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Tuesday, September 21, 2021

Explained: Why this September could turn out to be Delhi’s wettest month ever

Delhi rains explained: The national capital has recorded nearly 70 per cent of its monsoon season's rainfall during 10 days of September. Here's what’s happening

Written by Anjali Marar , Edited by Explained Desk | Pune |
Updated: September 14, 2021 8:23:43 am
delhi rains, delhi weather forecast, delhi rain today, will it rain in delhi today, delhi news, delhi temperature, delhi monsoon, indian expressVehicles on a waterlogged road in New Delhi. (Express Photo: Amit Mehra)

In July, the rainfall over Delhi had remained dry and significantly rain deficient. But with the monsoon turning active due to multiple weather systems and favourable atmospheric conditions, the national capital recorded nearly 70 per cent of its monsoon season’s rainfall during 10 days of September. Here’s what’s happening.

What is the normal rainfall over Delhi for June to September period?

Delhi’s mean rainfall for the monsoon season is 553.8mm — 34 per cent of the rainfall occurs in August and is followed by July contributing 31 per cent, September with 22 per cent and June at 13 per cent. The mean annual rainfall for Delhi is 670.7mm of which 553.8mm (about 83 per cent) is received during June to September period.

As monsoon arrives towards the end of June, the normal rainfall for the month is anywhere between 62 to 64mm. Normal rainfall for July is around 193.5mm. For August it is anywhere between 182 – 200mm and September normal is about 115.6mm.

The maximum number of rainy days over Delhi ranges between 25 and 26 and the minimum number is 21 to 22 days during June to September period. Likewise, the maximum number of dry days over Delhi can range between 87 to 91 days in the monsoon season.

Annually, there are about a maximum of 33 to 35 rainy days and a minimum of 29 to 32 rainy days over Delhi.

delhi rains, delhi weather forecast, delhi rain today, will it rain in delhi today, delhi news, delhi temperature, delhi monsoon, indian express An underbridge is flooded due to the rains in Delhi. (Express Photo: Amit Mehra)

How much rainfall has Delhi recorded this year?

In June, Delhi recorded 29.6mm and ended 54 per cent below normal. After the monsoon onset, July rainfall improved and ended with 42 per cent surplus with 338.8mm. Being the month when the national capital records the season’s maximum rainfall, this year’s August was particularly deficient. Last month, Delhi’s rainfall stood at 214.5mm and 14 per cent deficient.

Why is this September exceptionally wet for Delhi?

The 24-hours rainfall recorded at Delhi (Safdarjung) was 112.1mm and 117.7mm on September 1 and 2, respectively. This is exceptionally high rainfall amount given that the state’s September normal rain is just 115.6mm.

Due to the 10 rainy days out of the total 13 days this month, Delhi’s rainfall figures for the month have touched 386.5mm (as on September 13). In a better perspective, this is about 70 per cent of the seasonal rainfall (553.8mm) of the state.

“The rainfall is mainly due to consecutive and persistent low pressure systems that formed in the Bay of Bengal in September,” said Dr K Sathi Devi, head, National Weather Forecasting Centre, New Delhi.

Such consecutive low pressures — the main rain-bearing systems during the monsoon season — was possible due to favourable large-scale features like the Madden Julian Oscillation (MJO) and an active west Pacific Ocean, she added. This was further supported by the position of the monsoon trough, which remained to the south of its normal position.

“This led to moisture incursion and strong incoming of the easterlies. All these together acted in favour and led to enhancement in rainfall activity over Delhi and the overall northwest India so far this month,” she said.

Hence, despite a delayed onset in July and a 14 per cent deficient August, Delhi has recorded excess rainfall as on September 13 with most of it coming during the first 13 days this month.

An analysis of past September rainfall using IMD’s data between 2011 and 2021 shows that September over Delhi (Safdarjung) remains largely dry. Daily rainfall of light to moderate intensity (2.4mm to 64.4mm in 24-hours) occurs on an average for 2 to 6 days in the entire month. However, 2018, 2014, 2011 and now in 2021.

Delhi received rainfall for 14, 9, 8 and 10** days, respectively, in September. The month’s total rainfall remained above normal only in 2018 (237.8mm) and 2011 (225mm) and in now 2021.

With more rainfall forecast in the coming days this week, September rainfall over Delhi could break all previous records. The current rainfall this month has made September 2021 the second wettest in 122 years for Delhi.

Highest rainfall in September (1901 – 2021)

delhi rains, delhi weather forecast, delhi rain today, will it rain in delhi today, delhi news, delhi temperature, delhi monsoon, indian express ** Progressive figures (Source: IMD)

Is Delhi rainfall showing any trends?

The IMD’s report titled ‘Observed Rainfall Variability and Changes over Delhi State’ stated that neither monsoon seasonal or annual rainfall have shown any significant increasing or decreasing trend during the past 30 years (1989 – 2018).

In these three decades, the highest June rainfall of 146mm was recorded in 2008, July was 456.2mm in 2003, August was 457.8mm in 1995 and of 325.3 mm in September received in the year 2010.

Central and New Delhi receive the maximum rainfall during the monsoons whereas East and Northeast Delhi areas record the lowest rainfall amounts in these four months.

What is the forecast for Delhi and neighbourhood?

A deep depression, lying over coastal Odisha close to Keonjhargarh, prevails. This system, which is expected to move west-northwestwards across north Chhattisgarh and Madhya Pradesh, will bring rainfall along its path before weakening into a depression by Tuesday.

However, its remnants will persist near north Madhya Pradesh and west Rajasthan and as such cause widespread rainfall over parts of Delhi and adjoining areas of northwest India starting mid-week, the IMD officials said.

Besides, the remnants of the earlier low pressure formed in the Bay of Bengal on September 6 — which on Monday evening lay over south Gujarat — continued to influence northwest India on the day. However, it will not sustain beyond 24 hours over the region, the IMD officials said.

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