Most people live in places where it rains much more often than it does in the Sahara Desert. For this reason, it can be challenging to make sense of news reports of major storms in the Sahara. NASA satellite data and rainfall statistics can give the needed context.

Below is a discussion of three news stories about rainfall in or near the Sahara Desert during August and September 2024. These news stories described storm runoff, infrastructure damage, and the rain's impact on the ecosystems at the edge of the desert. Areas with significant impacts are shown in green in Figure 1.

Figure 1. A map of North Africa showing areas with notable rainfall in August and September 2024. The boundaries of the Sahara and Sahel are based on Olson et al. (2001).

Deeper in the desert, there are no rain gauges or weather radars for hundreds of miles and there is little plant life or infrastructure to be affected by rain. Satellites do fly over these locations and collect data. Satellite data suggest that, in August and September 2024, the greatest deviation from normal conditions occurred inside the Sahara Desert rather than in the less arid areas at its margins. The relevant statistic is calculated by dividing the rainfall in August and September of 2024 by the amount of rain that falls on average during a full year. High values for this statistic occurred in the red rectangles in Figure 1.

Northern Sudan Flooding in August 2024

In late August 2024, infrastructure was damaged by floods in northeast Sudan, primarily between the Nile River and the Red Sea. Storm runoff caused the collapse of the Arba'at dam late on August 24 (19.8338°N, 36.9396°E; Aljazeera 2024). The dam had supplied drinking water to the city of Port Sudan (United Nations 2024). The late-August floods damaged buildings and roads, making travel difficult between the towns in the region (IFRC 2024; ReliefWeb 2024; Guardian 2024). These impacts occurred, for the most part, between the city of Dongola and the Arb'at Dam, whose locations are marked with a "D" and "A," respectively, in Figure 2.

Humanitarian agencies described how floods impacted infrastructure but did not specify how much rain fell. For data from prior decades, one can turn to an international rain-gauge archive (GHCNd 2024) to find Sudanese daily rainfall measurements either along the Nile River or Red Sea coast. This archive, however, lacks recent-year data for these rain gauges.

Satellite-based estimates of rainfall are available over most of the globe, including northern Sudan. For example, such estimates are calculated by NASA's IMERG algorithm and are shown in Figure 2. IMERG stands for Integrated Multi-SatellitE Retrievals for Global Precipitation Measurement (GPM) (Huffman et al. 2023). The IMERG algorithm is used to estimate precipitation rate on a global grid with approximately 7-mile resolution every 30 minutes since January 1998.

Figure 2. IMERG precipitation estimates over northeastern Africa.

Considering how arid the region is, an impressive amount of rain fell during the 24 hours prior to the collapse of the Arba'at dam. IMERG estimated that, upstream of the dam, the Arba'at ravine received an inch of rain in one hour and a total of 2 inches that day. Additional rain had fallen earlier in the month (August 3–11) in northern Sudan. As a result, the monthly total reached 2 to 12 inches (56–316 mm) in many places (left panel of Figure 2). The monthly total for August 2024 was greater than the average-annual accumulation (center panel of Figure 2).

During August 2024, there was also unusually large rain accumulation west of the Arba'at Dam near the Sudan-Libya border. On average, July and August are the rainiest months in this region (Kelley 2014), but the amount that fell in August 2024 was well above average. The statistically unusual rain fell mostly in the 275-by-600-mile red rectangle in Figure 2, an area with no weather radars and no rain-gauge data in the GHCNd international archive. Elsewhere in Figure 2, small white squares mark where there is a rain gauge that has its observations routinely transmitted to the international archive or used in a global-gridded rain-gauge-only data product(GPCC 2024).

Unusually high rainfall totals occurred during August 2024 in the orange, red, and pink areas of the right panel of Figure 2. That month, these areas received over five times as much rain as usual falls during a whole year. Other instances of similarly high rain accumulation are known to have occurred elsewhere in very arid regions (Warner 2004, pg. 348–349). In contrast, it would be inconceivable for a location with a moderate climate to receive five times its average-annual accumulation in a single month. For example, it was considered a 1,000-year storm in 2017 when Hurricane Harvey brought sixty inches of rain to a location near Houston, Texas (Blake and Zelinsky 2018, pg. 6 and 7). Somewhat less, but still large, storm totals occurred over a larger area near Houston (GPM 2020). Even the sixty-inch storm total was not much greater than Houston's average-annual accumulation. As shown in the center panel of Figure 2, northwestern Sudan has a much lower average-annual accumulation: no more than an inch or so (3–32 millimeters). This statistic means that northwestern Sudan's average-annual accumulation could be exceeded by just one, fairly mild storm.

