EXTENDED RANGE FORECAST OF ATLANTIC SEASONAL HURRICANE ACTIVITY AND U.S. LANDFALL STRIKE PROBABILITY FOR 2006

 

We foresee another very active Atlantic basin tropical cyclone season in 2006.  However, we do not expect to see as many landfalling major hurricanes in the United States as we have experienced in 2004 and 2005. 

 

(as of 6 December 2005)

 

 

 

By Philip J. Klotzbach[1] and William M. Gray[2]

 

with special assistance from William Thorson[3]

 

 

 

This forecast as well as past forecasts and verifications are available via the World Wide Web at http://hurricane.atmos.colostate.edu/Forecasts


Brad Bohlander and Emily Wilmsen, Colorado State University Media Representatives, (970-491-6432) are available to answer various questions about this forecast

 

 

 

Department of Atmospheric Science

Colorado State University

Fort Collins, CO 80523

Email: amie@atmos.colostate.edu

 

 

 

 

ATLANTIC BASIN SEASONAL HURRICANE FORECAST FOR 2006

 

Forecast Parameter and 1950-2000

Climatology (in parentheses)

6 December 2005

 Forecast for 2006

Named Storms (NS) (9.6)

17

Named Storm Days (NSD) (49.1)

85

Hurricanes (H) (5.9)

9

Hurricane Days (HD) (24.5)

45

Intense Hurricanes (IH) (2.3)

5

Intense Hurricane Days (IHD) (5.0)

13

Net Tropical Cyclone Activity (NTC) (100%)

195

 

 

 

 

 

 

PROBABILITIES FOR AT LEAST ONE MAJOR (CATEGORY 3-4-5) HURRICANE LANDFALL ON EACH OF THE FOLLOWING COASTAL AREAS:

 

1)      Entire U.S. coastline - 81% (average for last century is 52%)

 

2)      U.S. East Coast Including Peninsula Florida - 64% (average for last century is 31%)

 

3)      Gulf Coast from the Florida Panhandle westward to Brownsville - 47% (average for last century is 30%)

 

4)      Above-average major hurricane landfall risk in the Caribbean

2005 Season update

 

Since the issuing of our forecast verification of November 18, three more tropical cyclones have formed in the Atlantic basin (Gamma, Delta and Epsilon).  Table A provides an update of Atlantic basin tropical cyclone activity for the 2005 season through December 4, and Figure A updates the tracks taken by all tropical cyclones that formed during the 2005 season.  A total of 26 named storms, 14 hurricanes and 7 major hurricanes have now formed during the record-breaking 2005 hurricane season.

 

 

 

Table A: Observed 2005 Atlantic basin tropical cyclone activity through December 4.

 

Forecast Parameter and 1950-2000

Climatology (in parentheses)

Observed 2005

 Tropical Cyclone Activity

Named Storms (NS) (9.6)

26

Named Storm Days (NSD) (49.1)

115.50

Hurricanes (H) (5.9)

14

Hurricane Days (HD) (24.5)

47.50

Intense Hurricanes (IH) (2.3)

7

Intense Hurricane Days (IHD) (5.0)

16.75

Net Tropical Cyclone Activity (NTC) (100%)

263

 

 

Figure A: 2005 Atlantic basin tropical cyclone tracks through December 4.

Notice of Author Changes

By William Gray

Beginning with this forecast, the order of the authorship of these forecasts has been reversed from Gray and Klotzbach to Klotzbach and Gray.  After 22 years (since 1984) of making these forecasts, it is appropriate that I step back and have Phil Klotzbach assume the primary responsibility for our project’s seasonal, monthly and landfall probability forecasts.  Phil has been a member of my research project for the last five years and has been second author on these forecasts for the last four years.  I have greatly profited and enjoyed our close personal and working relationships.

 

Phil is now devoting more time to the improvement of these forecasts than I am.  I am now giving more of my efforts to the global warming issue and in synthesizing my projects’ many years of hurricane and typhoon studies.

 

Phil Klotzbach is an outstanding young scientist with a superb academic record.  I have been amazed at how far he has come in his knowledge of hurricane prediction since joining my project five years ago.  I foresee an outstanding future for him in the hurricane field.  I expect he will make many new forecast innovations and skill improvements in the coming years.  I plan to continue to be closely involved in the issuing of these forecasts for the next few years. 


