What caused heavy rains in Tamil Nadu

FROM THE BLOG EDITION: http://www.downtoearth.org.in/blog/what-caused-heavy-rains-in-tamil-nadu-51998

DATE OF UPLOAD: 4th December 2015

An independent weather forecaster tries to explain the record-breaking December rains in Tamil Nadu

(NOTE: Views expressed in this blog are personal and they don't reflect the views of any organisation)

 

It wasn't a monsoon depression or a tropical cyclone. Then what was it? The 24-hour rainfall recorded in Chennai, parts of Tamil Nadu and Puducherry on December 1-2, 2015, raises this big question for researchers. Heavy rains and flooding potential in Chennai and other areas were anticipated, but the record-breaking rainfall figures have stumped everyone. 

 

In just 24 hours (till 08:30 am IST on December 2, 2015), Chembarambakkam, a lake southwest of Chennai, received a whopping rainfall of 475 mm. In the same period, Kattankulathur (near Chennai) received 445 mm rainfall, followed by 429 mm rainfall at Kattupakkam. The astonishing fact is that as per a report, 24-hour rainfall in at least three places in Tamil Nadu is more than maximum 24-hour rainfall of 380 mm recorded at Banki, Odisha during Tropical Cyclone Phailin. 

 

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24-hour rainfall in Tamil Nadu and Puducherry ending on 08: 30 am IST on December 2, 2015 (Courtesy: Vagaries of Weather)

 

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Daily mean rainfall in Tamil Nadu and Puducherry between October 1 and December 2, 2015. Fifty-eight per cent above normal rainfall has been recorded in this period (Courtesy: IMD RMC Chennai)

Chennai airport got 345 mm rain in a day whereas city-side got 294 mm rain. Rainfall statistics too present an unusually heavy rainfall picture with the star mark representing December's heavy rainfall event. 

 

Tropical cyclones are mostly responsible for bringing such heavy rainfall but Tamil Nadu's heavy rain wasn't caused by any tropical cyclone. It was caused by an ordinary low pressure system in the Bay of Bengal which performed extraordinarily. 

 

Monsoon low pressure systems 

 

India (in fact South Asia) experiences two monsoon seasons in a year. We are often familiar about the southwest monsoon season (June to September) which is a primary source of rain in most of India. For residents in parts of India (excluding southern) it's winter period right now. But, for southern parts of India such as Tamil Nadu, it's the real monsoon season. The state receives about 48 per cent of its annual rain in the northeast monsoon period (October to December). Coastal districts of the state get upto 60 per cent of their annual rain in this period. So, it's completely natural for rain in these areas at this time of the year and there isn't any "unseasonal" factor involved. 

 

Northeast monsoon (NEM) is known to be more variable (both in space and time) than the southwest monsoon (SWM). Technically, co-efficient of variation of NEM is 25 per cent as against SWM's 10 per cent which makes its forecasting even more challenging. The most important drivers of both these monsoon seasons are low pressures (known as monsoon low pressure systems), particularly those which form in the Bay of Bengal and move towards various parts of India. Every year, India witnesses many such monsoon low pressures some of which even intensify into monsoon depressions or even a tropical cyclone. Intensity-wise, tropical cyclones are the strongest systems followed by monsoon depressions and then monsoon low pressures. 

 

This year, around 11 low pressure systems were reported during the SWM season. Such monsoon low pressure systems usually have a life span of about 3-4 days and they bring heavy rain (often more than 100 mm in a day). This often inundates the areas through which they pass. But, when one compares Tamil Nadu low pressure system with other monsoon low pressure systems, some puzzling questions arise.  

 

How a weaker system brought so much rain? 