Having five years' worth of rain fall during a single month is a rare occurrence globally, as shown in Figure 3. The black and red areas are where one or two months during the past 24 years had rainfall that exceeded five times average-annual accumulation. Green areas are where a less extreme threshold was reached: at least one 12-month period (January through December) had five times more rainfall than the average year. The top panel of Figure 3 shows where these thresholds were exceeded during 2001 to 2022, and the bottom panel covers January 2023 through September 2024. Figure 3 omits places where IMERG never saw these thresholds exceeded, such as China, Australia, and the nearby ocean.

Figure 3. Locations where IMERG found that a single month or year had precipitation in excess of five times that location's 2001–2022 average-annual accumulation.

Evaluating the whole globe during the past 24 years, IMERG reports that it is primarily within the southeast portion of the Sahara Desert, along the southern tip of the Arabian Peninsula, and over nearby bodies of water that a monthly accumulation is ever greater than five times a location's average-annual accumulation. The places where this threshold is reached tend to be very arid and near less-arid regions where strong storms are more common. For example, the Sahara Desert is impacted by a few of the intense convective systems that move westward across tropical Africa. The southern end of the Arabian Peninsula is occasionally impacted by tropical cyclones that form over the Indian Ocean (e.g., GPM 2015, 2018).

So far, it has been shown that the August 2024 rain accumulation in the Sahara Desert near the Sudan-Libya border was a globally rare event because it delivered five times that region's average-annual accumulation.

Next, we take a look at one of the rainy days in August 2024 to see if the storm clouds over northern Sudan look any different than ordinary storms that occur frequently elsewhere in the world. An easy way to compare satellite observations of storms is to use NASA's WorldView website. Figure 4 shows a true-color satellite image taken from WorldView and subsequently annotated using other datasets.

Figure 4. A true-color MODIS snapshot of northern Sudan during several rainy days in late August 2024. The southeast boundary of the Sahara Desert is marked in orange or pink based on Olson et al. (2001) or AAG (2013).

Figure 4 shows the clouds over northern Sudan at about 2 PM local time (1158 UTC) on August 26, 2024. The isolated white blobs that cast dark shadows are tall convective systems that are producing rain and that are under 50 miles across. The larger areas of less bright clouds are generally not raining and not as tall. This true-color imagery and cloud-height information come from the MODIS instrument on NASA's Aqua satellite and the VIIRS instrument on NOAA's NPP satellite. The rainfall rate was estimated using Japan's Dual-frequency Precipitation Radar (DPR) that flies on NASA's Global Precipitation Measurement (GPM) satellite.

The GPM satellite's instruments estimated that the Sudanese convective cells were raining 3–10 millimeters per hour, which are rates typical of run-of-the-mill convective cells in locations with moderate climates. At 2 PM on August 26, the NPP satellite's OMPS instrument saw airborne dust to the west of the northern Sudan storms (GES DISC 2024).

Two facts suggest that at least some of the storm cells in this system were vigorous. For one thing, GPM radar observations suggested that one of these convective cells had strong updrafts. Strong updrafts were likely present because the radar saw ice-phase hydrometeors lifted to the tropopause, which means a height of about 16.5 km above sea level over Sudan (Tegtmeier et al. 2020, Fig. 8b). The location of this strong convective cell is marked with an "X" in Figure 4.

The other relevant fact is that lightning was observed in many of these cells. Strong updrafts are typically one of the necessary ingredients for lightning to form. At 2 PM on August 26, the European MTG-1 geostationary satellite saw lightning in many convective cells over northern Sudan (small yellow crosses in Figure 4), but no lightning in the clouds further north over Egypt (Weather.us 2024). In Figure 4, black circles mark Sudanese cities impacted by the late-August rainfall. The red circle and line through it represent the Arb'at dam and the stream it dams. As mentioned previously, the Arb’at dam collapsed on August 24 due to large rain accumulation.

It is unclear precisely which of the Sudanese storm cells were in the Sahara Desert because the desert's boundaries are subject to debate. Orange or pink lines in Figure 4 mark the southeastern edge of the Sahara Desert according to the work of Olson et al. (2001) or AAG (2013), respectively.

Flooding Near the Atlas Mountains in September 2024

Two weeks later and 2,000 miles away, there was another multi-day rain episode with large accumulations on the opposite side of the Sahara Desert. During September 5–8, 2024, IMERG estimated that about 1–4 inches (32–100 millimeters) fell over the Atlas Mountains of Morocco.

The Associated Press reported a similar or greater amount of rain than did IMERG in two locations in the southern foothills of the Atlas mountains (Metz and Ahmed 2024). The Associated Press reported that the Moroccan government measured 100 millimeters of rainfall in one day at Tagounite (29.978°N, 5.583°W) and 250 millimeters over two days at Tata (29.754°N, 7.973°W). Presumably, these measurements were made by the government-operated rain gauges in these cities. These rain gauges are the only government-operated gauges in a region spanning 300 miles (Directorate of Meteorology 2024).

