFB)

Note how on Wednesday’s plot, the red and green lines are fairly close together throughout the lower part of the atmosphere. In essence, that shows the lowest several miles were moist that morning, but a significant change is noted by Thursday morning, with a gap appearing between those two lines, indicating some drier air roughly one to two miles above the surface. That’s evidence of the SAL arriving locally. It also represents a profile supportive of strong and damaging winds should any storms develop. I’ll spare you a deep dive into the meteorology for now, but suffice it to say that layer of dry air causes evaporation within a storm, with the resultant cooler, heavier (more dense) air being transported to the ground in the form of damaging winds. That’s why I think the SAL contributed to Thursday’s severe weather; it produced a layer of dry air one to two miles above the surface.

On the final plot, you’ll note the layer of dry air had grown even deeper as of Friday morning, indicating the SAL was even a little stronger over the Baton Rouge area. Fortunately, other conditions were much less supportive of storm development Friday, preventing a repeat of the severe weather.

Moving Forward

The good news is the SAL intensity appears to have peaked as of Friday, meaning both the health and weather impacts should be reduced as we get into the weekend. Forecasts do indicate another slug of dust currently moving through the Caribbean could reach the Baton Rouge area by early next week, but it looks like concentrations may be considerably lower next time around.

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