# Presentation: 423159

## Refinements to Atlantic Basin Seasonal Hurricane Prediction from 1 December

**Refinements to Atlantic Basin Seasonal Hurricane Prediction from 1 December**

**Phil Klotzbach**
**33****rd**** Annual Climate Diagnostics and Prediction Workshop**
**October 21, 2008**

## Slide 2

## Gray et al. (1992)

**Gray et al. (1992)**

## Problems with Earlier December Forecast Schemes

-  Too many predictors utilized in forecasting each parameter – likely overfit the data
-  - Tenuous links between individual predictors and tropical cyclone activity during the following year’s hurricane season (e.g., QBO)
-  - Prediction scheme developed over the full data period – no “independent” testing done

**Problems with Earlier December Forecast Schemes**

## Linear correlation between previous year’s QBO and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)

**Linear correlation between previous year’s QBO and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)**

## Objectives

-  - To design this forecast utilizing NCEP/NCAR reanalysis
-  - To determine physical relationships between predictors utilized in the forecast and Atlantic Basin tropical cyclones

**Objectives**

## Methodology

-  - Add predictors to the forecast using a stepwise regression technique
-  - Only keep predictors that explain an additional 3% in NTC variance over the periods of 1950-1989, 1990-2007 and 1950-2007

**Methodology**

## October-November Correlation between SST and the following year’s Atlantic NTC

**October-November Correlation between SST and the following year’s Atlantic NTC**

## Slide 9

## Predictor 1 Physics (+)

**October-November SST (55o-65oN, 60o-10oW)**

- - Warm North Atlantic related to positive phase of the AMO
- - Strong auto-correlation between late fall North Atlantic SSTs and following summer/fall SSTs, both in the tropical and North Atlantic
- - Positive phase of the AMO is associated with a warm tropical Atlantic, low sea level pressures and reduced levels of vertical wind shear during the following August-October period

## Linear correlation between October-November SST in the North Atlantic (Predictor 1) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)

**Linear correlation between October-November SST in the North Atlantic (Predictor 1) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)**

## Predictor 2 Physics (+)

**November 500 mb Geopotential Height  (67.5o-85oN, 50oW-10oE)**

- - Predictor correlates at -0.73 with November Arctic Oscillation (AO) index
- - Negative AO values imply weaker westerlies, more blocking, a weaker Azores high, weaker trades and warmer tropical Atlantic SSTs
- - Warm tropical Atlantic SSTs, weaker vertical wind shear and lower pressures are clearly evident during the August-October period

## Linear correlation between November 500 mb heights in the far North Atlantic (Predictor 2) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)

**Linear correlation between November 500 mb heights in the far North Atlantic (Predictor 2) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)**

## Predictor 3 Physics (+)

**November SLP (7.5o-22.5oN, 175o-125oW)**

- - High pressure in the subtropical Northeast Pacific drives stronger trade winds encouraging mixing and upwelling – helping to initiate or prolong La Niña conditions
- - Strong trades also inhibit discharge of the warm pool in the western Pacific
- - La Niña is associated with anomalous upper-level easterlies during August – October, thereby reducing Atlantic basin vertical wind shear

## Linear correlation between November SLP in the subtropical Northeast Pacific (Predictor 3) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)

**Linear correlation between November SLP in the subtropical Northeast Pacific (Predictor 3) and the following year’s August-October sea surface temperature (a), August-October sea level pressure (b), August-October 200 mb zonal wind (c), and August-October 925 mb zonal wind (d)**

## Individual Predictor Correlations with Seasonal NTC

| Predictor | 1950-1989
(r) (40 Years) | 1990-2007
(r) (18 Years) | 1950-2007
(r) (58 Years) |
| --- | --- | --- | --- |
| 1 | 0.48 | 0.56 | 0.55 |
| 2 | 0.52 | 0.48 | 0.51 |
| 3 | 0.33 | 0.53 | 0.36 |

**Individual Predictor Correlations with Seasonal NTC**

## Stepwise Equation Regression Development with Seasonal NTC

| Predictor | 1950-1989
(r2) (40 Years) | 1990-2007
(r2) (18 Years) | 1950-2007
(r2) (58 Years) |
| --- | --- | --- | --- |
| 1 | 0.23 | 0.31 | 0.30 |
| 1, 2 | 0.32 | 0.45 | 0.38 |
| 1, 2, 3 | 0.40 | 0.53 | 0.46 |

**Stepwise Equation Regression Development with Seasonal NTC**

## Ranking Methodology

- - Rank all statistical hindcast values from 1950-2007
- - Assign observed NTC values based on the rankings assigned
- - Assign final hindcast NTC based on rank value
- - Hindcast NTC values restricted to range from 40 – 200

## Slide 19

## Top 15 NTC Hindcasts

**Top 15 NTC Hindcasts**

**Bottom 15 NTC Hindcasts**

**1950-2007**

**1950-2007**

**7 Major **
**Hurricane Landfalls**

**2 Major**
**Hurricane Landfalls**

**Florida Peninsula and East Coast**

**Florida Peninsula and East Coast**

## Summary and Conclusions

- - The scheme was developed over 1950-1989 and then tested over 1990-2007
-  - Utilizing these NTC hindcasts, one can issue United States landfall probability forecasts with skill well-exceeding climatology

**Summary and Conclusions**

- Full discussion of forecast model available in: Klotzbach, P. J., 2008:  Refinements to Atlantic basin seasonal hurricane prediction from 1 December.  *J. Geophys. Res., ***113, **D17109, doi: 10.1029/2008JD010047

## Questions???

**Questions???**