Guwahati: Conditions are likely to become favourable for further advance of southwest monsoon over some parts of NE states during the next 4-5 days, IMD said on Tuesday. While incessant rain wreaked havoc in several parts of Assam and Meghalaya with landslides in last 24 hours, the Met office issued forecast of fairly widespread rainfall in northeast in the next seven days.The heavy rain warning persists over several NE states in the next four days mostly due to moisture incursion from the Bay of Bengal, Met officials said.In the last 24 hours, Williamnagar in Meghalaya recorded extremely heavy rainfall (between 23cm and 28 cm), whereas Mawsynram in Meghalaya received very heavy rain in the range of 17 to 19 cm. At 10 cm, Guwahati experienced heavy rainfall, RMC said in its bulletin.The RMC of the IMD has warned that thunderstorm and lightning, accompanied with gusty winds reaching up to 40-50 kmph, are very likely to occur at isolated places over Tripura on Wednesday. Thunderstorm and lightning, accompanied with gusty winds reaching up to 30-40 kmph, are very likely to occur at isolated places over Assam, Meghalaya, Manipur and Mizoram. While thunderstorm and lightning is very likely to occur at isolated places over Arunachal and Nagaland, heavy to very heavy rain is very likely to occur at isolated places over Meghalaya and heavy rain is very likely at isolated places over Assam and Tripura.