When a low pressure area (LPA) forms in bay of Bengal (BOB) denoted as L1 in the illustration, trough usually run on plains close to Himalaya as denoted by top green line. Under these conditions, often, fairly widespread spread (FWS) rain occurs along foothills of Kashmir and NE Punjab for few days. The resultant cooling and persistent outflow from the spell pushes trough more Southward to central parts of the country. In the meantime LPA travels up to MP (Madhya Perdesh) India shown as L2.
With L2 position of the system trough moves to central parts of the country and trough strengthens very often at this position and can extend up to 2.1 KM ASL. However trough often start tilting S or SW with hight there. Isolated to scattered rainfall activity over Central parts of the country and resultant outflow from the spell toward South displace trough further south to Sindh and LPA also does shift close to Gujrat at L3.
At L3 position of LPA trough often moves to Sindh. It further strengthens and can extend up to 3.1 KM ASL or greater but at the same time it often start tilting South and relatively hot and dry wind influx from NE toward Western end of the trough decreases its rain giving capacity.
Fahim Malik Dhurnal.
