Written by Arghya ChakravortyA low-pressure area over the North Bay of Bengal is likely to strengthen and move northwestwards, with the India Meteorological Department (IMD) sounding red alerts for extremely heavy rainfall in parts of South Bengal and an orange alert for Kolkata and its neighbouring districts. The IMD has forecast heavy to very heavy rainfall of about 7-20 cm, with isolated extremely heavy rainfall of more than 20 cm, over parts of South Bengal on Thursday. South 24 Parganas and East Midnapore face the most severe threat, with a red alert for heavy to very heavy rainfall and isolated extremely heavy rainfall. On Thursday, the extreme rainfall risk moves westwards, with red alerts issued for parts of West Midnapore and Bankura. Cooch Behar is also expected to remain under yellow warnings, with heavy rainfall risk persisting later in the forecast period.