A Flash-Flooding Storm at the Steep Edge of High Terrain: Disaster in the Himalayas

Abstract

Flash floods on the edge of high terrain, such as the Himalayas or Rocky Mountains, are especially dangerous and hard to predict. The Leh flood of 2010 at the edge of the Himalayan Plateau in India is an example of the tragic consequences of such storms. The flood occurred over a high mountain river valley when, on three successive days, diurnally generated convective cells over the Tibetan Plateau gathered into mesoscale convective systems and moved off the edge of the Plateau over Leh. An easterly midlevel jet associated with a midlevel monsoon vortex over northern India and a high over Asia helped the convection organize into propagating mesoscale systems that moved over the edge of the Plateau. On the third day the mesoscale system moving off the plateau was greatly invigorated when it suddenly drew on moisture flowing upslope over the terrain. It gained maximum strength from this intake of moisture near Leh, and the heavy rains washed over the surrounding mountains and down and over the town.