India Meteorological Department (IMD) has officially got up a watch for the North-East monsoon that could likely make an onset during the course of the week.
In an update on Monday and valid until Saturday, it seemed to suggest that conditions would become favourable for the monsoon on retreat as the week progresses.
It has predicted fairly widespread rainfall over South Peninsular India and the Andaman and Nicobar Islands.
WEATHER WARNING
A weather warning valid for Monday said that isolated heavy rainfall would occur over Coastal Andhra Pradesh and South Coastal Orissa.
The causative upper air cyclonic circulation over West-central Bay of Bengal and adjoining Coastal Andhra Pradesh has been persisting over the past couple of days.
This system could be the fulcrum around which the seasonal transition of weather would come about over Peninsular India with westerlies getting increasingly replaced by northeasterlies to monsoon easterlies.
Additionally, the IMD has picked the crucial “shear zone” formation along the 15 deg latitude, which defines the playground for northeast monsoon weather systems.
The shear line is a line or narrow zone across which there is an abrupt change in the horizontal wind component; a line of maximum horizontal wind shear.
ONSET CONDITIONS
According to IMD specifications, commencement of north-east monsoon assumes withdrawal of southwest monsoon up to 15ยบ latitude, onset of persistent surface easterlies over Tamil Nadu coast to required depth, fairly widespread rainfall over coastal Tamil Nadu, South Coastal Andhra Pradesh and adjoining areas.
These conditions are likely to be met as the week progresses, with surface easterlies notching up cruising speed by Friday. Rainfall would also have become more widespread over the peninsula by then.
An IMD forecast until Thursday spoke about the build-up with fairly widespread rain or thundershowers being forecast for Coastal Andhra Pradesh, South Coastal Orissa and Andaman and Nicobar Islands.
CONVECTIVE CLOUDS
Scattered rain or thundershowers have been forecast for Rayalaseema, remaining parts of Orissa, Konkan and Goa.
Satellite cloud imagery on Monday revealed the presence of convective clouds over parts of Coastal Andhra Pradesh and West-central Bay of Bengal.
International Research Institute (IRI) for Climate and Society at Columbia University has indicated wetter than average weather for Rayalaseema, parts of Telangana, North and South Interior Karnataka and parts of North and Interior Tamil Nadu during the six days ending Friday.
Other international models signalled to the setting up of a strong “pulse” entering the Bay of Bengal from upstream South China Sea later in the week.
They also indicated that, onset of northeast monsoon during the week may not have the back-up of a wet phase of the periodic Madden-Julian Oscillation (MJO) wave as forecast earlier.
The onset, if at all, would happen on the monsoon's own inherent strength and dynamics. The MJO wave is now forecast to settle over Equatorial Indian Ocean and adjoining peninsular seas around November 7.
Transiting the upper levels of the atmosphere, the wave has been known to set up monsoon onsets, low-pressure areas, depressions and even cyclones.
Given this, the northeast monsoon could likely see a pick up after November 7, according toas per various MJO models surveyed.
The wave, which has implications for ground weather, is seen particularly strong during November 12 and 16.
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