Monsoon may not Advance in Copybook Pattern
Though the advance of the South west monsoon (SWM) across the country historically follows a predictable timeline, it sometimes gets stuck due to changes in upper level atmospheric dynamics.
The advance is closely linked to the northward migration of the thermal equator or the inter tropical convergence zone (ITCZ). By June 21st the Sun is overhead 23°N Lat and technically the ITCZ has to follow the Sun. But in reality, the north-west to south – east monsoon trough does not get established simultaneously as it is a complex interaction among various factors.
The Arabian sea branch of monsoon has to deepen, and it has to meet with the Bay of Bengal branch heading north- west after it gets deflected by the north eastern Himalayas. The diurnal heat flux, the position of the subtropical jet etc are the main factors that determine the position of the monsoon trough.
Killer heatwave conditions to return to North India
An upper level anticyclonic circulation has caused a thermal inversion cap over north India resulting in heatwave conditions. This has hampered the northward advance of the monsoon temporarily. The heatwave may prevail for 4-5 days and it may dissipate by 23rd June.
Though the south west monsoon has covered more areas over coastal Andhra Pradesh, north interior Tamil Nadu, north interior Karnataka, south interior Maharashtra, Telengana and some parts of south Chattisgarh, its further advance would be stuck by the current upper atmospheric dynamics.
An abrupt thermal inversion cap has developed over most parts of north India in the wake of an unseasonal upper level anticyclonic circulation. This may not aid the monsoon advance and hence it may be delayed in its entry to Kolkotta, Bihar and Jharkhand.
However due to a mid tropospheric low over Telengana and Andhra Pradesh, and east- west and north- south monsoon shear zones running from south Madhyapradesh to interior Tamil Nadu through interior Maharashtra, heavy thundershowers may be expected over Vidarbha region of Maharashtra, Telengana and Andhra Pradesh in the coming week.
Heavy monsoon induced thunder showers may be expected over pockets like Nasik, Sholapur, Pune, Nagpur in interior Maharshtra. The western offshore monsoon trough has not strengthened sufficiently which has resulted in less than normal monsoon rains in Kerala. This is likely to strengthen by 17th or 18th June and the monsoon would be getting vigorous over Kerala from 20th June.
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