Regardless of how much rain actually fell in early-September, drone and satellite images showed significant runoff from these storms (Earth Observatory 2024). It is clear that significant rainfall did occur, at least by the standards of this arid region (Armon et al. 2024, Fig. 6). Runoff from these storms filled riverbeds and lakebeds that had been dry for decades south of the Atlas Mountains near the Morocco-Algeria border. Photographs of some of these lakes were taken by drones nearly a month after the rain storms and widely circulated online (Metz and Ahmed 2024).

The MODIS instrument on NASA's Terra satellite collected the two false-color images in Figure 5. The color table is designed to highlight surface water such as streams, lakes, and flooded land. These false-color images are generated from bands 7, 2, and 1 of the MODIS instrument (Earthdata 2024). The top image was observed a few days after the early-September rain storms, and it shows streams in cyan and lakes in dark blue or black. The bottom image was observed a month after the rain storms, and it shows streambeds dry once again but lakes still filled. In Figure 5, dotted white lines mark intermittent streams and white circles mark eight dry lakebeds filled by runoff from the early-September rain storms.

Figure 5. False-color MODIS imagery of the Morocco-Algeria border that was observed either a few days (top) or a month (bottom) after the early-September 2024 storms. The Sahara Desert extends further to the northwest in the classification system of AAG (2013) (yellow contour) vs. that of Olson et al. (2001) (orange contour).

News stories did not tell the full story of the September 2024 rain storms, and strident headlines might mislead people into thinking that a large portion of the Sahara flooded. For example, CNN lead with "Sahara Desert flooded for the first time in decades" (CNN 2024). An internet search on the phrase "Sahara Desert flooding" found news stories with headlines that claim the early-September rain in Morocco's Atlas Mountains caused the first floods in the Sahara Desert in 50 years.

For one thing, the eight lakes filled by runoff south of the Atlas Mountains covered less than 1/1000^th of this region, as can be seen in Figure 5.

Second, the boundaries of the Sahara Desert are subject to debate. It is open to interpretation how many of these eight lakes were actually within the Sahara Desert rather than in the surrounding steppes. In some land-classification systems, only one of the eight lakes was in the Sahara Desert (Olson et al. 2001; Burgess et al. 2006; Dinerstein et al. 2017; OneEarth 2024). In other land-classification systems, most or all of these lakes were in the Sahara Desert (AAG 2013; Peel et al. 2007, Fig. 4; Warner 2004, pg. 80 and 84; and Nicholson 2011, pg. 294).

Last, the early-September floods in the southern foothills of Morocco's Atlas Mountains were not the first that the Sahara Desert had experienced in 50 years. On the contrary, flooding had occurred just two weeks earlier in northeastern Sudan, as discussed in the previous section of the present blog post. Northeastern Sudan and the southern foothills of the Atlas Mountains are both at the edge of the Sahara Desert or in the steppes just outside it, depending on what land-classification system you use.

News stories about the early-September rainfall in the vicinity of the Morocco-Algeria border did not comment on the total accumulation for the whole month. In the left panel of Figure 6, the monthly rainfall is seen to reach about 4 inches (100 millimeters) about 300 miles south of the Atlas Mountains, according to IMERG. This area is marked by the red rectangle in Figure 6, and it is far south of the impacts that the media covered from the early-September storms. In the red rectangle, the September 2024 rainfall was twice average-annual accumulation, which is shown in the center panel. The right panel of Figure 6 shows the multiple of average-annual accumulation that fell during September 2024.

Within the red box in Figure 6, there are salt flats (dry lakebeds) that flooded with storm runoff between September 4 and 9, 2024. The resulting lakes are visible in MODIS imagery on the WorldView website and are located within 27.081–27.606ºN, 5.782–4.865ºW. One of the salt flats is called Sebkha Ain Belbela (27.556ºN, 5.295ºW, Teach with GIS 2024). Perhaps, news stories did not mention this flooding because the region appears to be uninhabited. The location of these salt flats is marked by "S" in Figure 6. Also marked are locations further north that were mentioned in the news stories about these storms: Merzouga and Lake Sriji (M), storm runoff reported by the NASA Earth Observatory (E) (Earth Observatory 2024), Tagounite (T), Lake Iriqui (I), and Tata (A).

Figure 6. IMERG precipitation estimates near the Morocco-Algeria border.

The Partial "Greening" of North Africa in 2024

One way to measure the significance of large rain accumulation is by its impact on vegetation. Greater-than-average rainfall may increase crop yields or damage crops. Unusually large accumulation may also increase how green the Earth looks from space. Since the early 1980s, satellites have routinely measured vegetation density using a variable called NDVI, the normalized difference vegetation index (Tucker et al. 1991; NCEI 2024). To calculate NDVI, satellite observations at two wavelengths are compared. NDVI typically has values of 0.1–0.2 for barren ground and 0.7–0.8 for dense vegetation (Ding et al. 2016).