 

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Comparison of monsoon depression in central India (left) and Tamil Nadu monsoon low pressure (Image courtesy: University of Dundee)

 

 

To bring out the aforementioned question, I have compared it with one monsoon depression which was observed in central India during September 16-19, 2015. As per reports, more than 150 places in Maharashtra recorded more than 100 mm rain in a day because of this monsoon depression but there isn't any observation of rainfall as high as 475 mm of Chembarambakkam. Silod, a city in Aurangabad district of Maharashtra got 262 mm rain on September 17, while Vakadi (a place in Jalgaon district) got 220 mm rain. The low pressure system which brought rains in Tamil Nadu was very feeble (in terms of air pressure) as compared to the Septembersystem. The thumb rule is that the stronger a system is, the lower will be its air pressure and such a system will produce lot of rain under favourable conditions. From preliminary investigation, it looks like Tamil Nadu low pressure system had mean sea level pressure of around 1,008 millibar unlike around 997 millibar in the case of the September system. But naturally, maximum moisture content with the September system appears to be higher than that of Tamil Nadu system as reflected by total cloud water and total precipitable water data (which measures total atmospheric water vapour content in a vertical column of air between two levels). In the above image, compactness of clouds of monsoon depression (photographed on September 17) is another indicator of its strength over Tamil Nadu system (photographed on December 1). Thus, its definitely intriguing to see such a feeble system producing more rainfall than a powerful system. 

 

Internet is already flooded with several articles where without providing substantial evidences, people have linked this heavy rain event with factors like climate change or global warming. Below, we have a look at what could have made this low pressure system so powerful.

 

Variations in North-East Monsoon 

 

Dr. M Rajeevan, director of Indian Institute of Tropical Meteorology, Pune, and a senior monsoon researcher says that there is an increasing trend in the amount of northeast monsoon rainfall (NEMR) in the recent years and this rainfall posseses a lot of variability on inter-annual scale . But, he also agrees to the fact that it's not normal for a feeble low pressure (which impacted Tamil Nadu) to bring so much rain. He says that the total precipitable water for this system was about 1.4 times higher than the normal levels. According to him, even the weather models couldn't precisely capture the high rainfall associated with this system. He asserts that it will be too early to link this heavy rain event with climate change and more evidences will be needed to prove climate change's role. 

 

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21-year moving average of northeast monsoon (NEM) rainfall during 1901 to 2010. Dashed horizontal line indicates long-term average whereas recent increase in the rainfall is shown by red circle (Source: M Rajeevan et al 2012)

When asked about the influence of El Niño on Tamil Nadu rains, Dr. Rajeevan said that probability of getting a normal or above normal NEMR in an El Niño year is more than getting below normal NEMR. An analysis by Sreekala and others in 2011 states that there is 22 per cent chance of El Niño year being deficient NEMR as against 41 per cent chance of it being an excess NEMR year. So record El Niño in the Pacific is a sort of necessary but not sufficient condition to explain this year's excess NEMR. Between 2001 and 2010, there were four El Niño events (2002, 2004, 2006 and 2009). In 2002 and 2004, NEMR was below normal but in 2006 and 2009, it was slightly above normal. Also, in this decade, the statistical relationship between El Niño Southern Oscillation and NEMR has weakened as per Dr. Rajeevan. So El Niño's does have a role to play in this year's NEMR but its role isn't that straightforward. 

 

Role of vortex suspected 

 

Weather forecaster Rajesh Kapadia who writes a blog called "Vagaries of Weather" believes that Tamil Nadu heavy rains were caused by development of a vortex within the low pressure near surface levels. Vortex basically refers to rotating motion observed in a fluid (say air in this case). According to him, such vortex (or called vortices when in high number) developing within a low pressure can bring sudden heavy rain. Usually such vortex lasts till 3-4 hours after it's formed but in rare cases (like July 26, 2005, Mumbai rain), it can last even for a day which then brings such a huge rainfall. As such vortices have high unpredictability, weather models don't easily catch them which unnecessarily underestimates any rainfall forecast (say in the case of Chennai). 

 

A cocktail of weather conditions 

 

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GFS model map of winds at 20,000 feet above ground simulating the situation on December 1, 2015 (Courtesy: earth.nullschool.net)

 

The atmospheric setup on December 1, 2015, appears to be more complex than was anticipated. It's a cocktail of various atmospheric conditions. As can be seen from above image, a high pressure "H" (region of clockwise winds) anchored over eastern Bay of Bengal caused an inflow of moisture rich warm air from Bay of Bengal in Tamil Nadu. To represent this, I have used red dotted arrows. "C" refers to a cyclonic circulation (low pressure) which was observed east of Chennai. As monsoon has withdrawn from most parts of India, the air over central and northern parts of India is drier and colder than the warm moist air coming from southeast. Such an interaction of warm moist air from south-east and cold dry air from northeast crates a lot of instability in the atmosphere which is known to triggers strong thunderstorms. 