ABSTRACT

 

Information obtained through November 2005 indicates that the 2006 Atlantic hurricane season will be much more active than the average 1950-2000 season.  We estimate that 2006 will have about 9 hurricanes (average is 5.9), 17 named storms (average is 9.6), 85 named storm days (average is 49.1), 45 hurricane days (average is 24.5), 5 intense (Category 3-4-5) hurricanes (average is 2.3) and 13 intense hurricane days (average is 5.0).  The probability of U.S. major hurricane landfall is estimated to be about 55 percent above the long-period average.  We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2006 to be about 195 percent of the long-term average.  This forecast is based on our recently developed 6-11 month extended range statistical forecast procedure which utilizes 52 years of past data.  These include five selective measures of September-November North Atlantic and Pacific surface pressure and 500 mb height fields and a measure of the stratospheric quasi-biennial oscillation (QBO).  All predictors are calling for an active season.  Analog predictors have also been utilized.  The influences of El Niño conditions are implicit in these predictor fields, and therefore we do not utilize a specific ENSO forecast as a predictor.  We do not expect El Niño conditions during the 2006 season.  It is more likely that neutral or even La Niña conditions will develop. 

 

Acknowledgment


We are grateful to the National Science Foundation (NSF) and Lexington Insurance Company (a member of the American International Group (AIG)) for providing partial support for the research necessary to make these forecasts.  We also thank the GeoGraphics Laboratory at Bridgewater State College (MA) for their assistance in developing the Landfalling Hurricane Probability Webpage (available online at http://www.e-transit.org/hurricane).

 

The second author gratefully acknowledges valuable input to his CSU research project over many years by former graduate students and now colleagues Chris Landsea, John Knaff and Eric Blake.  We also thank Professors Paul Mielke and Ken Berry of Colorado State University for much statistical analysis and advice over many years. 


DEFINITIONS


1        Introduction


This is the 23rd year in which the CSU Tropical Meteorology Project has made forecasts of the upcoming season’s Atlantic basin hurricane activity.  Our research team has shown that a sizable portion of the year-to-year variability of Atlantic tropical cyclone (TC) activity can be hindcast with skill exceeding climatology.  These forecasts are based on a statistical methodology derived from 52 years of past data and a separate study of analog years which have similar precursor circulation features to the current season.  Qualitative adjustments are added to accommodate additional processes which may not be explicitly represented by our statistical analyses.  These evolving forecast techniques are based on a variety of climate-related global and regional predictors previously shown to be related to the forthcoming seasonal Atlantic basin tropical cyclone activity and landfall probability.  We believe that seasonal forecasts must be based on methods that show significant hindcast skill in application to long periods of prior data.  It is only through hindcast skill that one can demonstrate that seasonal forecast skill is possible.  This is a valid methodology provided that the atmosphere continues to behave in the future as it has in the past. 

A variety of atmosphere-ocean conditions interact with each other to cause year-to-year and month-to-month hurricane variability.  The interactive physical linkages between these many physical parameters and hurricane variability are complicated and cannot be well elucidated to the satisfaction of the typical forecaster making short range (1-5 days) predictions where changes in the momentum fields are the crucial factors.  Seasonal and monthly forecasts, unfortunately, must deal with the much more complicated interaction of the energy-moisture fields with the momentum fields. 

We find that there is a rather high (50-60 percent) degree of year-to-year hurricane forecast potential if one combines 4-5 semi-independent atmospheric-oceanic parameters together.  The best predictors (out of a group of 4-5) do not necessarily have the best individual correlations with hurricane activity.  The best forecast parameters are those that explain the portion of the variance of seasonal hurricane activity that is not associated with the other variables.  It is possible for an important hurricane forecast parameter to show little direct relationship to a predictand by itself but to have an important influence when included with a set of 4-5 other predictors. 

