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. 

 

Forecast Parameter and 1950-2000 Climatology (in parentheses)

New Statistical Scheme

 

Analog

Scheme

Adjusted

Final

Forecast

Named Storms (9.6)

11.2

12.0

17

Named Storm Days (49.1)

64.5

73.1

85

Hurricanes (5.9)

7.7

7.6

9

Hurricane Days (24.5)

36.7

40.5

45

Intense Hurricanes (2.3)

3.2

4.2

5

Intense Hurricane Days (5.0)

8.4

13.6

13

Net Tropical Cyclone Activity (100%)

142.2

170.4

195

 

 

 

6                   Skill and Verification of 1 December Forecasts

 

We define forecast skill as the degree to which we are able to predict the variation of seasonal hurricane activity parameters above that specified by a long-term climatology.  The latter is expressed as the ratio of our forecast error to the observed difference from climatology or:

 

Forecast Error / Seasonal Difference from Climatology

 

For example, if there were a year with five more tropical storms than average and we had predicted two more storms than average, we would give ourselves a skill score of 2 over 5 or 40 percent.  By this measure, each of the seven parameters of our seasonal forecasts has shown some degree of skill from 1 December.  Table 5 shows our skill based on 52 years of hindcasts from 1950-2001, and Table 6 displays our skill score in real-time forecasting for the last seven years.  All parameters of our real-time forecasts have shown skill from 1 December.

 

Table 5:  Average percent of variation explained of 1 December hindcasts above that of climatology (in percent) for the 52-year period 1950-2001.  A value of 40 means that we hindcast 40 percent of the variability from climatology or that we were unable to explain 60 percent of the variability from climatology. 

 

Tropical Cyclone

Parameter

Early December

Hindcast Skill

NS

31

NSD

29

H

35

HD

37

IH

41

IHD

29

NTC

44

Table 6:  Last seven years’ (1999-2005) average percent of variation explained of our ‘real-time’ forecasts issued on 1 December above that of climatology (in percent).  A value of 30 means that we hindcast 30 percent of the variability from climatology or that we were unable to explain 70 percent of the variability from climatology. 

 

Tropical Cyclone

Parameter

Early December

Forecast Skill

NS

28

NSD

21

H

7

HD

20

IH

17

IHD

18

NTC

22

 

 

Another way to consider the skill of our forecasts is to evaluate whether the forecast for each parameter successfully forecast above- or below-average activity.  Table 7 displays how frequently our forecasts have been on the right side of climatology in hindcasts from 1950-2001 and in real-time forecasts for the past seven years (1999-2005).  Note that our early December scheme has been successful at determining whether various hurricane parameters will be above- or below-average over 70% of the time at the extended lead time of 1 December in both hindcasts and real-time forecasts.

 

Table 7:  The number of years that our tropical cyclone forecasts issued on 1 December has correctly predicted above- or below-average activity for each predictand in (A) hindcast mode (1950-2001) from Klotzbach and Gray (2004) and in (B) real-time forecast mode (1999-2005). 

 

Tropical Cyclone

Parameter

(A)

Hindcast

(B)

Forecast

NS

39/52

6/7

NSD

42/52

6/7

H

40/52

5/7

HD

37/52

5/7

IH

40/52

5/7

IHD

39/52

5/7

NTC

44/52

5/7

 

Total

 

281/364

 

37/49

 

Correct Prediction of Above

 or Below Climatology

 

 

77%

 

 

76%

 

 

Of course, there are significant amounts of unexplained variance in a number of the individual parameter forecasts.  Even though the skill for some of these parameter forecasts is quite low, there is a great curiosity in having some objective measure as to how active the coming hurricane season is likely to be.  Therefore, even a forecast that has shown only modestly skill in past years should be considered worthwhile when the only other information available is climatology.

 

 

7                   Landfall Probabilities for 2006

 

A significant focus of our recent research involves efforts to develop forecasts of the probability of hurricane landfall along the U.S. coastline.  Whereas individual hurricane landfall events cannot be accurately forecast months in advance, the total seasonal probability of landfall can be forecast with statistical skill.  With the observation that, statistically, landfall is a function of varying climate conditions, a probability specification has been developed through statistical analyses of all U.S. hurricane and named storm landfall events during the last 100 years (1900-1999).  Specific landfall probabilities can be given for all tropical cyclone intensity classes for a set of distinct U.S. coastal regions.

