Why the lull in Mumbai rains after early onset

Data procured from BMC shows that between Sunday and Monday morning, eastern suburbs received 2.90 mm rain followed by the Island City division at 1.28 mm rain and western suburbs where 0.76 mm rainfall was recorded.

This week is likely to fare better with India Meteorological Department (IMD) forecasting that Mumbai and its neighbouring districts are expected to witness a revival of monsoon currents. However, as of Monday, IMD’s Colaba coastal observatory recorded NIL rain while Santacruz station registered 0.3 mm drizzle. Amidst absence of rain, maximum temperatures in suburbs and the city are hovering between 33 –34 degree Celsius.

Furthermore, IMD data showed that between June 1 and 12, Mumbai city division registered a departure of 20 per cent from its long period average, which falls in the category of ‘deficient’. Falling in the normal category, meanwhile, Mumbai suburban district has also registered a departure of 16 per cent.

Why has rain eluded Mumbai after early onset?

As per senior IMD scientists, the south west monsoon which entered into Kerala on May 30, arrived ahead of scheduled onset in Mumbai owing to a combination of favourable conditions including interaction of winds and presence of a shear zone. For the record, while the typical date of onset is June 11 for Mumbai, this year monsoon onset was declared on June 9.

However, soon after, the shear zone weakened alongside weakening of monsoon currents.

Speaking to The Indian Express, Sushma Nair, IMD scientist said that westerlies, which bring rainfall, had also weakened. “The strength of the westerly winds had declined in the past week. The presence of westerlies imply presence of moisture, which thereby brings rainfall. Since the westerlies had weakened, there was no rainfall,” explained Nair.

Rajesh Kapadia from Vagaries of Weather said that usually when monsoon arrives in Mumbai there is an offshore trough which is absent currently. “Owing to the absence of a strong offshore monsoon trough along Maharashtra coast, the region is experiencing a lull. Furthermore, even the monsoon access trough which is normally present over central India is absent and is likely to develop after June 20–21.”

When can Mumbai expect a revival of rain?

After a fairly dry week, weather experts said Mumbai would experience an uptick in rain activity after June 19. Sunil Kamble, director of IMD Mumbai told Express that the intensity of the rain will be determined on the basis of the weather developments over the next couple of days. Nair said, “Monsoon currents have strengthened considerably and this week, Mumbai can expect at least moderate showers.”

For Wednesday, IMD has sounded a yellow alert indicating that heavy rains are likely to sweep pockets of the district. While no warning has been issued for thereafter, the city is slated to receive moderate rains throughout the rest of the week.

In what marked a harbinger for what lies ahead in the week, the city witnessed cloudy skies on Monday while the IMD upgraded to a yellow warning for the city.

What is the overall forecast on rain this season?

In its long range forecast for monsoon season, the weather bureau has indicated that the entire country is in for an above average rainfall this year.

In light of this, Mumbai is also expected to get above normal rains this season, which includes June. While Mumbai receives an average of 2,300 mm rainfall in four monsoons each year, senior IMD scientists said the city is likely to exceed this average figure.