In a five-predictor empirical forecast model, the contribution of each predictor to the net forecast skill can only be determined by the separate elimination of each parameter from the full five predictor model while noting the hindcast skill degradation.  When taken from the full set of predictors, one parameter may degrade the forecast skill by 25-30 percent, while another degrades the forecast skill by only 10-15 percent.  An individual parameter that, through elimination from the forecast, degrades a forecast by as much as 25-30 percent may, in fact, by itself, show much less direct correlation with the predictand.  A direct correlation of a forecast parameter may not be the best measure of the importance of this predictor to the skill of a 4-5 parameter forecast model.  This is the nature of the seasonal or climate forecast problem where one is dealing with a very complicated atmospheric-oceanic system that is highly non-linear.  There is a maze of changing physical linkages between the many variables.  These linkages can undergo unknown changes from weekly to decadal time scales.  It is impossible to understand how all these processes interact with each other.  It follows that any seasonal or climate forecast scheme showing significant hindcast skill must be empirically derived.  No one can completely understand the full complexity of the atmosphere-ocean system or develop a reliable scheme for forecasting the myriad non-linear interactions in the full-ocean atmosphere system.

 

 

 

2        Early December Forecast Methodology

 

Our initial 6-11 month early December seasonal hurricane forecast scheme (Gray et al. 1992) demonstrated hindcast skill for the period of 1950-1990 but did not give skillful results when utilized on a real-time basis for forecasts between 1995-2001.  This was due to the discontinuation of the strong relationships we had earlier found between West African rainfall and the stratospheric quasi-biennial oscillation (QBO) with Atlantic basin major hurricane activity 6-11 months in the future.  We did not expect these relationships that had worked so well for 41 years to stop working from 1995 onward.  We do not yet have a good explanation as to why these relationships have failed.  We have discontinued this earlier 1 December forecast scheme and have developed a new 1 December forecast scheme.

Beginning with the 2002 December forecast for the 2003 season, we have relied on a new early December forecast scheme (Klotzbach and Gray 2004) which does not utilize West African rainfall and gives less weight to the QBO.  This new extended range forecast scheme shows significantly improved hindcast skill and better physical insights into why such precursor relationships have such an extended period memory.  The location of each of these new predictors is shown in Figure 1.  The pool of six predictors for the new extended range forecast is given in Table 1.  Strong statistical relationships can be extracted via combinations of these predictors (which are available by 1 December) and the Atlantic basin hurricane activity occurring the following year.

 

Figure 1: Location of predictors for our early December extended range statistical prediction for the 2006 hurricane season. 

 

 

 

Table 1: Listing of 1 December 2005 predictors for the 2006 hurricane season.  A plus (+) means that positive values of the parameter indicate increased hurricane activity the following year, and a minus (-) means that positive values of the parameter indicate decreased hurricane activity the following year.  All predictors are positive for an active season giving us increased confidence in forecasting a very active season.

 

Predictor

2005 Values for 2006 Forecast

1) November 500 mb geopotential height (67.5-85°N, 10°E-50°W) (+)

+0.6 SD

2) October-November SLP (45-65°N, 120-160°W) (-)

-0.4 SD

3) September 500 mb geopotential height (35-55°N, 100-120°W) (+)

+0.1 SD

4) July 50 mb U (5°S-5°N, 0-360°) (-)

-0.7 SD

5) September-November SLP (15-35°N, 75-95°W) (-)

-1.4 SD

6) November SLP (7.5-22.5°N, 125-175°W) (+)

+0.3 SD

2.1              Physical Associations among Predictors Listed in Table 1

 

The locations and brief descriptions of our 6-11 month predictors follow:

 

Predictor 1.  November 500 mb Geopotential Height in the far North Atlantic (+)

 

(67.5-85°N, 10°E-50°W)

 

Positive values of this predictor correlate very strongly (r = -0.7) with negative values of the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO).  Negative AO and NAO values imply more ridging in the central Atlantic and a warm North Atlantic Ocean (50-60°N, 10-50°W) due to stronger southerly winds during this period.  Also, on decadal timescales, weaker zonal winds in the subpolar areas (40-60°N, 0-60°W) across the Atlantic are indicative of a relatively strong thermohaline circulation.  Positive values of this November index (higher heights, weaker mid-latitude zonal winds) are correlated with weaker tropical Atlantic 200 mb westerly winds and weaker trade winds the following August-October.  The associated reduced tropospheric vertical wind shear enhances TC development.  Other following summer-early fall features that are directly correlated with this predictor are low sea level pressure in the Caribbean and a warm North and tropical Atlantic.  Both of the latter are also hurricane-enhancing factors. 

