A fourth low-pressure system is likely to form over the southeast Bay of Bengal around Nov 13 and intensify into a depression within 48 hours. However, whether it will bring heavy northeast monsoon rain to Chennai and neighbouring districts remains uncertain. Heavy rain forecast for Sivaganga, Ramanathapuram, Thoothukudi and Tirunelveli today as a low-pressure area may form over the Bay of Bengal on November 13.