“Although on average the western highlands receive between 10 and 15 inches of rain per year,” NASA writes, “much of eastern Yemen receives less and 5 inches per year with coastal areas often getting less than 2 inches per year.” And the region is certainly inexperienced with the flooding rains of a tropical cyclone; Chalapa was just the third hurricane-strength storm to make landfall anywhere on the Arabian Peninsula, preceded only by cyclones Phet in 2010 and Gonu in 2007.
Rainfall forecasts from the high-resolution hurricane model HWRF were dire in the days leading up to landfall. Generated every six hours, some forecasts were more extreme than others, but all were forecasting some areas to receive more than 24 inches of rain as Chapala came ashore. In particular, it was even suggesting a large swath of rainfall totals over 32 inches for the area around Mukalla, a large port city surrounded by steep mountains.
When the storm arrived, the Yemeni island of Socotra was ravaged by Chapala’s heavy rains as it swept by the island as the equivalent of a Category 4 hurricane. The port city of Mukalla was also heavily damaged by Chapala’s flooding. As torrential rain fell on the dry mountainsides it poured down into coastal communities unimpeded by vegetation, and overwhelmed the manmade canal that runs through the center of Mukalla.
Though we may never know exact rainfall totals, we can use NASA’s precipitation measuring satellites to estimate how much rain fell and where.
Based on these estimates, the HWRF model did fine on the location of the heaviest rain over coastal Yemen. It was always forecasting the Mukalla region and areas to the southwest to see the largest rainfall totals, but there’s not a lot of skill involved there — the heaviest rain is likely going to fall on the right side of the cyclone and in the region with the highest terrain. As air follows the terrain, hills and mountains act to wring out even more of the cyclone’s tropical moisture by lifting the air very quickly.
What it didn’t predict well was how much rain fell in that area. The highest rainfall total around Mukalla, according to the satellite estimate, was about 16 inches, with widespread totals over 8 inches. It’s possible that this product is underestimating the amount of rain that fell, but it’s likely not underestimating it by 10 to 12 inches. Even so, this mountainous region still saw anywhere from one to four years-worth of rain over the course of 24 to 36 hours.
Perhaps the biggest surprise from Chapala was the incredible amount of rain that fell over the island of Socotra. Even in the hours before the cyclone made its closest pass to Socotra, where there mountain tops peak at 5,000 feet, the largest amount of rain that the HWRF was forecasting was a small area of 8 to 16 inches at the very top of those mountains.
If you trust NASA satellite estimates, the western half of Socotra saw widespread rainfall totals of 16 to 24 inches, which is supported by the photos and video of flooding that came from the island.