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Tuesday, October 20, 2020

Explained: Tracking the weather system that gave Hyderabad its rainiest day ever

Hyderabad Rain: Normally, cyclones lose steam upon making their landfall. This particular system, however, clocked a long east-west track cutting across Andhra Pradesh, Telangana, north-interior Karnataka and Maharashtra.

Written by Anjali Marar | Pune | Updated: October 19, 2020 10:25:07 am
A man struggles to stay afloat in gushing floodwater following heavy rains in Hyderabad, on October 14. (Photo: PTI)

Earlier this week, three days of extremely heavy rainfall led to massive floods that killed over 70 people in Telangana, Andhra Pradesh and Maharashtra. Hyderabad recorded its rainiest day in 117 years, flooding over 20,000 houses. In Pune, Sangli and Solapur, nearly 20,000 people were evacuated.

This was caused by a weather that formed in the Bay of Bengal, hit the east coast and moved westward, weakening on the way. On Friday, it re-emerged in the Arabian Sea and is set to intensify further as it moves farther into the sea.

The journey

On October 9, a low pressure system developed in the North Andaman Sea. During its landward journey, it intensified multiple times — first to form a well-marked low pressure area, then a depression, and later a deep depression while at sea. Low pressure area, depression and deep depression are part of a classification based on wind speed.

On October 13, the system crossed over land near Kakinada, Andhra Pradesh, as a deep depression. As the system moved west-north-westward, it brought extremely heavy rainfall all along Telangana and some parts of Andhra Pradesh on October 13 and 14. In the last 48 hours, the system weakened into a well-marked low pressure area, and moved along south Madhya Maharashtra, causing widespread heavy rain over several areas of Maharashtra. Early on Friday, the system left the west coast.

Coast to coast, the journey

Record rainfall

Hours after the deep depression hit land, both Andhra Pradesh and Telangana experienced extremely heavy rainfall. The 24-hour rainfall ending 8.30 am on October 14 recorded at Hyderabad city was 191.8 mm. This is the heaviest spell Hyderabad has ever experienced in October. The previous record was 117.1 mm on October 6, 1903. Many districts along the central and western regions of Telangana received surplus rain — 150% to 400% above normal during the first fortnight this month. Hyderabad district recorded 411% above normal.

Met officials noted that the rainfall was not very high when compared to the rainfall that is received during the southwest monsoon. However, when such spells are recorded in so short a duration in October, these are largely associated with low pressure systems and result in urban flooding.

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Why it was so severe

Normally, cyclones lose steam upon making their landfall. This particular system, however, clocked a long east-west track cutting across Andhra Pradesh, Telangana, north-interior Karnataka and Maharashtra.

“All these states experienced above-normal rain during the recent monsoon season. As a result, the soil in these regions has retained significant moisture content… The high moisture availability even on land propagated this large embedded system all along its path so far,” said Mrutyunjay Mohapatra, Director General, India Meteorological Department (IMD).

In addition, vertical wind shear — the result of a significant difference in wind speed between higher and lower atmospheric levels — helped the system maintain its intensity as a deep depression or a well-marked low pressure area even on land, he said.

Hyderabad rains, Hyderabad floods, Hyderabad flooding, Telangana rains, Telangana floods, Hyderabad news, city news, Indian Express Floodwater gushes through a street following heavy rains, at Falaknuma, in Hyderabad, Wednesday, Oct. 14, 2020. (PTI Photo)

There are two seasons — March-May and October-December — when cyclones or depressions form in the North Indian Ocean (the Arabian Sea and the Bay of Bengal). The latter season witnesses four of the five cyclones that form over this region every year. Thus, the formation of the present depression or low pressure system is common, Mohapatra said.

In fact, stronger systems — Cyclones Phailin (2013) and Hudhud (2014) — formed in the Bay of Bengal had made their landfall in October, when the withdrawal of the southwest monsoon was still in progress.

On its last legs

On Friday, the well-marked low pressure area lay over the east-central Arabian Sea, off the Maharashtra coast. By Sunday, IMD officials said, the system would strengthen into a depression while continuing to head westward. Fishermen have been advised to stay away from the Sea until October 19.

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