Abstract
This study seeks to diagnose the development and intensification of a wintertime extratropical weather system that persisted from 16th to 21st December, 2008 and caused moderate to heavy downpours over various regions of Pakistan. The rainfall was widespread and the rainfall system moved from southwest to northeast, affecting Balochistan, Sindh, Punjab, Kashmir and KPK provinces and Northern Areas of Pakistan. The spatial and temporal features of various parameters of this westerly weather system have been analyzed using surface and upper air station-based observed data, acquired from the Climatic Data Processing Centre (CDPC) of Pakistan Meteorological Department (PMD) and NCEP/ NCAR reanalysis. It has been diagnosed that overall the Positive Vorticity (PV)persisted from 18th to 20th December, 2008 up to 500 hPa level and especially its presence at lower levels over Persian Gulf and northeast Arabian Sea supported the weather system as constant source of moisture incursion. The presence of cut off lows over central parts of Pakistan at 925 and 850 hPa levels further enhanced the moisture convergence over Pakistan resulting in moderate to heavy rainfall. The core of Subtropical Westerly Jet Stream (STJ) was located over the central parts of Pakistan from 17th to 19th of December 2008 and then gradually shifted eastward. The temperature profile revealed the interaction of two contrasting air masses comprising of comparatively cold dry air from the west and warm moist air from the Arabian Sea towards Pakistan which triggered the rainfall activity. Considering these spatial-temporal features of various meteorological parameters, timely forecast for such heavy rainfall systems can be issued in future during winter season.