Models and Observations
Modeling Storm Surge
NOAA utilizes SLOSH and P-Surge computational models for storm surge from tropical systems, ETSS and ESTOFS models for extratropical systems, and WAVEWATCH III® for modeling waves. The models allow NOAA to simulate many different storms in order to understand the risks involved and forecast storm surge.
The SLOSH (Sea, Lake, and Overland Surges from Hurricanes) model is the computer model developed by the National Weather Service for coastal inundation risk assessment and the prediction of storm surge. It estimates storm surge heights resulting from historical, hypothetical, or predicted hurricanes. SLOSH computes storm surge by taking into account a storm's atmospheric pressure, size, forward speed, track, and winds. The calculations are applied to a specific locale's shoreline, incorporating the unique bay and river configurations, water depths, bridges, roads, levees, and other features.
The model accounts for astronomical tides (which can add significantly to the water height) by specifying a constant tide level to coincide with landfall, but does not include rainfall amounts, riverflow, or wind-driven waves (those riding on top of the surge). The SLOSH model is the basis for the "hazard analysis" portion of coastal hurricane evacuation plans.
The SLOSH model is computationally efficient, resulting in fast computer runs. It is able to resolve flow through barriers, gaps, and passes and models deep passes between bodies of water. It also resolves inland inundation and the overtopping of barrier systems, levees, and roads. It can even resolve coastal reflections of surges, such as coastally trapped Kelvin waves.
However, the SLOSH model does not explicitly model the impacts of waves on top of the surge; it does not account for normal river flow or rain flooding; nor does it explicitly model the astronomical tide (although operational runs can be made with different initial water level anomalies). Future advancements in the SLOSH model will allow for the resolution of some of these limitations.
The SLOSH Display Program is software developed as a tool to aid emergency managers in visualizing storm surge vulnerability. The SLOSH model and the display program are two different tools. The National Weather Service uses the SLOSH model to forecast storm surge and model storm surge vulnerability; emergency managers and others use the display program to visualize the SLOSH data.
Graphical output from the model shows color-coded storm surge heights for a particular area in either feet above ground level (inundation) or feet above a specific reference level.
The SLOSH Display Program displays the results of the SLOSH model in order to assist emergency managers planning for evacuations and to educate decision-makers.
Numerical storm surge models depend on an accurate forecast of the hurricane’s track, intensity, and size. Even the best hurricane forecasts still have considerable uncertainty. The National Hurricane Center's forecast landfall location, for example, can be in error by tens of miles even during the final 12 to 24 hours before the hurricane center reaches the coast. These limitations can make the single, deterministic SLOSH surge forecasts incorrect. To help overcome these limitations, forecasters use probabilistic storm surge (P-surge) forecasts.
The Probabilistic Hurricane Storm Surge (P-Surge) model predicts the likelihood of various storm surge heights above a datum or above ground level based on an ensemble of SLOSH model runs using the official hurricane advisory. Graphical output shows:
- storm surge heights which have a certain probability of being exceeded
- the probability of storm surge exceeding a certain height
These storm surge heights and probabilities are based on the historical accuracy of hurricane track and wind speed forecasts, and an estimate of storm size. P-Surge also computes the probability of surge above ground to more clearly communicate where the surge will occur.
The graphical outputs created by P-Surge show tropical cyclone storm surge exceedance and the tropical cyclone storm surge probabilities (below).
The Tropical Cyclone Storm Surge Exceedance (Probabilistic Hurricane Storm Surge - Exceedance) graphical product for all of the Gulf or Atlantic coasts is always available on the Meteorological Development Laboratory website. The Tropical Cyclone Storm Surge Probabilities greater than 2-25 feet (Probabilistic Hurricane Storm Surge - Probabilities) graphical product is available on the National Hurricane Center website whenever a hurricane watch or hurricane warning is in effect.
Take all warnings and advisories seriously
Probabilities that may seem relatively small may still be quite significant. Even a small probability indicates there is a chance that danger may be heading your way. Consider the potentially immense cost (in lives and property) of not preparing for an extreme event, even if the chances are only 1 in 20 (5%) or 1 in 10 (10%).
To predict the surge accompanying an extratropical storm, the National Weather Service runs the Extratropical Storm Surge Model (ETSS). This model was developed by the Meteorological Development Laboratory and is a variation on the Sea, Lake and Overland Surges from Hurricanes (SLOSH) model, which is used for tropical (hurricane) storm surge forecasting. ETSS predicts storm surge flooding along U.S. coastlines, but is not able to predict the extent of overland flooding.
The ETSS model provides storm surge forecasts for extratropical cyclones. Because extratropical storms have larger time and length scales than tropical storms, the ETSS model uses the basic SLOSH model, but with winds and pressures generated from the Global Forecast System atmospheric model. The model performs well for both positive and negative surge events (a negative surge occurs when the winds are strong offshore, and the water level is driven below the normal level at that time). The model is particularly useful for forecasting storm surge associated with East Coast Nor’easters, and in western Alaska where storm surge associated with extratropical cyclones can devastate low-lying coastal communities.
Coverage for the model includes the east, west, and Gulf coasts of the continental United States and the Bering Sea and Arctic regions of Alaska. The Ocean Prediction Center website has area forecasts, while the Meteorological Development Laboratory website has station (point)-specific forecasts.
The ETSS model runs four times daily, out to 96 hours. The Extratropical Water Level Forecast website combines the model's results with tidal predictions and compares the results to observed water levels. It then combines the model results, the tidal predictions, and an adjustment from recent observations to create a graph of the total predicted water level. Presently, the ETSS model does not simulate tides, waves, or river effects on storm surge.
The Extratropical Surge and Tide Operational Forecast System, a new generation hydrodynamic modeling system, was developed for the Atlantic and Gulf coasts, using the ADvanced CIRCulation model. ESTOFS provides real-time forecasts of surges with tides.
ESTOFS provides water levels to WAVEWATCH III® for coastal surge + tide + wave predictions, using Global Forecast System forcing to create a 6-hour nowcast, followed by a 180-hour forecast, four times per day. The ESTOFS provides the National Weather Service with a second extratropical surge system that includes tidal simulations, in addition to the Extratropical Storm Surge model.
ESTOFS outputs the combined water level caused by the surge and tide, but also the surge and tide signals individually in order to enhance use by forecasters. ESTOFS output is available from the Ocean Prediction Center.
NOAA uses wind wave modeling to predict the evolution of waves caused by severe coastal storms which can cause flooding. The wave modeling simulations consider multiple factors to describe wave heights, periods, and directions. Coastal engineering efforts have led to the development of wind wave models specifically designed for coastal applications. The resulting wave hindcasts and forecasts help predict storm surge.
The National Weather Service computes the influence of wind waves using the WAVEWATCH III® numerical ocean-wave prediction computer model, which covers the entire globe. Experts couple the results with high-resolution local wave models to increase detail along the coast. WAVEWATCH III® provides important information about potentially dangerous surf conditions resulting from storms hundreds or even thousands of miles away. Using information from this model, forecasters from the NWS can alert the public to possible unsafe conditions.
The NOAA Environmental Modeling Center created WAVEWATCH III®. It provides consistent and reliable predictions of potentially dangerous wave heights, including those occurring with storm surge, that could have a devastating impact on lives and property along the shore. The model gives meteorologists and many other users a better understanding of coastal swells and surf conditions, and a better insight on probable effects from hurricane and tropical storm surge.
The model is run four times a day. Each run produces forecasts of every three hours from the initial time out to 180 hours (84 hours for the Great Lakes). The wave model suite consists of global and regional nested grids.