 

 

Predictor 2.  October-November SLP in the Gulf of Alaska (-)

 

(45-65°N, 120-160°W)

 

Negative values of this predictor are strongly correlated with a positive “Alaskan pattern” (Renwick and Wallace 1996) as well as a slightly eastward shifted positive “Pacific North American Pattern” (PNA) which implies reduced ridging over the central Pacific with increased heights over the western United States.  The negative mode of this predictor is typically associated with warm current eastern Pacific equatorial SST conditions and a mature warm ENSO event.  Low sea level pressure is observed to occur in the Gulf of Alaska with a weakening El Niño event (Larkin and Harrison 2002).  Negative values of this predictor indicate a likely change to cool ENSO conditions the following year.  Cool ENSO conditions enhance Atlantic hurricane activity. 

 

Predictor 3.  September 500 MB Geopotential Height in Western North America (+)

 

(35-55°N, 100-120°W)

 

Positive values of this predictor correlate very strongly (r = 0.8) with positive values of the PNA.  PNA values are usually positive in the final year of an El Niño event (Horel and Wallace 1981).  Therefore, cooler ENSO conditions are likely during the following year.  Significant lag correlations exist between this predictor and enhanced 200 mb geopotential height anomalies in the subtropics during the following summer.  Higher heights in the subtropics reduce the height gradient between the deep tropics and subtropics resulting in easterly anomalies at 200 mb throughout the tropical Atlantic during the following summer.  Easterly anomalies at 200 mb provide a strong enhancing factor for tropical cyclone activity. 

 

 

Predictor 4.  July 50 MB Equatorial U (-)

 

(5°S-5°N, 0-360°)

 

Easterly anomalies of the QBO during the previous July indicate that the QBO will likely be in the west phase during the following year’s hurricane season.  The west phase of the QBO has been shown to provide favorable conditions for development of tropical cyclones in the deep tropics according to Gray et al. (1992, 1993, 1994) and Shapiro (1989).  Hypothetical mechanisms for how the QBO effects hurricanes are as follows: a) Atlantic TC activity is inhibited during easterly phases of the QBO due to enhanced lower stratospheric wind ventilation and increased upper-troposphere-lower stratosphere wind shear, and b) for slow moving systems, the west phase of the QBO has a slower relative wind (advective wind relative to the moving system) than does the east phase.  This allows for greater coupling between the lower stratosphere and the troposphere. 

 

 

Predictor 5.  September-November SLP in the Gulf – SE USA (-)

 

(15-35°N, 75-95°W)

 

This feature is strongly related to the following year’s August-September sea level pressure in the tropical and subtropical Atlantic.  August-September SLP in the tropical Atlantic is one of the most important predictors for seasonal activity, that is, lower-than-normal sea level pressure is favorable for more TC activity.  Low pressure in this area during September-November correlates quite strongly with the positive phase of the PNA.  In addition, easterlies at 200 mb throughout the tropical Atlantic are typical during the following year’s August-September period with low values of this predictor.

 

 

Predictor 6.  November SLP in the Subtropical NE Pacific (+)

 

(7.5-22.5°N, 125-175°W)

 

According to Larkin and Harrison (2002), high pressure in the tropical NE Pacific appears during most winters preceding the development of a La Niña event.  High pressure forces stronger trade winds in the East Pacific which increases upwelling and helps initiate La Niña conditions which eventually enhance Atlantic hurricane activity during the following summer.  This predictor correlates with low geopotential heights at 500 mb throughout the tropics the following summer, indicative of a weaker Hadley circulation typical of La Niña conditions.  Also, high pressure in November in the tropical NE Pacific correlates with low sea level pressure in the tropical Atlantic and easterly anomalies at 200 mb during the following August through October period.  

 

2.2              Hindcast Skill

 

Table 2 shows the degree of hindcast variance explained by our new 1 December forecast scheme based on our 52-year developmental dataset between 1950-2001.  To reduce overfitting, the 1 December forecast picks the best combination of five predictors from a pool of six predictors or until the jackknife variance explained no longer increases.