 

After a preliminary investigation, to me, Tamil Nadu's heavy rain event seems to be a result of a Mesoscale Convective System (MCS) which was pretty intense given the above unstable conditions. Mesoscale in meteorology refers to a scale or horizontal extent of about 1-100 km. We know about thunderstorms and some of which we come across are single cell thunderstorms (meaning their influence is restricted to a certain area only). Such thunderstorms mostly pop up during the afternoon hours, produce bad weather and die. But, when such individual thunderstorms cluster together, they become organised on a larger scale and can last for several hours. Such a large cluster of thunderstorms is known as MCS which is observed in many parts of world like United States of America. In satellite or doppler radar images, such MCS can be linear or round in shape (appearing sort of round in Tamil Nadu's case) and it often produces severe weather events such as heavy rains which lead to flooding. 

 

In Tamil Nadu's case, role of MCS can't be ruled out. In addition to this, development of vortices can't be ruled out as well which will be clear after investigation of doppler radar images. In the case of the September monsoon depression, such a large instability in the atmosphere or a cocktail of atmospheric conditions weren't observed. That's how Tamil Nadu low pressure system is different from other low pressure systems and perhaps this caused it to produce so much rain.

More rain for South India; storms return to central region

FROM THE NEWS EDITION: http://www.downtoearth.org.in/news/storms-to-return-to-central-india-rain-to-the-south-51919

(FIRST UPLOAD NOVEMBER 28TH 2015, UPDATED 30TH NOVEMBER 2015)

A new upper air trough has hit India, changing the weather conditions across the central and southern regions of the country

Thunderstorms are set to return in parts of Central India after they brought severe weather a month ago. A new upper air trough (see image below) is set to reach India, November 30 onwards, which will create conditions favourable for outbreak of storms. As the week progresses, this trough and associated thunderstorms will move eastwards, covering states like Rajasthan, Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Jharkhand and possibly Bihar. 

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Map showing winds in upper parts of troposphere on 1st December 2015. The axis of trough is shown as dashed red lines over Western India. (Courtesy- weatheronline.co.uk )

Troughs are elongated low pressure regions in Earth’s troposphere (lowermost part of atmosphere where weather systems exist) that are powerful enough to change a bright sunny morning into a stormy afternoon. 

The round of thunderstorms in Central India will begin from November 30, Monday. These will be experienced in the northern and western parts of Madhya Pradesh and South-eastern Rajasthan (Kota-Sawaimadhopur) for two days.

On December 2, a large region comprising of North-eastern Madhya Pradesh, Southern Uttar Pradesh and parts of Jharkhand and Bihar can get some rain. December 3 onwards, the weather will start normalising in Central India. These thunderstorms would be capable of bringing localised heavy downpours, lightning and some winds.

Possibility of hailstorms

The thunderstorms may turn severe, given the availability of required conditions. The freezing level (pressure level in troposphere with temperature of 0°C) has dropped below normal in Madhya Pradesh which will be ideal for occurrence of hailstorms. Hailstorms are anticipated in south-east Rajasthan (around Kota), Northern Madhya Pradesh (Guna-Gwalior belt) and neighbouring districts of Uttar Pradesh. They are expected to progress towards North-western Madhya Pradesh and adjoining parts of Uttar Pradesh on Wednesday.

As per agricultural experts, this round of rain will be beneficial to Rabi as it stands to improve the soil moisture. However, localised heavy downpours (or hailstorms, if they occur) will have a potential to damage the horticulture and harvested crops in these areas, especially Madhya Pradesh. After December 3, northerly winds will dominate in many parts of India including Central India and enable drop in temperature in these areas, which have been above normal in November.  

