On NOAA’s Hurricane outlooks Muthuvel Chelliah Climate Prediction Center/NCEP

1. On NOAA’s Hurricane outlooks Muthuvel ChelliahClimate Prediction Center/NCEP NOAA's 33rd Climate Diagnostics and Prediction Workshop/CLIVAR Drought Workshop (CDPW), October 20-24, 2008 in Lincoln, Nebraska

2. NOAA has been issuing Long lead Hurricane outlooks for about a decade now. • The basis of these outlooks, lessons learned and some preliminary thoughts on science versus public expectations.

3. Hurricane seasons, in different ocean basins around the world, are “regular” seasonal/annual feature. • It happens every year, just like, say Summer/Winter Monsoon over Asia. • But how do the variability of the “basic characteristics” of these seasonal features compare? • Let’s very briefly compare the North Atlantic Hurricane season and the Indian summer monsoon.

4. Source: Monsoon On Line All-India Summer Monsoon (June-September) Rainfall (AISMR) Anomalies during 1871-2003 One of the worst summer monsoon failures and droughts in India in the past century. It is also the start of the famous “most severe Sahel drought in N.Africa” • Asian monsoon: REGULAR annual feature. • Will definitely happen every year. • Long-term Mean = 853 mm • Standard Deviation = 85 mm (about 10% of Mean) • Normal season: within 10% of Mean • Range~ 70% - 120% of Mean

5. 1951-2000 ACE: Mean: 93 Std:54 (58% of Mean) Range: 17-227 ACE: Accumulated Cyclone Energy (Bell et al. 2000) [similar to Bill Gray’searlierHDP: Hurricane Destructive Potential. ] - It is an accumulated sum of squared max. sustained wind speed every 6hours for all named storms (and hurricanes) for the whole season.

6. 2.2 • 1.1 • 0.9 • 0.7 • -0.8 MORE ACTIVE seasons? May be, Yes! Transition to generally MOREINACTIVE seasons. (exactly when ? ‘68? ‘70? ‘72? Late 60’s? Early 70s?) Sharp transition in 1995 to MORE ACTIVE seasons The reasons for this ‘sudden’ transition are less clear. But IT happened, and we have been in an ‘active phase’ since 1995 (but until when ?).This very fact is very much THE basis our NOAA’s hurricane outlooks. The atmospheric conditions associated with, or attributable to, this transition were better understood and discussed (Chelliah &Bell, 2004; Bell & Chelliah, 2006)

7. MAY OutlookAUG Outlook CPC’s ONI (Oceanic Nino Index) Year/Season A N B OBS A N B 1999 A A A 2000 75 50 25 A 4040 20 2001 35 45 20 N 10 50 40 2002 35 45 20 B 10 50 40 2003 55 35 10 PA60 35 5 2004 50 40 10 A4545 10 2005 70 20 10 PA95-100 2006 80 15 5 N 70 20 5 2007 75 20 5 N 85 10 5 2008 65 25 10 P85 10 5 NOAA’s Official Hurricane Outlooks (Probability Forecasts)

8. August 2006: Expected Activity - 75% chance above normal, 20% chance near normal, 5% chance below normal. .. Multi-decadal fluctuations in Atlantic hurricane activity Atlantic hurricane seasons exhibit prolonged periods lasting several decades of generally above-normal or below-normal activity. … ENSO is not expected to impact this hurricane season ( so, activity will be high) August 2007: Expected Activity -85% chance above normal, 10% chance near normal, 5% chance below normal Expected Climate Conditions – Active multi-decadal signal, La Niña-like pattern of tropical convection, warmer western tropical Atlantic and Caribbean Sea: The prediction for an above-normal 2007 hurricane season reflects the combination of two main climate factors: 1) the continuation of conditions that have been conducive to above –normal conditions since 1995 and 2) the continued La Niña-like pattern of tropical convection. All features of the multi-decadal signal are already in place across the tropical Atlantic, as is a persistent La Niña-like pattern of tropical convection across the eastern half of the equatorial Pacific. ( so, activity will be high) August 2008: Expected Activity -85% chance above normal, 10% chance near normal, 5% chance below normal Expected Climate Conditions – 1) The multi-decadal signal, which has been conducive to above-normal Atlantic hurricane activity since 1995, and which includes above-normal Atlantic sea-surface temperatures, and 2) a lingering La Niña signal as reflected in Pacific Ocean temperatures, convection and winds ( so, activity will be high)  so, activity will be high for every year may be appropriate for a “decadal type outlook”, but not for seasonal outlook, especially when the ‘causal factors’ and the influence of ENSO in a changing climate are not well known.

9. NOAA’s Atlantic hurricane season classifications -Official NOAA uses : 1) ACE index, combined with 2) the #s of named storms (NS), hurricanes (H), & major hurricanes (MH) to categorize hurricane seasons as being Above-Normal, Near-Normal, or Below-Normal. • Above-normal season: 1) An ACE index value well above 103 x 104 kt2 (corresponding to 117% of the median ACE value or 110% of the mean), OR 2) an ACE value slightly above 103 x 104 kt2combined with at least two of the following three parameters being above the long-term average: number of named storms, hurricanes, and major hurricanes. • Near-normal season:An ACE index value in the range 66-103 x 104 kt2 (corresponding to 75%-117% of the median or 71%-110% of the mean), OR an ACE index value slightly above 103 x 104 kt2 but with less than two of the following three parameters being above the long-term average: numbers of named storms, hurricanes, and major hurricanes. • Below-normal season:An ACE index value below 66 x 104 kt2, corresponding to below 75% of the median or 71% of the mean. • -------Definitions of Above/Normal/Below are complex, it should be more simple and objective, say based on std. alone of ACE. The results are not too different, if at all they are only slight that too only in “normal”

10. In closing …. • NOAA outlooks – from a public perspective may be ok. CPC is doing a good job. - Imagine if we had predicted a ‘normal or below normal season this year (2008) with all the landfalls and so on (which we do not predict anyway), - even though the media criticized the outlooks last couple of years at the end of the season, as the totals came in, but the public/media attention of scrutiny was not high during the season, due to lack of many landfalling systems. • So as we move to the second decade of NOAA hurricane outlooks, we have to not only worry about media, but also the science behind the outlooks, the scientific scrutiny, -Objective classification of seasons -Objective forecast methods (reproducible results, etc.) -More optimal use of high resolution GCM based guidance/outlooks for actual systems (TS/H/MH) and of course the ACE itself with proper bias corrections, which the Europeans have already begun to do and implement. The ensemble runs should give a good idea of the uncertainties in the outlook and hence a more objective probabilities based hurricane outlook.

11. ACE forecast by Objective methods using CFS hindcasts for the Entire season (from Hui Wang) • Root mean square error of ACE, in these forecasts ( whether it is from April (IC04) initial conditions or August (IC07) conditions) is higher than the total ACE itself in some years., suggesting probability outlook is of course the way to go thus clearly expressing the possible uncertainties.

14. CPC’s Oceanic Nino Index by season

15. 2002 Hurricane/Tropical Data for Atlantic Individual Storm Summary Winds in knots, pressure in millibars, category is based on Saffir-Simpson scale.