# Presentation: 476494

## Probabilistic Guidance for Hurricane Storm Surge (P-surge)

***Arthur Taylor and Bob Glahn***
***Meteorological Development Laboratory, National Weather Service***
***January 22, 2008***

## Hurricane Storm Surge Damage

**Hurricane Storm Surge Damage**

**Galveston 1900 – 6,000 to 12,000 deaths**
**Okeechobee 1928 – more than 2,500 deaths**
**Florida Keys, Labor Day 1935 – 408 deaths**
**New England 1938 – 600 deaths**
**Audrey 1957 – 390 deaths**
**Camille 1969 – 256 deaths**
**Hugo 1989 – 50 deaths**
**Opal 1995 – 9 deaths**
**Katrina 2005 – more than 1,800 deaths**

**Aerial Photo overlay of ****Katrina 2005 storm surge over ****Hancock County, Mississippi**

**“****The greatest potential for loss of life related to a hurricane is from the storm surge.”**

## Richelieu Apartments - Before Camille 1969

**Richelieu Apartments - Before Camille 1969**

## Richelieu Apartments - After Camille 1969

**Richelieu Apartments - After Camille 1969**

## Storm Surge Forecasting

**The Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model is the National Weather Service’s (NWS) operational hurricane storm surge model.**
**The NWS uses composites of its results to predict potential storm surge flooding for evacuation planning**
**The National Hurricane Center (NHC) begins operational SLOSH runs 24 hours before forecast hurricane landfall**
**The operational runs are based on a single NHC forecast track and its associated parameters.**
**When provided accurate input, SLOSH results are within 20% of high water marks.**
**Track and intensity prediction errors cause large errors in SLOSH forecasts and can overwhelm the SLOSH results.**

## Hurricane Ivan: A Case Study

## Probabilistic Storm Surge Methodology

**Use an ensemble of SLOSH runs to create probabilistic storm surge (P-surge) **
**Intended to be used operationally so it is based on NHC’s official advisory.**
**P-surge’s ensemble perturbations are determined by statistics of past performance of the advisories**
**Hurricane forecast errors which impact storm surge:**
**Cross track errors (impacts Location)**
**Along track errors (impacts Forward Speed)**
**Intensity errors (impacts Pressure)**
**Size of the storm errors.**

## Katrina Advisory 23

## Varying Katrina’s Tracks

**Varying Katrina’s Tracks**

-  Include 90% of possible cross track error (roughly 3 times the size of the cone of error).
-  Spacing based on size of the storm

## Varying the Other Parameters

**Size: Small (30%), Medium (40%), Large (30%)**
**Forward Speed: Fast (30%), Medium (40%), Slow (30%)**
**Intensity: Strong (30%), Medium (40%), Weak (30%)**
**The weight of a run is: cross track weight * along track weight * intensity weight * size weight**

## Is P-surge Statistically Reliable?

**If we forecast a 20% chance of storm surge exceeding 5 feet numerous times, then on 20% of those times storm surge should exceed 5 feet.**
**Create a reliability diagram comparing the ratio of occurrence with forecast probability.**
**Problem: Insufficient observations**
**Number of hurricanes making landfall is relatively small. **
**Observations are made where there has been surge. **
**340 observations for storms between 1998-2005**

## SLOSH Hindcast

**Used SLOSH hindcast runs for “observations”.**
**NHC used best historical information for input**
**Given accurate input, model results are within 20% of high water marks.**
**Advantage: **
**Uniform “observations” everywhere, even where there is little or no surge.**
**Disadvantage: **
**Same surge model used in analysis as in P-surge.**

## Reliability Diagrams for Forecasts > 5 feet

**48hr**

- 21589

- 5010

**36hr**

## Probability of > X feet of Storm Surge

**To calculate the probability of exceeding X feet:**
**For each cell, add the associated weights of the hypothetical storms whose maximum surge values are greater than X feet.**
**Example: **
**Five hypothetical storms have weights of 0.1, 0.2, 0.4, 0.2, and 0.1**
**The first two exceeded X feet in a given cell.**
**The probability of exceeding X feet in that cell is: 30% (0.1 + 0.2 = 30%)**

## Probability of > 5 feet of Storm Surge for Katrina Adv 23

## Height Exceeded by X percent of the Ensemble of Storms

**To calculate the height exceeded by X percent of the ensemble runs:**
**For each cell, find the surge value where the weights of the surge values which are higher add up to a value **_**<**_** X.**
**Example: **
**Five hypothetical storms have maximum surge values of 6, 5, 4, 3, 2 feet and respective weights of 0.2, 0.4, 0.1, 0.1, 0.2.  **
**The height exceeded by 60% of the ensemble is 4 feet, since the 6 foot value represents the top 20% of the storms, and the 5 foot value represents the next 40%.**

## Height Exceeded by 10% of the Ensemble for Katrina Adv 23

## http://www.weather.gov/mdl/psurge

**When is it available?**
**Beginning when the NHC issues a hurricane watch or warning for the continental US**
**As close to the  advisory release time as possible**

## Current Development

**We were “experimental” in 2007**
**The model is running in NCEP’s job stream.**
**The data are flowing to the National Digital Guidance Database (NDGD)**
**The data will soon be available to NWS forecast offices.**
**A decision will be made soon on becoming “operational” in 2008.**
**We continue to develop training material.**
**We continue to update the error statistics.**