Wet week for Southern states

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District rainfall map of Tamil Nadu (1st October to 25th November period) showing districts with normal and excess rainfall. Courtesy- Indian Meteorological Department (Courtesy- Indian Meteorological Department)

Coastal and interior parts of Tamil Nadu recently witnessed heavy north-east monsoon rain which caused massive flooding. North-east monsoon period (October to December) is a major period of rain activity in Southern India and accounts for 48 per cent of Tamil Nadu’s annual rainfall as per a India Meteorological Department report.

Under the influence of a new low pressure system in southern Bay of Bengal, parts of Andhra Pradesh, Pondicherry and Tamil Nadu will witness rain in this week. Places in Tamil Nadu will get occasional moderate to heavy rain in this week that can create fresh water logging in cities like Chennai and add to existing problems due to flooding. There will be flooding potential in coastal areas of Andhra Pradesh as well.

Hailstorm lashes parts of Rajasthan, Madhya Pradesh and Uttar Pradesh

FROM THE NEWS EDITION: http://www.downtoearth.org.in/news/hailstorm-lashes-parts-of-rajasthan-madhya-pradesh-and-uttar-pradesh-51643 (DATED 29TH OCTOBER 2015)

Temperatures dip in a number of places, experts say occurrence good for Rabi crops

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Hailstorm in Jhansi area of Uttar Pradesh. According to weather experts, hailstorm at this time of the year is not a normal phenomenon Credit: Ritesh Garg

An upper air circulation brought on by western disturbance caused hailstorm in parts of eastern Rajasthan, Madhya Pradesh and Uttar Pradesh on Wednesday.

Lightning due to severe weather claimed six lives in Mathura, Aligarh, Allahabad and left over a dozen injured.

As per initial reports, hailstorm was reported from Bharatpur, Alwar, Jaipur and Sikar in Rajasthan. Available Doppler radar images taken on Wednesday afternoon indicated more incidents of hailstorm in isolated areas of east and north-east Jaipur.

Following hailstorm, temperatures dipped in Jaipur. The pink city recorded 21.7 mm rainfall on Wednesday and the minimum temperature touched 12.6°Celsius, 4°Celsius below normal, on Thursday morning.

The Bundelkhand region of Madhya Pradesh was severely affected. Areas like Ashoknagar, Shivpuri, Khurai, Malthon, Damoh, Chattarpur, Dabra, Gwalior, Anchal Kheda, Sevda and Datia witnessed severe hailstorm on Wednesday. Coin-sized hails fell for a long time in many areas of the state. Here also, temperatures registered a dip.

Jhansi, Mathura, Aligarh and Baghpat districts in Uttar Pradesh witnessed hailstorm mostly on Wednesday afternoon. Heavy rainfall was reported from Jhansi, Lucknow, Kanpur, Farrukhabad and Banda. Strong winds uprooted several trees and electric poles as a result of which telecommunication and power supply were disrupted.

Jhansi's maximum temperature on Wednesday touched 22.1°Ceslius, registering a departure of -10°Celsius from normal.

Was the hailstorm normal?

According to weather experts, hailstorm at this time of the year is not a normal phenomenon. If one looks at the climatological table (1971-2000) of these places, the mean number of days with hailstorm in October is zero.

What is even more striking is that as per climatology, Jhansi never gets hailstorm (the mean number of days with hailstorm is zero for all months).

However, observations clearly suggest that hailstorms are becoming more and more frequent. Since 2011, parts of Rajasthan such as Sikar and nearby areas have been experiencing hailstorm practically every year in the January-April period, but perhaps it is for the first time that it occurred in October.

Some amount of rainfall will be beneficial for Rabi crops at this time. According to Bharatiya Kisan Union general secretary Anil Malik, hailstorm at this time of the year will prove to be beneficial for Rabi crops. "It would have proved disastrous if the hailstorm had occurred during February and March," he said.

N Chattopadhyay, deputy director general of meteorology (agricultural meteorology), India Meteorological Department, said as the Rabi sowing period is going on, Wednesday's rains in parts of Uttar Pradesh and Madhya Pradesh will be beneficial for agriculture. Such rain helps in increasing soil moisture which is very important for proper growth of crops.