 

 

Figure 2:  Flow diagram illustrating how forecasts of U.S. hurricane landfall probabilities are made.  Forecast NTC values and an observed measure of recent North Atlantic (50-60°N, 10-50°W) SSTA* are used to develop regression equations from U.S. hurricane landfall measurements of the last 100 years.  Separate equations are derived for the Gulf and for Florida and the East Coast (FL+EC). 

 

Figure 2 provides a flow diagram showing how these forecasts are made.  Net landfall probability is shown linked to the overall Atlantic basin Net Tropical Cyclone activity (NTC; see Table 8) and to climate trends linked to multi-decadal variations of the Atlantic Ocean thermohaline circulation as inferred from recent past years of North Atlantic SSTA*.

 

Higher values of SSTA* generally indicate greater Atlantic hurricane activity, especially for intense or major hurricanes.  Atlantic basin NTC can be skillfully hindcast, and the strength of the Atlantic Ocean thermohaline circulation can be inferred from the value of SSTA* which is North Atlantic SST anomalies (in the region 50-60°N, 10-50°W) from current and prior years.  See our previous papers located online at http://hurricane.atmos.colostate.edu/Forecasts for further discussion of SSTA*.  The forecast relationship we use to make probability estimates for U.S. landfall is as follows:

 

Landfall Probability = Forecast NTC + Measured SSTA*    (1)

 

The current (November 2005) value of SSTA* is 69.  Hence in combination with a prediction of NTC of 195 for 2006, a combination of NTC + SSTA* of (195 +69) yields a value of 264.

 

As shown in Table 8, NTC is a combined measure of the year-to-year mean of six indices of hurricane activity, each expressed as a percentage difference from the long-term average.  Long-term statistics show that, on average, the more active the overall Atlantic basin hurricane season is, the greater the probability of U.S. hurricane landfall.  For example, landfall observations during the last 100 years show that a greater number of intense (Saffir-Simpson category 3-4-5) hurricanes strike the Florida and U.S. East Coast during years of (1) increased NTC and (2) above-average North Atlantic SSTA* conditions. 

 

Table 8:   NTC activity in any year consists of the seasonal total of the following six parameters expressed in terms of their long-term averages.  A season with 10 NS, 50 NSD, 6 H, 25 HD, 3 IH, and 5 IHD would then be the sum of the following ratios: 10/9.6 = 104, 50/49.1 = 102, 6/5.9 = 102, 25/24.5 = 102, 3/2.3 = 130, 5/5.0 = 100, divided by six, yielding an NTC of 107.

 

1950-2000 Average

1)

Named Storms (NS)

9.6

2)

Named Storm Days (NSD)

49.1

3)

Hurricanes (H)

5.9

4)

Hurricane Days (HD)

24.5

5)

Intense Hurricanes (IH)

2.3

6)

Intense Hurricane Days (IHD)

5.0

 

 

Table 9 lists strike probabilities for the 2006 hurricane season for different TC categories for the entire U.S. coastline, the Gulf Coast and the East Coast including the Florida peninsula.  The mean annual probability of one or more landfalling systems is given in parentheses.  Note that Atlantic basin NTC activity in 2006 is expected to be well above its long-term average of 100, and North Atlantic SSTA* values are measured to be well above average (69 units).  The long-period SSTA* average is 0.  During periods of positive North Atlantic SSTA, a higher percentage of Atlantic basin major hurricanes cross the Florida and eastern U.S. coastline for a given level of NTC.  U.S. hurricane landfall probability is thus expected to be well above average owing to both above-average NTC and above-average North Atlantic SSTAs.

 

Please visit our website at http://www.e-transit.org/hurricane for landfall probabilities for 11 U.S. coastal regions, 55 subregions and 205 coastal and near-coastal counties from Brownsville, Texas to Eastport, Maine.

