Hurricane Ida made landfall just before 1 p.m. on Sunday near Port Fourchon in southeast Louisiana with sustained winds of 150 mph before weakening to a tropical depression Monday afternoon.

Prior to landfall, the National Hurricane Center warned that Hurricane Ida could bring “extremely life-threatening” storm surge flooding along the Louisiana coast, near the mouth of the Mississippi River.

Storm surges are abnormal rises in sea level generated by intense storms. Surges generally occur on the side of the storm where winds rotate into the shore. The strongest surges are where the winds are most powerful, usually near the storm’s eye.

Water is pushed toward land by strong wind and, to a lesser extent, by low pressure around the storm. The height of a storm surge can be affected by many factors, such as the size and speed of the storm, the angle of its approach, the shape of the coastline and the slope of the continental shelf.

Storm surges and large waves generated by hurricanes are among the greatest threats to life and property during a storm.