EXTENDED RANGE FORECAST OF ATLANTIC SEASONAL HURRICANE ACTIVITY AND
We have increased our forecast for the 2007 hurricane season, largely due to the rapid dissipation of El Niño conditions. We are now calling for a very active hurricane season. Landfall probabilities for the 2007 hurricane season are well above their long-period averages.
(as of 3 April 2007)
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
Emily Wilmsen, Colorado State University Media Representative, (970-491-6432) is
available to answer various questions about this forecast
Department of Atmospheric Science
Email: amie@atmos.colostate.edu
|
Forecast Parameter and 1950-2000 Climatology (in parentheses) |
Issue Date 8 December 2006 |
Issue Date 3 April 2007 |
|
Named Storms (NS) (9.6) |
14 |
17 |
|
Named Storm Days (NSD) (49.1) |
70 |
85 |
|
Hurricanes (H) (5.9) |
7 |
9 |
|
Hurricane Days (HD) (24.5) |
35 |
40 |
|
Intense Hurricanes (IH) (2.3) |
3 |
5 |
|
Intense Hurricane Days (IHD) (5.0) |
8 |
11 |
|
Accumulated Cyclone Energy (ACE) (96.2) |
130 |
170 |
|
Net Tropical Cyclone Activity (NTC) (100%) |
140 |
185 |
PROBABILITIES FOR AT LEAST ONE MAJOR (CATEGORY 3-4-5) HURRICANE LANDFALL ON EACH OF THE FOLLOWING COASTAL AREAS:
1) Entire
2) U.S.
East Coast Including Peninsula
3)
4) Above-average
major hurricane landfall risk in the
ABSTRACT
Information obtained through March
2007 indicates that the 2007 Atlantic hurricane season will be much more active
than the average 1950-2000 season. We
estimate that 2007 will have about 9 hurricanes (average is 5.9), 17 named
storms (average is 9.6), 85 named storm days (average is 49.1), 40 hurricane
days (average is 24.5), 5 intense (Category 3-4-5) hurricanes (average is 2.3)
and 11 intense hurricane days (average is 5.0).
The probability of
This early April forecast is based
on a newly devised extended range statistical forecast procedure which utilizes
40 years of past global reanalysis data and is then tested on an additional 15
years of global reanalysis data. Analog predictors are also utilized. We have increased
our forecast from our early December prediction due largely to the rapid
dissipation of El Niño which has occurred over the past couple of months. Currently, neutral ENSO conditions are
observed. We expect either neutral or
weak-to-moderate La Niña conditions to be present during the upcoming hurricane
season. Tropical and
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). We thank Jim Kossin and Dan Vimont of the University of Wisconsin-Madison for providing the data for the Atlantic Meridional Mode prediction used in this forecast.
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
Notice of Author Changes
By William Gray
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 six years and has been second author on these forecasts for the last five 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 six 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.
1 Introduction
This is the 24th 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 statistical methodologies derived from 55 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
April Forecast Methodology
Our initial early April seasonal hurricane forecast scheme demonstrated hindcast skill for the period of 1950-1995. We developed a new early April forecast scheme that used more hindcast years (1950-2001) and showed improved hindcast skill and better physical insights into why such precursor relationships have an extended period memory.
This year, we have focused on revamping our statistical prediction techniques. We debuted a new early December statistical prediction technique that used fewer predictors and only used data for the two months prior to the forecast issuance date. In addition, the scheme was developed on dependent data from 1950-1989 and then tested on “independent” data from 1990-2004. Predictors were only included in the forecast if they added skill in both the dependent dataset and the independent dataset. We also attempted to only predict the NTC index with our new statistical technique, instead of attempting to predict a multitude of predictands including named storms, named storm days, etc. See our early December 2006 forecast for more information on this new technique.
Our new early April forecast scheme that we are using for the first time this year utilizes a similar technique to what was used in developing our new early December forecast scheme. We only attempt to predict the NTC index and only use predictors from the two months prior to the forecast issuance date (i.e. February-March data). This scheme then derives predictions for our other predictands from this NTC prediction. For example, if a typical season has 10 named storms and the predicted NTC value is 120%, the predicted number of named storms for the season would be 12 (10 * 120%).
