KOLKATA: Kolkata could get the season's first pre-monsoon showers by mid-week next. The Met office is keeping close tabs on a low-pressure area brewing over the Bay of Bengal, which may also have a bearing on the monsoon onset in Bengal.

In the meantime, the city will continue to get scattered rain, which will keep the mercury in check.

A nor'wester spell that triggered 38.2 mm of rain on Wednesday night dragged the mercury down significantly below the normal mark. While the maximum temperature dipped from 36.3°C to 31.8°C, the minimum also scaled down to 22.4°C on Thursday. The two parameters were 3.6 and 4.5 notches, respectively, below the normal mark.According to a special bulletin from IMD, a low-pressure area is likely to form over the west-central and adjoining north Bay of Bengal around Tuesday next week and become well-marked in the next two days. "Light to moderate rainfall with isolated heavy rainfall is likely to commence on May 28 over coastal areas of West Bengal," said the bulletin.The Met office has issued a heavy rain warning for a few districts, including neighbouring North and South 24 Parganas and East and West Midnapore, on Wednesday.

"In Kolkata, we can expect light to moderate rain triggered by this likely low-pressure area," said Met scientist H R Biswas, head of the weather forecast section at Regional Meteorological Centre, Kolkata.Before the monsoon, Kolkata gets spells of rain known as the nor'wester, in which the clouds move over the city sky from land, mainly from the northwest. The pre-monsoon showers are triggered by cloud formation due to a low-pressure area over the Bay of Bengal.IMD has already forecast the onset of the southwest monsoon over Kerala within the next two to three days, while the normal date of arrival is June 1. Met officials said an error margin of four to five days is considered normal in Met parlance.RMC Kolkata officials said while there is no direct correlation between the Kerala onset date and its arrival in south Bengal, normally a 10-day gap is considered between the two onsets.

In Kolkata, June 11 is taken as the normal date for the monsoon's arrival. "We need to consider many factors for the monsoon onset in south Bengal after its arrival in Kerala. Often, the monsoon flow enters into a sluggish phase after its arrival in Kerala. There, we have to see the strength of the flow, how it sustains the flow, and other weather parameters for us to forecast its arrival in Bengal. We need to wait for its onset in Kerala," added Biswas.Another low-pressure area is developing over the Arabian Sea, which is likely to intensify into a depression. While the weather systems in the Arabian Sea have no direct impact over this region, Met officials said they can influence its advancement.