Thunderstorms in the Khumbu Himal, Spring 1999

In light of the fact that during the pre-monsoon period in the Northeast of the Indian subcontinent, thunderstorms are almost a daily occurrence, this study attempts to show how convective activity is initiated in the Khumbu Himal by relating the daytime up-valley flow to synoptic conditions. At the higher - over 5000 m - elevations in the Khumbu, the up-valley flow that delivers humid, warm air from the Terai by mid to late afternoon has been observed to combine with the prevailing upper air westerly flow, resulting in an orographically lifted field of convergence. This convective activity sometimes supports the formation of thunderstorms, and sometimes not. This behaviour appears to be dependent on upper air temperatures. Northerly flow at the surface and at the 6000 m level in the Khumbu have suppressed thunderstorm activity, even though conditions at Kathmandu further south were ideal. In addition, at elevations above 5000 m in the Khumbu, high daytime surface temperatures and dewpoints are not essential for thunderstorm formation, because convective activity is initiated above the surface where the core of the fast, moist valley flow appears to be located. At the surface above 5000 m, the up-valley wind has been observed to cease for long periods at midday. The goal of this research is to develop procedures for forecasting severe thunderstorm activity in the Khumbu Himal of eastern Nepal.