Rainfall characteristics of hurricane Mitch

Abstract

The hurricane or tropical storm known as Mitch struck Central America towards the end of October 1998. The subsequent flooding and landslides claimed approximately 11,000 lives. It was the most deadly hurricane to strike the Western Hemisphere in two centuries. We have measured rainfall totals during Mitch (from 1 to 48 hours) and find that they were not exceptional for hurricanes and tropical storms in the Atlantic basin. Rainfall totals and intensities measured over intervals of 1, 2, 5, 10, 30 and 60 minutes were less than values from the updated maximum potential rainfall curve¹,². The data suggest that extraneous factors, such as already saturated soils and denuded hillsides, were largely responsible for the damage caused.

Main

Detailed rainfall data from hurricanes are often lacking because rainfall-recording instruments are washed away. The emphasis has been on recording wind strengths, which can underestimate the damage potential. Rainfall distribution is often estimated from radar and satellite data with little ground-truth support. Data from an autographic rain gauge in southern Honduras, where flooding and landslides were extensive, provide a unique insight into the rainfall distribution during Mitch. The tipping-bucket rain gauge (ELE DRG-52) was located at 87° 04′ W and 13° 17′ N on a hill crest (100 m above sea level) in the foothills of Cerro Guanacaure (1,007 m above sea level).

Mitch formed on 21 October 1998 in the southwest Caribbean and reached category 5 hurricane status on the Saffir/Simpson scale as it moved towards northern Honduras. On 29 October, Mitch was downgraded to a tropical storm and moved southwards and inland. The storm was declared to be over on 1 November.

Mitch produced torrential rains despite being downgraded from a hurricane. Between 18:00 (Honduran time, which is 6 hours before GMT) on 27 October and 21:00 on 31 October, there was 896 mm of rainfall. The most intense and prolonged rainfall was between 15:00 on 29 October and 07:00 on 31 October (a total of 41 hours). During this period, 698 mm of rain fell. There were two distinct periods of extreme rainfall intensities: 186 and 245 mm of rain fell during six-hour periods from 16:00 to 22:00 on 29 and 30 October, respectively. Maximum intensities ranged from 138 mm h⁻¹ (2-minute period) to 58.4 mm h⁻¹ (60-minute period). Landslides affected approximately 20% of surrounding hillsides and typically occurred during the two periods of most intense rainfall. Simultaneously, the Choluteca river flooded adjacent settlements and parts of the city of Choluteca. The river also altered its course.

Although Mitch caused extensive flooding and loss of life, Fig. 1 shows that rainfall in southern Honduras was comparable to the severest hurricanes and tropical storms in the Atlantic basin^3,4,5. Rainfall was also substantially less than the updated maximum potential rainfall curve, which is defined largely by rainfall events from La Réunion, Indian Ocean¹,², where there might be greater topographic forcing. The data suggest that extensive damage in Honduras and Nicaragua was accentuated by several factors: the storm struck at the end of the rainy season when the soil was saturated, resulting in catastrophic flooding and landslides; agricultural extension caused by land pressures had left many hillsides denuded; and the population was ill prepared because, before making landfall, it had been predicted that Mitch would move northwards.

Figure 1: Plot of maximum rainfall amounts (squares) against duration from a rain gauge in southern Honduras during hurricane/tropical storm Mitch (y =51.64x ^0.674, R ²=0.99).

Also plotted are updated record rainfall events (diamonds) for different durations^1,2 that define the curve of maximum potential rainfall (y =353.07x ^0.519, R ²=0.99) and data (triangles) from recent major Atlantic hurricanes and tropical storms^3-5.