 

 

Table 2:  Variance explained based upon 52 years (1950-2001) of hindcasting.

 

Variables Selected

Variance (r2) Explained

Jackknife (r2)

NS – 1, 2, 3

0.40

0.29

NSD – 1, 3, 4, 5, 6

0.45

0.28

H – 1, 2, 3, 4, 5

0.53

0.38

HD – 1, 2, 3, 4, 5

0.53

0.35

IH – 1, 2, 3, 4, 5

0.69

0.57

IHD – 1, 3, 4, 5, 6

0.51

0.41

NTC – 1, 3, 4, 5, 6

0.62

0.46

 

 

3                   Analog-Based Predictors for 2006 Hurricane Activity

 

Certain years in the historical record have global oceanic and atmospheric trends which are substantially similar to 2005/2006.  These years also provide useful clues as to likely trends in activity that the forthcoming 2006 hurricane season may bring.  For this early December extended range forecast, we project atmospheric and oceanic conditions for August through October 2006 and determine which of the prior years in our database have distinct trends in key environmental conditions which are similar to current October-November 2005 conditions.  Table 3 lists our analog selections.

 

We select prior hurricane seasons since 1949 which have similar atmospheric-oceanic conditions to those currently being experienced.  Analog years for 2006 were selected primarily on how similar they are to conditions that are currently observed such as very warm tropical and North Atlantic sea surface temperatures and neutral ENSO conditions.  In addition, we look for analogs with similar conditions to what we project for August-October 2006 including warm Atlantic sea surface temperatures, neutral to cool ENSO conditions and west phase QBO conditions. 

 

There were five hurricane seasons since 1949 with characteristics similar to what we observe in October-November 2005 and what we project for August-September 2006.  The best analog years that we could find for the 2006 hurricane season are 1961, 1967, 1996, 1999, and 2003.  We anticipate that 2006 seasonal hurricane activity will have slightly more activity than what was experienced in the average of these five years.  We believe that 2006 will be a very active season in the Atlantic basin. 

 

 

Table 3:  Best analog years for 2006 with the associated hurricane activity listed for each year.

 

Year

NS

NSD

H

HD

IH

IHD

NTC

1961

11

70.75

8

47.50

6

20.75

211

1967

8

58.00

6

36.25

1

3.25

93

1996

13

79.00

9

45.00

6

13.00

192

1999

12

78.50

8

41.00

5

14.25

182

2003

16

79.25

7

32.75

3

16.75

174

Mean

12.0

73.1

7.6

40.5

4.2

13.6

170.4

 

2006 Forecast

17

85

9

45

5

13

195

 

 

4                   ENSO

 

We believe that neutral or weak La Niña conditions are likely to be present during August-October 2006.  During the summer/early fall of 2005, sea surface temperatures have cooled in all Niño regions, and the Southern Oscillation Index (SOI) has been trending positive over the past few months.  Trade winds in the central Pacific have also been increasing in strength.  These features will likely contribute to keeping waters from becoming anomalously warm over the next few months.  In addition, most forecast models call for neutral conditions to persist for the next 6-9 months.  When the tropical Atlantic is warm and neutral or La Niña conditions are present, Atlantic basin hurricane activity is greatly enhanced. 

 

5                   Adjusted 2006 Forecast

 

Table 4 shows our final adjusted early December forecast for the 2006 season which is a combination of our derived full 52-year statistical forecast, our analog forecast and qualitative adjustments for other factors not explicitly contained in either scheme.  We foresee another very active Atlantic basin hurricane season.  We anticipate that ENSO will likely be neutral or slightly cool and will therefore play a minor enhancing role for the 2006 season.  Warm sea surface temperatures are likely to continue being present in the tropical and North Atlantic during 2006, due to the fact that we are in a positive phase of the Atlantic Multidecadal Oscillation (AMO) (i.e., a strong phase of the Atlantic thermohaline circulation).

 

Table 4:  Summary of our new early December statistical forecast, our analog forecast and our adjusted final forecast for the 2006 hurricane season.