NOAA’s Seasonal Hurricane Forecasts: Climate factors influencing the 2006 season and a look ahead for Eric Blake / Richard Pasch / Chris Landsea(NHC) Gerry Bell / Muthuvel Chelliah (CPC) Stan Goldenberg (HRD) Todd Kimberlain (HPC) Eric Blake / Richard Pasch / Chris Landsea(NHC) Gerry Bell / Muthuvel Chelliah (CPC) Stan Goldenberg (HRD) Todd Kimberlain (HPC)

Outline Summary of seasonal forecast methodology Verification of 2006 Atlantic basin seasonal forecast Climate conditions during the season Reasons for the incorrect forecast Long-term verification Thoughts on 2007

NOAA Forecast Methodology 1)Assess states of the multi-decadal signal, El Niño, and Atlantic SSTs. 2)Use available CPC/CDC forecasts for El Niño/Atlantic SSTs, incorporate any analog techniques and assume persistence of upper- level conditions. 3)Predict range of overall activity and probabilities of above-, near-, and below- average seasons. 4)Qualitative/Quantitative process. 5)No forecast of hurricane landfalls, just the total seasonal activity for the entire basin.

A measure of seasonal hurricane activity: “Accumulated Cyclone Energy”, or “ACE” ACE is defined as the sum of the squares of the wind speed every six hours for all tropical storms, subtropical storms and hurricanes. One ACE unit is equal to one 100 kt system lasting for 6 hours. For comparison, a 50 kt system would have last for 24 hours to earn one ACE unit. Therefore, ACE is maximized for long-lived, major hurricanes, such as Ivan (2004).

Season and May 2006 Aug Observed Activity Type Outlook Outlook Activity Climatology Chance Above Average 80% 75% 33% Chance Near Average 15%20% Near Average 33% Chance Below Average 5% 5% 33% Tropical Storms Hurricanes Major Hurricanes ACE % of Median ~100 NOAA’s 2006 Atlantic Hurricane Outlooks

Vertical wind shear generally lower than average Lower than average Higher than average Tropical Storm genesis points during August-October 2006 (m/s)

August-October sea-surface temperature anomalies in the green box were the second warmest since 1871.

In mid-June the atmosphere made a rapid transition out of La Niña conditions Dry Wet More clouds and rain showers than average in the Atlantic Ocean. Less clouds and rain showers than average in the same areas.

Rapid El Niño warming after early August forecast

2006 ASO 200 mb anomalies for Velocity Potential and Divergent Wind versus (average activity (above average activity Note all of the converging wind vectors in the Green Box. Note the the lack of converging wind vectors in the Green Box.

Vertical Motion at 300 mb in the Atlantic Basin during August-October 2006 More rising than average More sinking than average Overall, too much sinking air was present in the Atlantic basin during the heart of hurricane season. The sinking process also produces drying and warming in the middle to upper troposphere, which can be detrimental for developing disturbances.

Stability and dry air over the tropical Atlantic Ocean and Caribbean Sea was greater than average during the hurricane season (figures courtesy CIRA) Stable Unstable Moist Dry

Available dynamical and statistical ENSO forecast models generally underdid the strength of the event, even in July. Observed

500 mb anomalies for September 2006 Large trough during September over the Central and Eastern US prevented any hurricane landfalls.

Current Conditions Atlantic SSTs are warmer than average. Early March 2006 Warmer than 2006, and nearly as warm as Early March 2007 Early March 2005

The demise of the 2006 El Niño

Climate Forecast System (CFS) –Coupled global model (T62L64) –Integrates for 10 months with 40 ensemble members using initial conditions. –Objective predictions of 200 mb streamfunction, vertical wind shear and SSTs. –Shown to have skill comparable with statistical models in Nino 3.4 SST hindcasts. –Some skill in forecasting year-to-year changes in important parameters that control hurricane variability.

CFS Seasonal Forecasts from March 7 Vertical Shear (much lower than average in the deep tropics) SST (generally warm in the deep tropics)

La Niña conditions possible!

February Nino 3.4 SST guidance JF

February 2007 SST anomalies °C°C

Correlations of Feb SSTs to next season’s ACE

Vertical Wind Shear anomalies (m/s) 25 kt! Higher Lower

Conclusions NOAA’s 2006 Atlantic seasonal forecast over-predicted the activity. Rapidly developing El Niño seems to have been the primary cause, forcing sinking air over the Atlantic basin, leading to more stable and drier conditions than usual. Extremely warm waters and favorable atmospheric conditions in the Atlantic (related to multi-decadal oscillation) during September prevented a below average year. Seasonal forecasts are still more skillful than climatology over the long run. The main source of forecast error is the lack of El Niño forecast skill in the summer months. If current conditions and trends persist, 2007 appears active and maybe very active. Official NOAA forecast comes out in mid-May.

Dry air in the middle to upper levels over the tropical Atlantic Ocean and Caribbean Sea was more pronounced than average during the hurricane season (figures courtesy CIRA) Dry Moist