However, strong winds and hailstorm have reportedly damaged standing crops and vegetables in Madhya Pradesh and Uttar Pradesh. Some areas in these two states have witnessed back to back hailstorms since the past few years. Several areas of Madhya Pradesh were badly beaten by hailstorm during March-April this year.

Crop damage is a bad news for drought-hit farmers. This year, the south-west monsoon brought scanty rainfall in eastern Madhya Pradesh and Uttar Pradesh. The eastern part of Madhya Pradesh received 29 per cent below normal rainfall this time, eastern Uttar Pradesh received 47 per cent below normal whereas western Uttar Pradesh received 43 per cent below normal rainfall.

Fewer Doppler radars

Doppler radars play a vital role in detecting lightning and hailstorm. Based on these detections, people can be warned well in advance about such occurrences. Hence, for an efficient forecasting, there must be a high coverage of these radars. United States has an extensive grid of about 155 Doppler radars as against just 16 in India.

Out of these 16, three (in Hyderabad, Lucknow and Nagpur) are non-functional at present. Big states such as Madhya Pradesh and Uttar Pradesh have just two radars in Bhopal and Lucknow respectively. These installed (S band) Doppler radars work best only within a radius of 250 km.

As Lucknow's radar was non functioning on Wednesday, most areas in Uttar Pradesh (where both hailstorm and lightning were reported) were out of coverage area. Even Bhopal's radar could not properly track hailstorm beyond its 250-km range. In north-central Madhya Pradesh, large areas were out of coverage because of their distance from Bhopal's radar.

Last year during February-March when severe hailstorms hit many parts of India, more than half of the Doppler radars in India were not working. Some of them were broken like Nagpur's radar and others were undergoing maintenance. Thus, India has a long and unfortunate history of "out of order" radars.

India is not a weather-ready nation. As per records, lightning strikes killed over 2,500 people in India in 2014 as compared to 26 in the US during the same period.

Possibility of severe thunderstorms in parts of India

FROM THE BLOG EDITION : http://www.downtoearth.org.in/blog/possibility-of-severe-thunderstorms-in-parts-of-india-51602 (27TH OCTOBER 2015)

Existing weather system can even cause hailstorms in eastern Rajasthan, northern Madhya Pradesh and parts of Uttar Pradesh

This past weekend, an upper air trough (western disturbance) significantly impacted North India, especially Jammu and Kashmir. As anticipated, maximum temperatures dipped by 12-15°C in most places in the state on Sunday, October 25, due to rain and snow. Jammu-Srinagar highway was closed to traffic due to frequent landslides. Rain caused water logging in Srinagar on Sunday. Thundershowers were also reported in Punjab and Haryana. 

This upper air trough is set to move southwards till central India during this week and set the stage for thunderstorms and rain in parts of eastern Rajasthan, Madhya Pradesh, north-western Chhattisgarh, Uttar Pradesh, Vidarbha, Bihar and Jharkhand till Friday, October 30. 

What does this atmospheric pattern mean? 

 

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GIF showing the approach of 'western disturbance' (represented by the reddish patch) from Pakistan into north-central India between October 27 and 30, 2015 (Courtesy: COLA/IGES)

 

A section in my previous blog explains what upper air troughs are. Usually, such troughs don't dip as low as central India in October. The "unseasonal" rain India saw in February and March this year was also because these troughs were very intense and had dipped more than usual. Such troughs move from west to east and ahead of their arrival, they destabilise the troposphere (part of atmosphere where weather systems exist), creating conditions for thunderstorms. Under ideal conditions, these thunderstorms intensify into severe thunderstorms and are capable of producing heavy rain in a short time, strong lightning, hail and strong winds. 

Associated with such upper air troughs, which are typically observed at around 30,000 feet, are cyclonic circulations or low pressure regions with anticlockwise spin of air in the northern hemisphere, in the lower levels of troposphere (around 20,000 feet). These circulations typically develop because of upper air troughs and they move along with the troughs. The image above shows one such circulation coming from Pakistan to north-central India by October 30. This circulation will reach northern Madhya Pradesh by Wednesday, October 28, and travel south-east near Uttar Pradesh by Thursday, October 29.

