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Heavy rain has been forecast over the Andaman & Nicobar Islands over the next three to four days starting Wednesday as monsoon gathers momentum in the extreme South-East Bay of Bengal.
Heavy rain is also forecast for Assam, Meghalaya, and the rest of the North-Eastern States during this period, an India Met Department (IMD) outlook said on Tuesday evening.
Shear zone
A shear zone of monsoon turbulence, featuring opposing winds at height of three to six km in the atmosphere, is also in place around the southern Sri Lankan latitude and extending into the South Bay.
It represents the zone in the heights where the monsoon is most active, and sets up the platform for rain-bearing systems (low-pressure areas/depressions) to play around in. The shear zone strengthens in tandem with strength of the monsoon. Elsewhere over the mainland, the IMD sees a marked rise in thunderstorm activity accompanied by lightning, high winds, and hail as pre-monsoon weather peaks over North-West, East, North-East and Peninsular India.
Causative atmospheric features include remnants of a western disturbance in the form of a cyclonic circulation over South-West Rajasthan and peer circulations over North Interior Karnataka and Assam.
Joining them are troughs running down from North Interior Karnataka to the Comorin across South Interior Karnataka and Tamil Nadu, and a second one from Bihar to Interior Odisha.
Troughs join in
Warm and moist air fans into these circulations and troughs located above a heated up land, causing the airmass to rise abruptly into the atmosphere, cool, and rain down as rain or hail to the accompaniment of thunder and lightning. As for Wednesday, the IMD has forecast thunderstorm and lightning with (i) squall (wind speed of 60 km/hr) over the hills of Bengal and Sikkim; (ii) gusty winds (40 km/hr) over Chhattisgarh, plains of Bengal, Rayalaseema,South Interior Karnataka, and Kerala and (iii) with dust storm and gusty winds (40 km/hr) over Punjab, Haryana, Chandigarh, Delhi, West Uttar Pradesh and Rajasthan.
Heat wave conditions are forecast for Vidarbha and Madhya Maharashtra, Chhattisgarh, Odisha, Marathawada, Telangana, Rayalaseema and Tamil Nadu and Puducherry.
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