On Monday afternoon, only hours before Sandy struck, the National Hurricane Center in Miami, Fla., upgraded its risk assessment for south Bellmore and south Merrick, saying there was a 10 to 20 percent chance the area could be hit by a storm surge of at least eight feet.

A storm surge is different than the wave action caused by a hurricane. Surge is a steady rise in sea level itself, according to the National Oceanographic and Atmospheric Administration, which oversees the National Hurricane Center. So, an eight-foot storm surge would mean a sea level rise eight feet above normal levels.

Storm surge is a critical factor in determining whether water from the Atlantic will inundate an area. Many parts of Bellmore and Merrick south of Merrick Road lie at or only a few feet above sea level, so even a four-foot storm surge could cause street flooding in those sections of the community.

Hurricane Sandy marched up the East Coast at 15 miles per hour into Monday afternoon, but accelerated to 29 mph at mid-afternoon as it neared the New Jersey shoreline, gaining energy as it went, according to the National Hurricane Center.

Sandy started to build in intensity early on Monday, increasing the certainty that it would bring hurricane-force wind gusts up to 85 mph, drenching rain and heavy tidal surges from Maryland-Delaware to Long Island and beyond. In its 5 a.m. briefing that day, the National Hurricane Center predicted that there was an 80 percent chance the South Shore could see tropical storm-force winds during the storm, and there was a 50 to 70 percent chance that coastal areas like south Bellmore and south Merrick would see a tidal surge of at least four feet.

“Reports from an Air Force Hurricane Hunter aircraft indicate that the maximum sustained winds have increased to near 85 mph, with higher gusts. The combination of an extremely dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters,” the hurricane center noted in its briefing.