Diagnostic Study of Heavy Downpour in 2015 Flash Floods over Chitral Area, Northern Pakistan

Abstract

The weather patterns of the whole world have changed over last five decades as depicted by analysis of the past. This research work presents diagnosis of atmospheric conditions that resulted in the extreme flood events in the northern areas of Pakistan especially over Chitral valley from 28th to 31st July, 2015 by utilizing observed data as well as National Centre for Environmental Protection and National Centre for Atmospheric Research (NCEP/NCAR) and European Centre for Mediun Range Forecasting (ECMWF, ERA-Interim) reanalyses data. Analysis of geopotential height and temperature contours at 200 hpa indicated strong differential development which resulted in protraction of jet stream trough over Chitral valley. Easterly moisture interacted with the westerly moisture contributing to 80 % relative humidity causing heavy down pour. Persistence of heat wave over a week prior to the rain spell attributed to the development of low pressure cells over Chitral valley. Heavy precipitation was also recorded prior to and during the flash flood episode, having a total of 24.8 mm in the second fortnight of July. Another reason of rainfall was the presence of warm temperature conditions at the surface, leading to positive omega values on 28th July. The discrete analysis of the NCEP/NCAR datasets along available data of Pakistan Meteorological Department revealed that the entire interaction led to the development of low pressure system, penetration of moisture at much higher levels, protraction of monsoon over Northern areas, bringing about rain spell and flash floods over the region.