Shrayan Sen
West Bengal is currently in the midst of a significant weather phase as the state braces for what is expected to be the final intense spell of rains this monsoon season. The deep depression turned low-pressure area is expected to have a strong influence on the weather systems over entire eastern India, leading to enhanced moisture incursion and active monsoon conditions, particularly over northern districts of the West Bengal.
The deep depression, which has already weakened into a well marked low pressure area is positioned over Jharkhand. In the next two days it is likely to move further north northeastwards towards Bihar-Nepal border, very close to Sub-Himalayan West Bengal. As a result of which, the districts of Darjeeling, Kalimpong, Jalpaiguri and Alipurduar, is likely to experience enhanced rainfall activity till 6th October with isolated extremely heavy rainfall on 4th. Extremely heavy rainfall, in IMD terminology, refers to precipitation amounts exceeding 20 cm in a 24-hour period. Such intense rainfall episodes, especially in hilly terrain, can lead to serious impacts such as landslides, flash flooding in hill streams, and sudden rises in river water levels. Authorities have advised travellers and local administrations to remain on alert for possible disruptions, particularly in areas prone to slope failures and inundation.
Apart from the hills and foothills, the southern part of the state is also expected to receive moderate to heavy rain spells till 6th. The low pressure will attract huge amount of moisture from the Bay of Bengal that will pass through the districts of South Bengal, leading to these moderate spells of rain. Kolkata, Howrah, Hooghly, both North and South 24 Parganas, and parts of East Midnapore will receive rainfall mostly in the form of intermittent showers and thunderstorm activity, with a few short but intense spells that may lead to localised waterlogging.
Monsoon Withdrawal on the Horizon
As this rain-bearing system begins to weaken and move eastwards, conditions will start becoming favourable for the withdrawal of the southwest monsoon over West Bengal. Typically, the withdrawal process in India starts from the northwestern regions in late September and progresses southeastward over the following weeks. For West Bengal, monsoon withdrawal usually takes place in the first half of October.
This year, the monsoon has already withdrew itself of major parts of North India including Delhi. However back to back rain bearing low pressure systems has stalled its further withdrawal since the last week of September. But on a positive note, as the current rain bearing low pressure area moves eastwards, conditions will start becoming favourable for monsoon withdrawal.
The withdrawal of the monsoon is marked by a distinct change in the direction and nature of prevailing winds. Throughout the monsoon season, West Bengal is dominated by moist southerly to southwesterly winds from the Bay of Bengal. These winds are responsible for transporting moisture inland and sustaining the monsoon rainfall. Once the monsoon begins to retreat, these winds gradually weaken and are replaced by dry northerly to northwesterly winds that originate over the interior parts of the subcontinent.
This change in wind direction is a critical factor. The dry winds effectively cut off the moisture supply from the Bay, leading to a gradual reduction in cloudiness and rainfall. The skies begin to clear more frequently, humidity levels drop, and the overall weather turns drier and more stable. While daytime temperatures may remain on the warmer side during this transitional phase, night temperatures start to fall, especially in north Bengal and the adjoining plains. This drop in nighttime temperatures is often one of the first signs of the shifting season.
Once dry northerly flow persists over several days and rainfall activity remains subdued, the IMD formally declares the withdrawal of the monsoon from the region. This usually signals the end of the active monsoon season and the onset of the post-monsoon or retreating monsoon period, which is typically characterized by stable weather patterns and declining humidity.
Localised Thunderstorms Likely During Transition
Interestingly, the withdrawal phase is not entirely devoid of weather activity. In fact, meteorologists note that localised thunderstorms are quite common during this period of transition. This phenomenon occurs due to the intermingling of two contrasting wind systems: the retreating moist southwesterlies and the incoming dry northerly or northwesterly winds. When these opposing air masses meet, the resulting atmospheric instability can trigger the development of convective clouds, particularly during the afternoons and evenings.
These thunderstorms are typically short-lived but can be intense. They are often accompanied by gusty surface winds, frequent lightning, and sharp bursts of rain over a short duration. In urban areas, this can cause sudden but temporary waterlogging or disrupt local traffic. In rural areas, they are often welcomed as they mark the last spells of rainfall before the landscape transitions into the dry post-monsoon period.
Such transitional thunderstorms are considered a hallmark of the changing season. They usually occur for a brief window of time before the atmosphere stabilizes completely under the influence of persistent dry winds. Once this happens, the frequency of convective activity drops sharply, giving way to clearer skies, lower humidity, and calmer winds across most parts of the state.
Clearer Skies Ahead
If the current meteorological trends continue, it is expected that by the third week of October, most parts of West Bengal will experience a decisive shift to post-monsoon weather conditions. The moist southerly winds will be fully replaced by dry northerlies, leading to a marked decrease in humidity and rainfall. Skies will turn clearer, allowing for longer spells of sunshine during the day and cooler, more pleasant nights.
The transition to post-monsoon conditions is often seen as the arrival of the much-anticipated pre-winter season, bringing with it crisp mornings, comfortable evenings, and stable weather patterns. Farmers too look forward to this period as it allows for the harvesting of crops without the interruption of persistent rains.
While this final spell of rainfall is significant and needs to be watched carefully for potential hazards, particularly in north Bengal, it also signals the closing chapter of this year’s monsoon season. Once the system moves away and withdrawal is declared, West Bengal will enter its dry, stable post-monsoon phase, setting the stage for autumn and winter.
Thank you for the information.