What is expected from this setup? 

 

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Area circled in grey has the highest chance of witnessing severe thunderstorms on October 28 and 29

 

Such troughs are known to bring a lot of cold air to the region where they are present from regions which are to their north. Cold air is dense and reduces the height of the freezing level in that region. In the troposphere, the temperature of air decreases with an increase in height. Freezing level is that level in the troposphere which has a temperature of 0°C. Above that level, the temperature of air is negative and towards the Earth's surface, the temperature of air is positive.

If this level is further away from the Earth's surface, hailstones get enough time to melt after they have precipitated from thunder clouds. The positive temperatures between this freezing level and surface of the Earth are responsible for this melting. If this level is nearer the ground, hailstones reach the ground without evaporating much and what we get is hail. 

On Wednesday, October 28, severe thunderstorms capable of producing rain and lightning can be expected in eastern Rajasthan (areas east of Jaipur, Sawaimadhopur), parts of Madhya Pradesh (particularly around Jhansi-Gwalior) and parts of Uttar Pradesh (areas west of Kanpur-Allahabad) adjoining northern Madhya Pradesh and Rajasthan. The freezing level in these areas has reduced below normal and owing to high instability, some  thunderstorms in these regions can even produce hail. 

On Thursday, October 29, the region of thunderstorms and rain will shift slightly eastwards. Eastern Madhya Pradesh, north-western Chhattisgarh, eastern Uttar Pradesh (around Allahabad), and parts of Bihar and Jharkhand will get rain. Some areas may also get hail. People living in these areas must remain alert. 

Climatologically speaking (based on 1971-2000 data), these areas had not seen a single day of hail. Hence, if the anticipated hail arrives, it will be considered a bit unusual. 

Update on low pressure systems

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Approximate positions of low pressure systems (denoted by 'L') in the Arabian Sea and the Bay of Bengal according to a satellite image as of the afternoon on October 27 (Courtesy: India Meteorological Department)

According to IMD's update on Tuesday afternoon, the Arabian Sea low pressure has become a "well-marked low pressure" system. Data from the Thai Meteorological Department suggests that this system has a mean sea level pressure of 1006mb while the system in the Bay of Bengal has mean sea level pressure of 1008mb. But, as per present weather models, the low pressure in the Bay of Bengal isn't expected to intensify much due to unfavourable conditions. It will lead to the onset of northeast monsoon in parts of South India on October 28 and won't cause any threat to India. 

However, due to marginally favourable conditions, the Arabian Sea low pressure can intensify slightly as it moves towards Gulf of Aden. Present weather models don't expect this low pressure to track towards India and hence, it isn't expected to threaten India.

Rainfall, snow expected in northern India over weekend

From the blog edition: http://www.downtoearth.org.in/blog/rainfall-snow-expected-in-northern-india-over-weekend-51565

A variety of weather events will be witnessed in India in the coming week 

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The satellite image taken on October 23 shows the position of the western disturbance (circled in red) and the low pressure system (denoted by L) in the Bay of Bengal Credit: IMD

A powerful upper air trough (popularly called western disturbance by the India Meteorological Department) is all set to hit northern India by this weekend.

As a result of it, the hilly areas of Jammu and Kashmir, Himachal Pradesh and Uttarakhand will receive snowfall from Saturday (October 24) till Wednesday (October 28). 

The maximum amount of rainfall (to be witnessed in the low-altitude areas of these three states) and snowfall will be witnessed from Saturday to Monday. During this period, landslides will be witnessed on the Jammu-Srinagar highway, which will likely to remain shut for a few days.

Temperatures in much of the Kashmir Valley (including Srinagar) are already above the freezing point. Hence, the expected rainfall may create water logging and flooding. Travellers should remain cautious.

Along with rainfall and snowfall, thundershowers are also expected in Punjab, Chandigarh, parts of Haryana, Delhi and the National Capital Region from Sunday to Tuesday.

Arrival of winter

Winter has gradually started arriving in the hilly areas of north India. From Saturday onwards, maximum temperatures will decrease by an average 2-3° Celsius in Jammu and Kashmir, Himachal Pradesh, Punjab, Haryana and Chandigarh due to rainfall.

