|National Weather Service|
Sea Lake and Overland Surge from Hurricanes (SLOSH)
The SLOSH model computes storm surge heights from tropical cyclones using pressure, size, forward speed, and track data to create a model of the wind field which pushes the water around. (A portion of the surge is caused by the low pressure at the center of the hurricane, but this effect is small in comparison to the wind effects.) A sample output can be seen here for Hurricane Hugo.
SLOSH has been applied to the entire U. S. East coast and Gulf of Mexico coastlines. In addition coverage extends to Hawaii, Guam, Puerto Rico, and the U. S. Virgin Islands (coverage). An example of a typical compuational domain, or basin, is the Biloxi, Mississippi basin.
Based on tide gage observations and high water marks (HWM) measured after past hurricanes, the SLOSH model is accurate to within +/- 20%. However, there is considerable error in the HWM data. Using higher quality, "excellent" HWMs from Katrina, SLOSH accuracy approached +/- 5%.
The SLOSH model consists of a set of equations derived from the Newtonian equations of motion (shallow water equations) and the continuity equation applied to a rotating fluid with a free surface. The equations are integrated from the sea floor to the surface. The coastline is represented as a physical boundary. Subgrid-scale water features (cuts, chokes, sills and channels), and vertical obstructions (levees, roads, spoil banks, etc.) are parameterized. The model accounts for astronomical tides by specifying an initial tide level, but does not include rainfall amounts, riverflow, or wind-driven waves.
At the time emergency managers must make an evacuation descision, errors in the hurricane's track and characteristics are large. To aid emergency managers in planning for hurricanes, the potential surge for an area is computed. This is done by running SLOSH with hypothetical hurricanes with various landfall directions and locations, Saffir-Simpson categories, forward speeds, sizes, and tide levels. Each individual run generates an envelope of high water containing the maximum value a grid cell attains during the run. These envelopes are then combined by taking the maximum by category, speed and direction to create MEOWs (Maximum Envelope Of Water), or simply by category to create MOMs (Maximum Of MEOWs). The MEOWs and MOMs form the basis of the "hazard analysis" portion of coastal hurricane evacuation plans.
SLOSH Display Program and Web Page
The SLOSH Display Program is intended to display the results of the SLOSH model in order to assist Emergency Managers plan for evacuations, display the latest NHC real-time runs and help them educate decision makers. It was built for the use of trained Emergency Managers, FEMA personnel, and NWS forecasters.
To access the SLOSH Display Web Page and download the program, you will
need a username and password, which can be obtained at this
website. After you have the
user name and password, you can follow