What are the categories of monsoon?
A normal, or average, monsoon means rainfall between 96 and 104 percent of 89 cm, which is a 50-year average rainfall in four months starting in June.
The classification by the India Meteorological Department, says rainfall below 90 percent of the average would count as drought.
Rainfall above 110 percent of the average means an excessive monsoon, which is not as damaging as a drought, but could be potentially harmful for the yields of certain crops.
The monsoon season starts with rains on the southern Kerala coast around June 1, covering the whole country by the middle of July.
The monsoon delivers about 70 percent of India's annual rainfall and determines the yield of key crops such as rice, wheat, sugarcane and oilseeds such as soybeans. The farm sector accounts for about 15 percent of India's $2 trillion economy, but employs more than half of the country's 1.3 billion people.
If monsoon rains lift farm output, it can boost demand for consumer goods as it raises incomes of rural people.
A stronger economic outlook would lift equities, mainly for companies selling products in rural areas, including consumer goods, automobiles, fertilisers and pesticides.
Monsoon rains replenish reservoirs and groundwater, allowing better irrigation and more hydropower output. Higher rainfall can trim demand for subsidised diesel, which is used to pump water from wells for irrigation.
Food accounts for 50 percent of India's consumer price index, which the central bank closely monitors while deciding on monetary policy. A bumper farm output would keep food prices under control.
How reliable is the monsoon forecast?
The IMD issues its first forecast typically more than a month ahead of the monsoon's onset.
On an average, its forecast has been accurate only once every five years over the past two decades, even after taking into account an error band of plus or minus 5 percentage points.
The IMD's forecast for the 2017 monsoon was its most accurate since 2008. Last year, there was a difference of only 1 percentage point between the forecast and the actual rainfall. Even during normal monsoon years, some parts of India face drought, while some others suffer from floods.
The IMD will come out with a second forecast for the 2018 monsoon rains in June.
PM Modi, who has promised to double farmers' income over five years, remains popular nearly four years into his term. Farmer unrest, however, has flared in some states ruled by the BJP, catching regional leaders flat-footed.
A normal monsoon could lead to higher output of summer-sown crops, helping state leaders placate farmers.