 

Table 9: Estimated probability (expressed in percent) of one or more U.S. landfalling tropical storms (TS), category 1-2 hurricanes (HUR), category 3-4-5 hurricanes, total hurricanes and named storms along the entire U.S. coastline, along the Gulf Coast (region 1-4), and along the Florida Peninsula and the East Coast (Regions 5-11) for 2006.  The long-term mean annual probability of one or more landfalling systems during the last 100 years is given in parentheses. 

 

Coastal

Region

 

TS

Category 1-2

HUR

Category 3-4-5

HUR

All

HUR

Named

Storms

Entire U.S. (Regions 1-11)

91% (80%)

88% (68%)

81% (52%)

98% (84%)

99% (97%)

Gulf Coast (Regions 1-4)

74% (59%)

61% (42%)

47% (30%)

79% (61%)

95% (83%)

Florida plus East Coast (Regions 5-11)

64% (51%)

69% (45%)

64% (31%)

89% (62%)

96% (81%)

 

 

8        Is Global Warming Responsible for the Large Upswing in 2004-2005 US Hurricane Landfalls?

 

8.1       Background

 

The recent U.S. landfall of major hurricanes Dennis, Katrina, Rita and Wilma and the four Florida landfalling hurricanes of 2004 (Charley, Frances, Ivan and Jeanne) has raised questions about the possible role that global warming may be playing in these last two unusually destructive seasons.

 

The global warming arguments have been given much attention by many media references to recent papers claiming to show such a linkage.   Despite the global warming of the sea surface of about 0.3oC that has taken place over the last 3 decades, the global numbers of hurricanes and their intensity have not shown increases in recent years except for the Atlantic.

 

The Atlantic has seen a very large increase in major hurricanes during the last 11-year period of 1995-2005 (average 4.0 per year) in comparison to the prior 25-year period of 1970-1994 (average 1.5 per year).  This large increase in Atlantic major hurricanes is primarily a result of the multi-decadal increase in strength of the Atlantic Ocean thermohaline circulation (THC) which is not directly related to global temperature increase.   Changes in ocean salinity are believed to be the driving mechanism.  These multi-decadal changes have also been termed the Atlantic Multi-Decadal Oscillation (AMO). 

 

There have been similar past periods (1940s-1950s) when the Atlantic was just as active as in recent years.  For instance, when we compare Atlantic basin hurricane numbers of the last 15 years with an earlier 15-year period (1950-64), we see no difference in hurricane frequency or intensity even though the global surface temperatures were cooler and there was a general global cooling during 1950-64 as compared with global warming during 1990-2004. 

 

8.2       Discussion

 

There is no physical basis for assuming that global hurricane intensity or frequency is necessarily related to global mean surface temperature changes of less than ± 0.5oC.  As the ocean surface warms, so too does global upper air temperatures to maintain conditionally unstable lapse-rates and global rainfall rates at their required values.  Seasonal and monthly variations of sea surface temperature (SST) within individual storm basins show only very low correlations with monthly, seasonal, and yearly variations of hurricane activity.  Other factors such as tropospheric vertical wind shear, surface pressure, low level vorticity, mid-level moisture, etc. play more dominant roles in explaining hurricane variability than do surface temperatures.  Although there has been a general global warming over the last 30 years and particularly over the last 10 years, the SST increases in the individual tropical cyclone basins have been smaller (about half) and, according to the observations, have not brought about any significant increases in global major tropical cyclones except for the Atlantic which as has been discussed, has multi-decadal oscillations driven primarily by changes in Atlantic salinity.    No credible observational evidence is available or likely will be available in the next few decades which will be able to directly associate global surface temperature change to changes in global hurricane frequency and intensity. 

 

Most Southeast coastal residents probably do not know how fortunate they had been in the prior 38-year period (1966-2003) leading up to 2004-2005 when there were only 17 major hurricanes (0.45/year) that crossed the U.S. coastline.  In the prior 40-year period of 1926-1965, there were 36 major hurricanes (0.90/year or twice as many) that made U.S. landfall.  It is understandable that coastal residents were not prepared for the great upsurge in landfalling major hurricanes in 2004-2005.

