Weather Bee | Why IMD has forecast rain over most of India in the next five days
Monsoon’s arrival is no guarantee of rain. The forecast for the next week, therefore, details the immediate weather patterns responsible for rain
In a press release issued on June 26, the India Meteorological Department (IMD) has forecast some rain for almost all of India in the days leading up to June 30. In addition, it has issued warnings for isolated (around 25% stations of a region) heavy or very heavy rain in several regions for at least one day till June 30. These regions are Konkan and Goa, coastal Karnataka, south interior Karnataka, Kerala and Mahe, Tamil Nadu, Haryana, east Rajasthan, Uttarakhand, Uttar Pradesh, Bihar, Odisha, sub-Himalayan West Bengal and Sikkim, and all northeastern states. Why is this the case? The IMD has answered this question in its press release, but that answer is very technical. Here is an attempt at simplifying that answer.
Rain over large parts of India should not be surprising when the monsoon has already progressed over around half of the country and is expected to make more progress in the coming days. However, the monsoon’s arrival is no guarantee of rain, as current and past monsoon seasons are proof. The forecast for the next week, therefore, details the immediate weather patterns responsible for rain.
One of the patterns responsible for rain in the coming days, according to the IMD, is this: “A trough at mean sea level runs off Maharashtra-central Kerala coasts. A cyclonic circulation lies over central Gujarat in lower and middle tropospheric levels and a trough runs from this cyclonic circulation to east Vidarbha in lower tropospheric levels.” The second pattern is this: “A western disturbance as a trough in middle tropospheric westerlies runs roughly along Long. 64°E to the north of Lat. 28°N. A cyclonic circulation lies over northwest Rajasthan and a trough runs from this cyclonic circulation to Manipur in the lower tropospheric levels. Strong southerly/southwesterly winds prevail in the lower tropospheric levels from the Bay of Bengal into east and northeast India.”
While the reasons described by IMD may look complicated, even a casual reading should show that only two features are largely being described: troughs and cyclonic circulations. What are these features? Simply put, a trough is an area of relatively low pressure — the term generally used if the area is elongated — surrounded by areas of relatively higher pressure. Cyclonic circulation, on the other hand, is an area where the air is moving in a way similar to cyclones, but not as fast as in a cyclone. In the northern hemisphere, this means air moving in an anti-clockwise direction. Unsurprisingly, just like cyclones, such circulation happens around an area of relatively low pressure, such as troughs.
Both of these two features can be observed to some extent in the map shown below, which shows a forecast for mean sea level pressure (MSLP) and wind at a height of 850 mb (lower troposphere or around 1.5 kilometres above sea level) at 11:30 AM on June 26. The reason why forecasters look at these particular parameters is complicated, but the reason why troughs and cyclonic circulation cause rain are simple. Troughs and cyclonic circulations allow moisture to rise up, similar to the popular image of a cyclone. As this moisture rises, it cools down and brings rain. In addition, the map also shows southerly/southwesterly winds (those blowing from the south or southwest) from the Bay of Bengal into east and northeast India, as IMD’s forecast says, which is likely to bring moisture and rain into these regions. To be sure, these features are also not a guarantee of rain but usually support it.
An even better understanding of the map shown above can be gleaned from looking at the same parameters — MSLP and 850 mb wind — ten days ago, when rain was falling in only select places. This map shows higher pressure over most of India, fewer troughs, less cyclonic circulation, and slower winds. This is why there were fewer storms and less rain ten days ago.
Abhishek Jha, HT’s assistant editor-data, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.
In a press release issued on June 26, the India Meteorological Department (IMD) has forecast some rain for almost all of India in the days leading up to June 30. In addition, it has issued warnings for isolated (around 25% stations of a region) heavy or very heavy rain in several regions for at least one day till June 30. These regions are Konkan and Goa, coastal Karnataka, south interior Karnataka, Kerala and Mahe, Tamil Nadu, Haryana, east Rajasthan, Uttarakhand, Uttar Pradesh, Bihar, Odisha, sub-Himalayan West Bengal and Sikkim, and all northeastern states. Why is this the case? The IMD has answered this question in its press release, but that answer is very technical. Here is an attempt at simplifying that answer.
Rain over large parts of India should not be surprising when the monsoon has already progressed over around half of the country and is expected to make more progress in the coming days. However, the monsoon’s arrival is no guarantee of rain, as current and past monsoon seasons are proof. The forecast for the next week, therefore, details the immediate weather patterns responsible for rain.
One of the patterns responsible for rain in the coming days, according to the IMD, is this: “A trough at mean sea level runs off Maharashtra-central Kerala coasts. A cyclonic circulation lies over central Gujarat in lower and middle tropospheric levels and a trough runs from this cyclonic circulation to east Vidarbha in lower tropospheric levels.” The second pattern is this: “A western disturbance as a trough in middle tropospheric westerlies runs roughly along Long. 64°E to the north of Lat. 28°N. A cyclonic circulation lies over northwest Rajasthan and a trough runs from this cyclonic circulation to Manipur in the lower tropospheric levels. Strong southerly/southwesterly winds prevail in the lower tropospheric levels from the Bay of Bengal into east and northeast India.”
While the reasons described by IMD may look complicated, even a casual reading should show that only two features are largely being described: troughs and cyclonic circulations. What are these features? Simply put, a trough is an area of relatively low pressure — the term generally used if the area is elongated — surrounded by areas of relatively higher pressure. Cyclonic circulation, on the other hand, is an area where the air is moving in a way similar to cyclones, but not as fast as in a cyclone. In the northern hemisphere, this means air moving in an anti-clockwise direction. Unsurprisingly, just like cyclones, such circulation happens around an area of relatively low pressure, such as troughs.
Both of these two features can be observed to some extent in the map shown below, which shows a forecast for mean sea level pressure (MSLP) and wind at a height of 850 mb (lower troposphere or around 1.5 kilometres above sea level) at 11:30 AM on June 26. The reason why forecasters look at these particular parameters is complicated, but the reason why troughs and cyclonic circulation cause rain are simple. Troughs and cyclonic circulations allow moisture to rise up, similar to the popular image of a cyclone. As this moisture rises, it cools down and brings rain. In addition, the map also shows southerly/southwesterly winds (those blowing from the south or southwest) from the Bay of Bengal into east and northeast India, as IMD’s forecast says, which is likely to bring moisture and rain into these regions. To be sure, these features are also not a guarantee of rain but usually support it.
An even better understanding of the map shown above can be gleaned from looking at the same parameters — MSLP and 850 mb wind — ten days ago, when rain was falling in only select places. This map shows higher pressure over most of India, fewer troughs, less cyclonic circulation, and slower winds. This is why there were fewer storms and less rain ten days ago.
Abhishek Jha, HT’s assistant editor-data, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.