Rainfall will bring about a temporary rise in the minimum temperatures till October 26, but after that, minimum temperatures will significantly fall in these areas bringing colder nights.

Already several places in northern India are experiencing chilly conditions. Leh in Jammu and Kashmir recorded a minimum temperature of -1.9°Celsius on Friday morning.

Other places such as Gulmarg (J&K) recorded a minimum temperature of -1.4°Celsius (3° Celsius below normal), Srinagar recorded 5.5° Celsius, Pahalgam and Keylong (Himachal Pradesh) recorded 0.6° Celsius and Manali stood at 4.6° Celsius.

Not to be outdone, the plains in north India have also started witnessing mild winter conditions. Places in Punjab and Haryana are witnessing minimum temperatures in the range of 14-17°Celsius. Even Safdarjung in Delhi recorded a minimum temperature of 17.3°C on Friday morning.

Southern story

However, south tells a different story. If one travels a bit southwards, a contrast in the weather will be observed.

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The image shows maximum temperatures in several places across India on October 19, 2015. Red patches indicate maximum temperatures above 35°Celsius. Credit: RMC Delhi/IMD

The maximum temperatures in the past 15 days have been consistently in the range of 35-40°Celsius in states such as Maharashtra, parts of Karnataka, Telangana and Andhra Pradesh.

In western India, a few places in the Kutch area of Gujarat recorded maximum temperatures above 40°Celsius.

Mumbai is also "boiling" as was expected (read http://www.downtoearth.org.in/blog/tropical-depression-forms-in-the-arabian-sea-51447).

On October 16, a 43-year-old record was broken in Mumbai. The day was the hottest in October in Santa Cruz area. On this day, the maximum temperature stood at 38.6°Celsius, which was 5.2°C above normal. Earlier, 37.9°Celsius was recorded on October 23, 1972.

Generally, Mumbai is not known for so much heat, but the maximum temperatures have remained above 35°Celsius for several days. Places in Vidarbha are witnessing maximum temperatures in the range of 35-40°Celsius.

Such high temperatures are affecting Maharashtra's agriculture. The state's water stock is at its lowest in the past five years due to poor monsoon this year. 

Weather conditions will continue to stay hot in western and central India, as no significant change in weather conditions is anticipated in the remaining days of October.

The temperature contrast between north and south India is a typical feature the country witnesses every year in October after the south-west monsoon withdraws.

This year, the south-west monsoon withdrew from entire India on October 19, as per the IMD. Dry, northerly winds in north India are keeping the skies clear. This, in turn, is causing radiative cooling (a process in which heat from the Earth's surface is radiated back to the space). Central and western India witness high temperatures in October due to the prevalence of dry winds. The lack of any rain-bearing system during this period is also responsible for this situation. But, this time the duration of such high temperatures is more than normal.

Strong low-pressure system possible in the Bay of Bengal

At present, there are two low-pressure systems in the north Indian Ocean. One is in the Arabian Sea, off the shore of Lakshadweep and another is in the Bay of Bengal, off the shore of Sri Lanka.

Weather models suggest intensification of this system in the Bay of Bengal early next week. If we believe in the present forecast, the system will reach near the Tamil Nadu coast by Wednesday. It may bring heavy rainfall in the coastal areas of Tamil Nadu and Andhra Pradesh. With this, the much awaited north-east monsoon is anticipated to commence in southern India.

The southern peninsula (particularly the eastern coastal areas of South India) gets a large percentage of its annual rains because of the North-East Monsoon. Tamil Nadu is known to receive about 48 per cent of its annual rainfall between October and December, which is known as the North-East Monsoon period. There are several criteria for declaring the onset of the North-East Monsoon. IMD usually declares this around October 20, mainly after an increase in rainfall activity in the coastal areas of Tamil Nadu and Andhra Pradesh.

Also, we cannot rule out the possibility of the low pressure intensifying into a tropical depression or even a tropical storm. Hence, it will be monitored closely and an update will be given, if at all, it poses any significant risk for the south-east coastal areas of India.