 

We should interpret the last two years of unusual large numbers of U.S. landfalling hurricanes as natural but very low probability years.   During 1966-2003, the U.S. hurricane landfall numbers were substantially below the long-term average.  In the last two seasons, they have been much above the long-term average.  Although the 2004 and 2005 hurricane seasons have had an unusually high number of major landfall events, the overall Atlantic basin hurricane activity has not been much more active than five of the recent hurricane seasons since 1995 (i.e., 1995-1996, 1998-1999, 2003).  What has made the 2004-2005 seasons so unusually destructive is the higher percent of major hurricanes which moved over the U.S. coastline.  These landfall events were not primarily a function of the overall Atlantic basin net major hurricane numbers, but rather of the favorable broad-scale Atlantic upper-air steering currents which were present the last two seasons.  It was these favorable Atlantic steering currents which caused so many of the major hurricanes which formed to come ashore. 

 

It is rare to have two consecutive years with such a strong simultaneous combination of high amounts of major hurricane activity together with especially favorable steering flow currents.  The historical records and the laws of statistics indicate that the probability of seeing another two consecutive hurricane season like 2004-2005 is very low.  Even though we expect to see the current active period of Atlantic major hurricane activity to continue for another 15-20 years, it is statistically unlikely that the coming 2006 and 2007 hurricane seasons, or the seasons which follow, will have the number of major hurricane US landfall events as we have seen in 2004-2005.

 

 

9        Forecast Theory and Cautionary Note

 

Our forecasts are based on the premise that those global oceanic and atmospheric conditions which preceded comparatively active or inactive hurricane seasons in the past provide meaningful information about similar trends in future seasons.  It is important that the reader appreciate that these seasonal forecasts are based on statistical schemes which, owing to their intrinsically probabilistic nature, will fail in some years.  Moreover, these forecasts do not specifically predict where within the Atlantic basin these storms will strike.  The probability of landfall for any one location along the coast is very low and reflects the fact that, in any one season, most U.S. coastal areas will not feel the effects of a hurricane no matter how active the individual season is.  However, it must also be emphasized that a low landfall probability does not insure that hurricanes will not come ashore.  Regardless of how active the 2006 hurricane season is, a finite probability always exists that one or more hurricanes may strike along the U.S. coastline or in the Caribbean Basin and do much damage. 

 

 

10      Forthcoming Updated Forecasts of 2006 Hurricane Activity

 

We will be issuing seasonal updates of our 2006 Atlantic basin hurricane forecasts on Tuesday April 4, Wednesday 31 May (to coincide with the official start of the 2006 hurricane season on 1 June), Thursday 3 August, Friday 1 September and Tuesday 3 October 2006.  The 3 August, 1 September and 3 October forecasts will include separate forecasts of August-only, September-only and October-only Atlantic basin tropical cyclone activity.  A verification and discussion of all 2006 forecasts will be issued in late November 2006.  Our first seasonal hurricane forecast for the 2007 hurricane season will be issued in early December 2006.  All these forecasts will be available on the web at: http://hurricane.atmos.colostate.edu/Forecasts.

 

11      Acknowledgments

 

Besides the individuals named on page 2, there have been a number of other meteorologists that have furnished us with data and given valuable assessments of the current state of global atmospheric and oceanic conditions.  These include Arthur Douglas, Richard Larsen, Todd Kimberlain, Ray Zehr, and Mark DeMaria.  In addition, Barbara Brumit and Amie Hedstrom have provided excellent manuscript, graphical and data analysis and assistance over a number of years.  We have profited over the years from many in-depth discussions with most of the current and past NHC hurricane forecasters.  The second author would further like to acknowledge the encouragement he has received for this type of forecasting research application from Neil Frank, Robert Sheets, Robert Burpee, Jerry Jarrell, former directors of the National Hurricane Center (NHC), and from the current director, Max Mayfield and their forecast staffs.  Uma Shama and Larry Harman of Bridgewater State College, MA have provided assistance and technical support in the development of our Landfalling Hurricane Probability Webpage.  We also thank Bill Bailey of the Insurance Information Institute for his sage advice and encouragement.

