Storm surge is an abnormal rise in water level, over and above the regular astronomical tide, caused by forces generated from a severe storm's wind, waves, and low atmospheric pressure. Storm surges are extremely dangerous, because they are capable of flooding large coastal areas.
If you are currently experiencing a storm surge event,
from a hurricane is due to storm surge.
Several factors contribute to coastal floods:
- Severe weather events create meteorological conditions that drive up the water level, creating a storm surge. These conditions include strong winds and low atmospheric pressure. They can be caused by hurricanes, by extratropical storms such as Nor'easters, or by other severe storm conditions.
- Large waves, whether driven by local winds or swell from distant storms, raise average coastal water levels and individual waves roll up over land.
- High tide levels are caused by normal variations in the astronomical tide cycle.
- Other larger scale regional and ocean scale variations are caused by seasonal heating and cooling and ocean dynamics.
Coastal floods are extremely dangerous, and the combination of tides, storm surge, and waves can cause severe damage.
Note: Coastal flooding is different from river flooding, which is generally caused by severe precipitation. Depending on the storm event, in the upper reaches of some tidal rivers, flooding from storm surge may be followed by river flooding from rain in the upland watershed. This increases the flood severity.
Storm surge is a leading cause of coastal floods
Very intense storms - like hurricanes - can generate large and devastating surge. Storm surge occurs when the winds push water toward the shore. The low pressure associated with intense storms has a small effect on surge as well.
The size of a storm surge for a particular location depends on a number of factors. Storm surge is very sensitive to the shape of the coast, and to changes in storm track, intensity, forward speed, and size. Tidal height at the time of maximum storm surge is an important factor, too. (The combined effect of the storm surge and the astronomical tide is called the storm tide.) The slope of the sea floor also influences the level of surge in a particular area. Areas with a shallow slope of the sea floor off the coast will allow a greater surge. Areas with a steeper slope will not see as much surge, but will generally have large breaking waves that can destroy lower elevation buildings near the coast and open bays.
Because of these factors, surge prediction does not correlate well with one single factor, such as the category of a hurricane, nor are past events likely to be good predictors of the surge caused by future storms.
Forecasting storm surge and coastal flooding is vital to protecting lives
The most severe consequence of coastal floods is loss of life; flood-related deaths are the biggest cause of natural hazard-related deaths in this country. In addition, property damage from floods can be economically devastating.
NOAA forecasts coastal flood conditions so that communities can take action.
NOAA's National Weather Service monitors coastal flooding conditions 24 hours a day, 7 days a week. They issue forecasts, watches, and warnings like Hurricane Local Statements. These forecasts, watches, and warnings provide details on a storm's impact on an area, such as the onset of winds, rainfall, and storm surge, and preparedness actions. Using information provided by local officials, the Weather Service alerts also include evacuation notices and locations of emergency shelters.
NOAA's National Ocean Service monitors and distributes real-time water levels, which are used to assess storm surge conditions at stations throughout the United States and its territories. NOAA issues website alerts on high water conditions caused by severe storms.
NOAA models storm surge to improve determinations of the risks to specific areas
Because there are many factors that affect surge, scientists have developed computational models based on the processes at work. These models allow NOAA to simulate many different hypothetical storms in order to understand the risk of coastal flooding.
NOAA's hurricane storm surge model is called SLOSH, for "Sea, Lake and Overland Surges from Hurricanes." The National Hurricane Center and local Weather Forecast Offices use SLOSH to assess coastal flooding risks and to predict storm surge heights. NOAA's extratropical storm surge model, ETSS, is also based on the SLOSH model. Four times a day, ETSS is used to create storm surge forecasts for the continental U.S. and Alaska.
The National Weather Service computes the influence of wind waves using the model WAVEWATCH III (R). This model covers the entire globe. WAVEWATCH III (R) is being coupled with high-resolution local wave models to increase detail along the coast.
With this suite of models, the National Weather Service provides estimates of the extent of storm surge flooding. The National Hurricane Center uses this information when issuing its hurricane advisories. Emergency managers and other officials use this information to determine which coastal areas should evacuate due to potential storm surge threats. NOAA also continues to develop new models that will improve predictions in the future.
Planning for hurricane or storm surge evacuation could save your life
NOAA is part of a federal partnership between FEMA and the U.S. Army Corps of Engineers that develops and updates evacuation plans for the U.S. coast. These plans are based on modeling thousands of possible storms in SLOSH, mapping the results over the landscape, and working with local emergency managers to determine the most efficient ways to move people from harm's way.
Areas Most At Risk
Coastal areas and barrier islands: Because they have few evacuation routes, barrier islands are especially vulnerable to hurricanes and flooding. Local officials will usually ask people on barrier islands and in vulnerable coastal areas to evacuate well in advance of a storm's landfall. If they ask you to evacuate, do so IMMEDIATELY!
Low-lying inland areas: It is not often clear which parts of a community are most vulnerable to floods, since they may not be closest to the coast; however, if they are low-lying or near a waterway, they may be at a higher risk than assumed. Listen carefully to local authorities to determine what threats you can expect and take the necessary precautions to protect yourself, your family, and your property.
What can you do? Here are some storm surge safety actions
- Know the types of hazards that could affect your family, and know your home's vulnerability to storm surge, floods, and wind. Find out if you live in an evacuation zone and keep track of which zone it is. If you live close to the floodplain, consider flood insurance.
- Make plans for where you'll go, preferably outside the vulnerable area, or consider the closest possible public shelter. Have a single point of contact for your family members to keep in touch. Make a plan now for what to do with your pets if you need to evacuate.
- Stock non-perishable emergency supplies and keep a disaster supply kit within reach.
- Prepare your home prior to leaving: board up doors and windows, secure or move all yard objects indoors, and turn off all utilities. Fill your car with gas, withdraw extra money from the ATM, and be sure to take all prescription medicines and special medical items, such as glasses and diapers.
- If you live in an evacuation zone and are ordered to evacuate by state or local officials, do so as quickly as possible. Do not attempt to cross flowing water. As little as six inches of water may cause you to lose control of your vehicle.
- Stay tuned to a local radio or television station and listen carefully for any advisories or specific instructions from local officials. Monitor your NOAA weather radio. Remember to replace its battery every six months, as you do with your smoke detectors.