This page details the scientific and operational foundation of the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model¹.

1. MODEL PHYSICS AND GRID ARCHITECTURE

SLOSH is a numerical-dynamic tropical storm surge model that solves the transport equations of motion integrated from the sea floor to the water surface¹². It is designed to compute storm surge heights across continental shelves, coastlines, and inland water bodies, allowing for inland routing and the overtopping of barriers such as levees, dunes, and roads¹.

The model utilizes a polar, elliptical, or hyperbolic Arakawa-B grid system². This continuously varying grid allows for high spatial resolution in localized bays and coastlines while stretching to a coarser mesh in deep ocean waters to minimize computational load¹².

Topographic and bathymetric data are referenced to a common datum, traditionally the National Geodetic Vertical Datum of 1929 (NGVD-29) or the newer North American Vertical Datum of 1988 (NAVD-88)². SLOSH incorporates various sub-grid scale features, including:

• 1-dimensional flow for rivers and streams³.
• Channel flow with chokes and expansions³.
• Cuts between barriers³.
• Increased wind friction coefficients for dense vegetation such as trees and mangroves³.

Amongst the more recent developments for SLOSH has been the introduction of spatially varying bottom friction based on the Manning-n coefficient over land. This is now done operationally for all regions except south Florida, which instead uses a constant coefficient based on the basin’s original testing⁴.

2. METEOROLOGICAL DRIVING FORCES

Because forecasting the exact wind field of a hurricane is highly complex, SLOSH relies on a simplified parametric wind model to generate surface stresses². This wind model balances surface forces and is driven by standard forecast parameters provided by the National Hurricane Center (NHC)¹²:

• Storm position as a function of time².
• Central pressure².
• Peripheral (ambient) pressure².
• Radius of maximum winds².

A constant drag coefficient is applied to compute the transfer of momentum from the air to the water, which reduces the model’s sensitivity to minor wind speed errors by creating compensating convergence effects in the wind field¹².

3. ADDITIONAL PHYSICS: TIDES AND WAVES

Astronomical Tides: The model accounts for finite amplitude effects. Historically, SLOSH did not dynamically calculate astronomical tides; forecasters approximated high tide by selecting a single constant water level, which could introduce errors if a storm made landfall at low tide⁵. Recent operational advancements have resolved this by coupling a time-varying tidal component directly with the storm surge⁵.

Using harmonic constants derived from the high-resolution ADvanced CIRCulation (ADCIRC) ec2015 Tidal database and adjusted via the Tidal Constituent And Residual Interpolation (TCARI) method, predicted tidal water levels are now calculated for each grid cell and were initially superimposed onto the surge via tide method 1⁵. While method 1 remains available, we now have method 3 (tidal forcing on the boundary) and the more commonly used method 2 (addition and subtraction of the tide field from the total water field every timestep for grid cells with bathymetry deeper than a configurable depth threshold) to simulate fully dynamic interactions.

Wind Waves: Short-period wind waves are generally omitted from core SLOSH computations to focus purely on the long-period gravity wave of the storm surge. SLOSH has historically been coupled to various wave models to account for wave set-up. SLOSH was first coupled to SWAN; however, that code was not added to operations due to it taking too long to run. More recently SLOSH has been coupled to a second-generation wave model from the Great Lakes and that has been placed in operations. However, because that code is currently unstable, it has also been omitted from this repository.

4. OPERATIONAL FORECAST PRODUCTS

SLOSH serves as the foundational model for several critical NWS storm surge guidance products:

• Real-Time Forecasting: SLOSH is run by the NHC when a hurricane threatens the coast, providing localized guidance for advisories and warnings².
• MEOW (Maximum Envelopes of Water): A composite of the highest surge values at each grid location generated from thousands of hypothetical hurricane simulations with the same category, forward speed, and direction².
• MOM (Maximum of the MEOWs): A composite of the absolute maximum surge heights for all simulated hurricanes of a given category, regardless of direction and speed². MOMs are widely used by FEMA and the U.S. Army Corps of Engineers for evacuation planning².
• P-Surge (Probabilistic Storm Surge): An ensemble of SLOSH forecasts that vary the storm’s speed, direction, intensity, and size based on historical NHC forecast errors to establish the probability of surge heights².
• P-ETSS (Probabilistic Extra-tropical Storm Surge): Applies the SLOSH shallow-water equations to extra-tropical cyclones². Instead of the parametric wind model, P-ETSS uses 10-meter surface winds and sea level pressures generated by the NWS Global Forecast System (GFS), Global Ensemble Forecast System (GEFS), and Canadian Global Ensemble Prediction System (GEPS)².

5. MODEL ACCURACY AND VISUALIZATION

When precise meteorological driving parameters (track, intensity, size) are provided, the SLOSH model predicts significant surge heights to an accuracy of approximately +/- 20%¹².

SLOSH Display Program (SDP)
SLOSH model outputs—specifically rex files containing time-lapsed snapshots of surge elevations and wind data—are visualized using the SLOSH Display Program (SDP)³. To learn how to render these animations, view historical storm data, and interrogate grid-specific surge heights, please visit the SLOSH Display Program (SDP) Documentation.

References