 

The financial backing for the issuing and verification of these forecasts has in part been supported by the National Science Foundation and by the Research Foundation of Lexington Insurance Company (a member of the American International Group).  We also thank the GeoGraphics Laboratory at Bridgewater State College for their assistance in developing the Landfalling Hurricane Probability Webpage.

 

 

12      Citations and Additional Reading

 

13      Verification of Previous Forecasts

 

Table 10:  Summary verification of the authors’ six previous years of seasonal forecasts for Atlantic TC activity between 2000-2005. 

 

 

 

2000

 

8 Dec. 1999

Update

7 April

Update

7 June

Update

4 August

 

Obs.

No. of Hurricanes

7

7

8

7

8

No. of Named Storms

11

11

12

11

14

No. of Hurricane Days

25

25

35

30

32

No. of Named Storm Days

55

55

65

55

66

Hurr. Destruction Potential

85

85

100

90

85

Intense Hurricanes

3

3

4

3

3

Intense Hurricane Days

6

6

8

6

5.25

Net Tropical Cyclone Activity

125

125

160

130

134

 

 

2001

 

7 Dec. 2000

Update

6 April

Update

7 June

Update

7 August

 

Obs.

No. of Hurricanes

5

6

7

7

9

No. of Named Storms

9

10

12

12

15

No. of Hurricane Days

20

25

30

30

27

No. of Named Storm Days

45

50

60

60

63

Hurr. Destruction Potential

65

65

75

75

71

Intense Hurricanes

2

2

3

3

4

Intense Hurricane Days

4

4

5

5

5

Net Tropical Cyclone Activity

90

100

120

120

142

 

 

2002

 

7 Dec. 2001

Update

5 April

Update

31 May

Update

7 August

Update

2 Sept.

 

Obs.

No. of Hurricanes

8

7

6

4

3

4

No. of Named Storms

13

12

11

9

8

12

No. of Hurricane Days

35

30

25

12

10

11

No. of Named Storm Days

70

65

55

35

25

54

Hurr. Destruction Potential

90

85

75

35

25

31

Intense Hurricanes

4

3

2

1

1

2

Intense Hurricane Days

7

6

5

2

2

2.5

Net Tropical Cyclone Activity

140

125

100

60

45

80

 

 

2003

 

6 Dec. 2002

Update

4 April

Update

30 May

Update

6 August

Update

3 Sept.

Update

2 Oct.

 

Obs.

No. of Hurricanes

8

8

8

8

7

8

7

No. of Named Storms

12

12

14

14

14

14

14

No. of Hurricane Days

35

35

35

25

25

35

32

No. of Named Storm Days

65

65

70

60

55

70

71

Hurr. Destruction Potential

100

100

100

80

80

125

129

Intense Hurricanes

3

3

3

3

3

2

3

Intense Hurricane Days

8

8

8

5

9

15

17

Net Tropical Cyclone Activity

140

140

145

120

130

155

173

 

 

2004

 

5 Dec. 2003

Update

2 April

Update

28 May

Update

6 August

Update

3 Sept.

Update

1 Oct.

 

Obs.

No. of Hurricanes

7

8

8

7

8

9

9

No. of Named Storms

13

14

14

13

16

15

14

No. of Hurricane Days

30

35

35

30

40

52

46

No. of Named Storm Days

55

60

60

55

70

96

90

Intense Hurricanes

3

3

3

3

5

6

6

Intense Hurricane Days

6

8

8

6

15

23

22

Net Tropical Cyclone Activity

125

145

145

125

185

240

229

 

 

2005

 

3 Dec. 2004

Update

1 April

Update

31 May

Update

5 August

Update

2 Sept.

Update

3 Oct.

 

Obs.

No. of Hurricanes

6

7

8

10

10

11

14

No. of Named Storms

11

13

15

20

20

20

26

No. of Hurricane Days

25

35

45

55

45

40

48

No. of Named Storm Days

55

65

75

95

95

100

116

Intense Hurricanes

3

3

4

6

6

6

7

Intense Hurricane Days

6

7

11

18

15

13

16.75

Net Tropical Cyclone Activity

115

135

170

235

220

215

263

 


[1] Research Associate

[2] Professor Emeritus of Atmospheric Science

[3] Research Associate