Storm surge is an abnormal rise of water generated by a storm, over and above the predicted astronomical tides. Storm surge should not be confused with storm tide, which is defined as the water level rise due to the combination of storm surge and the astronomical tide. This rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 20 feet or more in some cases.
Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm. The impact on surge of the low pressure associated with intense storms is minimal in comparison to the water being forced toward the shore by the wind.
There are three mechanisms that contribute to the storm surge
- The action of the winds piling up water (typically more than 85% of the surge)
- Waves pushing water inland faster than it can drain off. This is called wave set-up. Wave set-up is typically 5-10% of the surge
- The low pressure of a hurricane sucking water higher into the air near the eye (typically 5-10% of the surge)
The storm surge depends greatly upon the size and intensity of a weather system, the angle that it approaches the shore at, how deep the water is close to shore (the slope of the seabed at the coastline) and how fast the storm is moving.
Tips to help understand storm Surge:
- Storm surge is often the greatest threat to life and property from a hurricane. It poses a significant threat for drowning. A mere six inches of fast-moving flood water can knock over an adult. It takes only two feet of rushing water to carry away most vehicles— including pickups and SUVs.
- Storm surge can cause water levels to rise quickly and flood large areas—sometimes in just minutes, and you could be left with no time to take action if you haven’t already evacuated as instructed.
- Storm surge values do not correspond well to the hurricane wind categories (of the Saffir-Simpson Hurricane Wind Scale) that range from 1 to 5. These categories are based only on winds and do not account for storm surge.
- Tropical storms, category 1 or 2 hurricanes, major (category 3 to 5) hurricanes, and post-tropical cyclones can all cause life-threatening storm surge.
- Storm surge can also occur with non-tropical storms like Nor’easters and other winter storms.
- Many U.S. Gulf and East Coast areas are vulnerable to storm surge, including areas up to several miles inland from the coastline. Find out today, well before a hurricane ever approaches, if you live in a storm surge evacuation zone.
- Storm surge can occur before, during, or after the center of a storm passes through an area. Storm surge can sometimes cut off evacuation routes, so do not delay leaving if an evacuation is ordered for your area.
- During the peak of a storm surge event, it is unlikely that emergency responders will be able to reach you if you are in danger.
- Even if your community is not directly affected by storm surge, it could experience other hazards from the storm and face dangerous conditions such as impassable roads, water and sewage problems, and power outages. If power remains on, downed electrical wires can pose an electrocution risk.
- Weather conditions and the forecast can change. Local officials could issue evacuation or other instructions for many reasons. Always follow the instructions of local officials