On September 19, 2024, the Washington Post reported that "parts of the Sahara Desert are dramatically greener than usual" because of higher-than-average rainfall (Stillman 2024). The Washington Post supported its claim by showing a map of where NDVI changed between the first half of September 2024 and one year earlier. The figure in the Washington Post article, however, showed that the Sahara Desert's vegetation was essentially the same in September 2024 and one year earlier. It was in the Sahel, not the Sahara, where the Washington Post figure showed increased vegetation.

To further explore the greening of North Africa, consider the top panel of Figure 7. That panel shows how much the early-October 2024 NDVI deviated from the long-term average for that time of year (USGS 2024). It makes sense to look at NDVI to measure the effect of excess rain from prior months. There can be a delay of a month or two between above-average rainfall and a subsequent increase in plant density (Ugbaje and Bishop 2020). One can check the timing of NDVI changes using the GIMMS data browser.

When discussing NDVI change specifically in the Sahara Desert, it is worth noting that various land-classification systems agree for the most part about what land falls within the Sahel or the Sahara Desert, but they disagree about the exact boundaries (e.g., Olson et al. 2001 vs. AAG 2013). Keeping this in mind, one can still say that, in early October 2024 (day of year 273–288), the vegetation over most of the Sahara Desert was slightly more scarce than average for that time of year, according to MODIS observations. Average conditions were calculated using data from 2000–2023. This decrease in vegetation is indicated by light red in the top panel of Figure 7. NDVI was higher than normal in two woodland areas that are surrounded by the Sahara Desert. These two woodland areas are in Chad's Tibesti Mountains and Algeria's Hogar Mountains. The increased vegetation is indicated by shades of green in the top panel of Figure 7.

Figure 7. Terra MODIS NDVI during the first two weeks of October 2024 compared to the median NDVI at the same time of the year during 2000–2023. The yellow and green lines indicate the boundaries of the Sahara Desert and the Sahel (Olson et al. 2001).

In early October 2024, the Sahel's NDVI was much higher than average for this time of year. The Sahel is the acacia-tree savanna south of the Sahara Desert. The large increase in vegetation is indicated by dark green in the top panel of Figure 7.

The bottom panel of Figure 7 shows the rainfall anomaly estimated by IMERG during the three-month period of July through September 2024. Blue or red indicate where there was more or less rain than usual during these three months of 2024.

Comparing the top and bottom panels of Figure 7, one sees that, in places that already had a moderate amount of vegetation, the excess rainfall during July through September 2024 were associated with denser-than-average vegetation in October 2024.  Such places are the savannas, steppes, and woodlands located in the Sahel, Atlas Mountains, or the Hoggar and Tibesti Mountains.

In contrast, October 2024 did not have denser vegetation than normal in some nearly barren locations, even though those places did receive more rain than usual during the preceding three months. Three such locations are labeled a, b, and c in the bottom panel of Figure 7. The imperfect correlation between the preceding three months of rain and vegetation density indicates that other factors are at work. Such factors might include longer-term rainfall, air temperature, evaporation rate, soil type, and differences in how various ecosystems respond to rainfall.

Conclusion

It can be difficult to make sense of major rain storms and flooding that is occasionally reported in the Sahara Desert. The preceding discussion used NASA data and other datasets to provide context for three news stories published about rainfall in August and September 2024. Long-term averages, single overflights of storms, and monthly statistics were discussed. The only way to see the big picture over this part of the world is to use satellite data because much of the Sahara Desert lacks rain gauges and weather radars. For seeing the big picture, NASA's IMERG rainfall-estimation algorithm is helpful because it combines infrared and microwave observations from many satellites and calibrates those estimates with rain gauges where available.

In one part of the Sahara Desert, the rain accumulation during just August 2024 was more than five times as great as what usually falls there during a full year. At the edge of the Sahara Desert or in the steppes adjacent to it, the rain accumulation was less statistically extreme (as a fraction of average-annual accumulation), yet it was still significant. In these edge cases, the rainfall was sufficient to produce floods that damaged infrastructure and runoff that filled dry lakebeds.

Satellites saw little change in the nearly barren Sahara Desert despite the higher-than-average rainfall there in 2024. In contrast, satellites saw a dramatic increase in vegetation in October 2024 in the Sahel. The Sahel is a less arid region of acacia-tree savanna that lies to the south of the Sahara Desert. Most of the Sahel received higher than normal rainfall during July through September 2024, preceded the increase in vegetation.

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Credits: Text and images by Owen Kelley (GMU/NASA). Several people provided feedback on an earlier version of this story including Jim Tucker (NASA), George Huffman (NASA), Jorge Pinzon (SSAI/NASA), Ed Pak (SSAI/NASA), and Jason West (KBR/NASA).

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