Storm Surge Modeling and Forecasting LTJG Jeffrey Pereira, NOAA Storm Surge Unit National Hurricane Center NOAA Storm Surge Workshop May 2011
Modeling Surge Statistical Utilize historical data to develop statistical relationships between surge and driving factors Necessary data is non-existent
Deterministic Numerical Models Forecast surge based on solving physical equations Strongly dependent on accurate meteorological input Current uncertainty in tropical cyclone forecasts render such methods inaccurate Numerical Model Ensemble
Many different runs of the same model but with different conditions (family of storms) Best approach for determining storm surge vulnerability for an area since it takes into account forecast uncertainty SLOSH Sea, Lake, and Overland Surges from Hurricanes
A computerized numerical model developed by the National Weather Service (NWS) to estimate storm surge heights (and winds) resulting from historical, hypothetical, or predicted hurricanes SLOSH SLOSH does include:
Flow through barriers/gaps/passes Deep passes between bodies of water Inland inundation (wet/dry cell) Overtopping of barrier systems, levees, and roads Coastal reflection (coastally trapped Kelvin waves) SLOSH does not include: Breaking waves/wave run-up
Astronomical tide Operational runs can be run at different tide levels via an initial water level (anomaly) Normal river flow and rain ADCIRC & SLOSH Overall flooding pattern very similar
Forecasting Surge All storm surge models are STRONGLY dependent on the accuracy of the meteorological input!!! Meteorological uncertainty will dominate over storm surge model specifications (physics, resolution, etc.) Different vertical datums/reference levels
Storm surge is only one component in the real water level rise. Total water rise = surge + tides + waves + freshwater flow Forecast Uncertainty
NHC TRACK ERROR 12 hr. OUT 133 mph, 933 mb. Hurricane Advisory Approximately 12 hr. before landfall Rmax=25 mi (forecast)
Surge Based on NHC -12 hr. Advisory ACTUAL TRACK TRACK FORECAST 133 mph, 933 mb. Actual Hurricane Track 30 mi. E of -12 hr. Advisory Forecast Track
Rmax=40 mi Surge Based on NHC Storm Best Track RMW = 25 mi., Average Size
RMW = 6 mi. Alternative to Single Runs Atlas of pre-computed surge maps based on: Different directions of motion Different landfall locations
Different intensities Different storm sizes Different forward speeds Ensemble Products 1. MEOWs Maximum Envelopes Of Water
2. MOMs Maximum Of the MEOWs 3. P-surge Probabilistic Storm Surge
MEOW Maximum Envelope Of Water Maximum Envelope Of Water User selects: 1)Category (Cat 3)
2)Landfall direction (wnw) 3)Forward speed (15 mph) 4)Initial tide (High) Maximum Envelope Of Water User selects:
1)Category (Cat 3) 2)Landfall direction (wnw) 3)Forward speed (15 mph) 4)Initial tide (High) Maximum Envelope Of Water
User selects: 1)Category (Cat 3) 2)Landfall direction (wnw) 3)Forward speed (15 mph) 4)Initial tide (High) MOM Maximum Of the MEOWs
MOM User selects: 1)Category (Cat 3) Probabilistic Storm Surge
Use an ensemble of SLOSH runs to create probabilistic storm surge (p-surge) Intended to be used operationally so it is based on NHCs official advisory P-surges ensemble perturbations are determined by statistics of past performance of the advisories
Error Incorporated in P-Surge Ensemble based on distributions of: Cross track error (impacts Location) Along track error (impacts Forward Speed, Timing) Intensity error (impacts Pressure) Rmax error (impacts Size)
Cross-Track Error Other Parameters Size: Small (30%), Medium (40%), Large (30%) Forward Speed: Fast (30%), Medium (40%), Slow (30%) Intensity: Strong (30%), Medium (40%), Weak (30%)
When is it Available? Whenever a hurricane watch or warning is in effect Available about 30 minutes after the
advisory release time Understanding/Using Probability The number one argument against using probability is that users do not understand how
to interpret low probabilities of an extreme event Would you offer to pick up free lunch if there is a 20% chance of you being involved in a fatal
car accident along the way? Deterministic SLOSH run shows limited surge threat to Pensacola area
Rmax=25 mi (forecast) Surge Based on NHC -12 hr. Advisory Probabilistic product shows considerable surge threat to Pensacola area
Actual storm caused highest surge in Pensacola area Surge Guidance Timeframe NHC Storm Surge Product Decision Support Wedge Tier 1 Response
< 48 h of landfall Tier 2 Readiness 48 h 120 h of landfall Tier 3
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