The NOAA SLOSH Model

Robbie Berg, NOAA National Hurricane

Center

1/31/2012

NOAA Sea, Lake and Overland Surges from

Hurricanes (SLOSH) Model 1

The NOAA SLOSH Model

Robbie BergNOAA / National Weather Service

National Hurricane Center

Forum on Hydrodynamic Modeling in the Hudson River Estuary

New York City, New York

31 January 2012

The SLOSH ModelSea, Lake, and Overland Surges from Hurricanes

A numerical model developed by the National Weather

Service to estimate storm surge heights from historical,

hypothetical, or predicted storms.

Purpose Guidance for storm surge forecasts during real-time events

Assess vulnerability to storm surge

Basis for Hurricane Evacuation Studies

Since storm surge is dictated by many different factors, each

with their own uncertainties, SLOSH (nor any other surge

model for that matter) cannot answer the question: What is the exact amount of surge an approaching storm will produce at

specific locations?


Center

1/31/2012



Model Inputs

Parameterization of a tropical cyclone wind field (track, pressure, radius of

maximum winds [RMW])

Real-time forecast (real-time forecast information)

Historical storm (validation and training)

Synthetic storms (assess vulnerability)

A SLOSH basin

Consists of a grid within which are included geographic features such as

height/depth, trees, barriers, etc. used to control, route, and impede the flow

of water (topography and bathymetry)

Polar, elliptical, or hyperbolic grid centered on an area of interest

Grid is arranged to provide fine resolution in the primary area of interest, and

coarse resolution along the boundary region

Currently 37 operational basins covering at-risk coastal communities

Model Basics

SLOSH does include:

Flow through barriers, gaps, and passes

Deep passes between bodies of water

Inland inundation (wet or dry cells)

Overtopping of barrier systems, levees, and roads

Coastal reflection (coastally trapped Kelvin waves)

SLOSH does not include

Breaking waves and wave run-up

Astronomical tidal cycle

Synthetic runs can be initialized at varying levels to account for different points

within the tidal cycle or account for a tidal anomaly

Normal river flow

Precipitation


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1/31/2012



Model Geography and Resolution

New York Basin (NY3) Recently updated: reference changed from NGVD29 to

NAVD88, increased resolution, inclusion of new Lidar data,

hypothetical tracks include average and large storms

30,832 individual cells

18,595 overland cells

Average cell resolution: ~ 3.1 km2

Average overland cell resolution: ~ 2.2 km2

Minimum overland cell resolution: 214 m2

Minimum anywhere in U.S. is 66 m2

Only 4 other SLOSH basins have higher resolution

than NY3

Covers Hudson River continuously up to Albany

Butdoes not include freshwater riverine flow

To be used in operational forecasting, a storm surge model

must be fast and robust

NHC produces new forecasts every 6 hours The NHC Storm Surge Unit has roughly 30 min 1 hour to generate SLOSH

output and storm surge forecast based on the hurricane forecast concurrently

being created by the Hurricane Specialist

Probabilistic storm surge product must be available within

30 minutes of advisory release 2000 to 6000 individual SLOSH simulations run on a supercomputer

Operational Time Constraints


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Assumes a perfect forecast

NHC does not condone its use during a real-time event

Only relevant in the response stage

SLOSH ProductsDeterministic

Takes storms of a particular category, direction of motion, and point within the tidal

cycle, and moves them inland at all locations along the coast

Useful for preparations before a storm arrives

NY3 basin has 288 MEOWs user selects:

Direction of motion: NE, NNE, N, NNW, NW, WNW

Category: 1, 2, 3, 4

Forward speed: 10 mph, 20 mph, 30 mph, 40 mph, 50 mph, 60 mph

Point within tidal cycle: mean tide, high tide

Category 2, moving NNW at 30 mph, at high tide

SLOSH ProductsMaximum Envelope of Water (MEOW)

NOT the flooding footprint for one storm


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Takes storms of a particular category and moves them inland at all locations along the

coast in different directions at different speeds

Useful in pre-season planning and many days before a storm arrives

Basis for coastal evacuation zones

NY3 basin has 8 MOMs user selects:

Category: 1, 2, 3, 4

Point within tidal cycle: mean tide, high tide

Category 2 at high tide

SLOSH ProductsMaximum of MEOWs (MOM)

NOT the flooding footprint for one storm

2000 6000 possible storms based around actual forecast and

recent forecast errors

Statistics computed from this set of storms

Varied parameters: Cross track error (landfall location)

Along track error (timing)

Intensity error

Radius of maximum winds error

SLOSH ProductsProbabilistic Storm Surge (P-surge)


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SLOSH Products: When to Use

Tier 1 Tier 1

ResponseResponse

< 48 h of landfall

Tier 3Tier 3

PPlanning/Mitigationlanning/Mitigation

> 120 h of landfall

Tier 2Tier 2

ReadinessReadiness

48 h 120 h of landfall

* Always available from the SLOSH Display Program or NHC website

** Available on the NWS and NHC websites when a hurricane watch/warning is in effect

Validation

Hurricane Irene

Validation uses best track

of Irene removes the

influence of meteorological

uncertainty

Tidal constituent of gauge

data removed since SLOSH

does not include tide

SLOSH appears to have done

well with maximum surge

and timing in NY area

Reference:

C. Forbes and J. Rhome, 2011: An automated

operational storm surge prediction system for

the National Hurricane Center. Estuarine and

Coastal Modeling XII, M. L. Spaulding [ed],

ASCE, submitted.


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Validation

Jelesnianski, C. P., J. Chen, and W. A. Shaffer, 1992: SLOSH:

Sea, lake, and overland surges from hurricanes. NOAA

Technical Report NWS 48, National Oceanic and

Atmospheric Administration, U. S. Department of

Commerce, 71 pp.

Glahn, B., A. Taylor, N. Kurkowski, and W. A. Shaffer, 2009: The Role

of the SLOSH Model in National Weather Service Storm Surge

Forecasting. National Weather Digest, Volume 33, Number 1, 3-

14.

1992 study found higher surge forecasts have a slight high bias; slight low bias for lower surge

forecasts; 20% for significant surges

2009 study found more accurate predictions for surge higher than about 12-13 ft.

Still some low bias at lower surge predictions

The use of deterministic approaches does not account for

hurricane forecast uncertainty and is therefore of little use

for real-time storm surge forecasting Forecast uncertainties: track, intensity, forward speed, size, radius of

maximum winds

Ensemble approaches are necessary in order to fully model

and simulate the spectrum of uncertainty NHC recommends pre-generated surge maps and atlases (MOMs and

MEOWs) and/or a real-time ensemble system (P-surge)

The need for an ensemble technique necessitates a computationally efficient

model (runtimes of minutes vs. hours)

Concluding Remarks


Center

1/31/2012



Contacts

Robbie Berg

Hurricane Specialist

[email protected]

Jamie Rhome

Storm Surge Specialist

[email protected]

Acknowledgments: Michael Lowry, Tarah Sharon, Cristina Forbes, John Cangialosi

SLOSH Reference:

Jelesnianski, C. P., J. Chen, and W. A. Shaffer, 1992: SLOSH: Sea, lake, and overland surges from hurricanes.

NOAA Technical Report NWS 48, National Oceanic and Atmospheric Administration, U. S. Department of

Commerce, 71 pp.

http://www.nhc.noaa.gov/ssurge/ssurge_slosh.shtml

http://slosh.nws.noaa.gov

Documents

The NOAA SLOSH Model