Our new early April forecast uses three predictors derived from the NOAA-NCEP reanalysis products. A combination of these three predictors, using data over the complete 1950-2004 period, is able to hindcast 55% of the variance in NTC activity. The location of each of these new predictors is shown in Fig. 1. The pool of three predictors for this extended range forecast is given in Table 1. Strong statistical relationships can be extracted via combinations of these predictors (which are available by the end of March) and the amount of Atlantic basin hurricane activity occurring later in the year. The combination of these three predictors is calling for an active season this year.
Figure 1: Location of predictors for the early April forecast for the 2007 hurricane season.
Table 1: Listing of 1 April 2007
predictors for this year’s hurricane activity.
A plus (+) means that positive values of the parameter indicate
increased hurricane activity this year, and a minus (-) means that positive
values of the parameter indicate decreased hurricane activity this year. The combination of these three predictors
calls for an active hurricane season this year.
|
Predictor |
Values for 2007 Forecast |
|
1) February-March SST (30-45°N, 10-30°W) (+) |
+1.7 SD |
|
2) February-March SLP (20-45°S, 100-160°W) (+) |
0.0 SD |
|
3) February-March SST (30-45°S, 20-45°W) (-) |
+1.1 SD |
2.1
Physical
Associations among Predictors Listed in Table 1
Brief descriptions of our early April predictors follow:
Predictor
1. February-March SST in the Subtropical
(30-45°N, 10-30°W)
Above-normal sea surface temperatures (SSTs) in the eastern
subtropical Atlantic are associated with a weaker-than-normal Azores high and
reduced trade wind strength during the boreal spring (Knaff 1997). These above-average SSTs in February-March
are strongly correlated with weaker trade winds, lower-than-normal sea level
pressures and above-average SSTs in the tropical
Predictor
2. February-March SLP in the Subtropical
Southeastern Pacific (+)
(20-45°S, 100-160°W)
Anomalously high sea level pressures in this portion of the
subtropical southeastern Pacific during February-March are associated with a
positive Southern Oscillation Index and stronger trade winds across the
tropical Pacific. Anomalously strong
trade winds drive increased upwelling in the eastern tropical Pacific and are
typically associated with cool ENSO conditions.
Lag correlations for the August-October period indicate that La Niña
conditions are much more likely with positive values of this predictor. Cool ENSO conditions are typically associated
with more active Atlantic basin hurricane seasons through a reduction of
vertical wind shear across the Caribbean and tropical
Predictor
3. February-March SST in the
(30-45°S, 20-45°W)
Above-average SSTs in February-March in the South Atlantic
are associated with higher sea level pressures throughout the tropical
3
Analog-Based Predictors for 2007 Hurricane Activity
Certain years in the historical record have global oceanic and atmospheric trends which are substantially similar to 2007. These years also provide useful clues as to likely trends in activity that the forthcoming 2007 hurricane season may bring. For this early April extended range forecast, we project atmospheric and oceanic conditions for August through October 2007 and determine which of the prior years in our database have distinct trends in key environmental conditions which are similar to current February-March 2007 conditions. Table 2 lists our analog selections.
We select prior hurricane seasons
since 1949 which have similar atmospheric-oceanic conditions to those currently
being experienced. For 2007, we searched
for years that had transitioning warm to neutral or cool ENSO conditions and
warm
There were five hurricane seasons since 1949 with characteristics most similar to what we observe in February-March 2007 and characteristics that we expect to see in August-October 2007. The best analog years that we could find for the 2007 hurricane season are 1952, 1964, 1966, 1995 and 2003. We anticipate that 2007 seasonal hurricane activity will have activity slightly more than what was experienced in the average of these five years. We expect the 2007 hurricane season to be very active.
Table 2: Best analog years for 2007 with the associated hurricane activity listed for each year.
|
Year |
NS |
NSD |
H |
HD |
IH |
IHD |
ACE |
NTC |
|
1952 |
7 |
39.75 |
6 |
22.75 |
3 |
7.00 |
87 |
103 |
|
1964 |
12 |
71.25 |
6 |
43.00 |
6 |
14.75 |
170 |
184 |
|
1966 |
11 |
64.00 |
7 |
41.75 |
3 |
7.75 |
145 |
137 |
|
1995 |
19 |
121.25 |
11 |
61.75 |
5 |
11.50 |
227 |
222 |
|
2003 |
16 |
79.25 |
7 |
32.75 |
3 |
16.75 |
175 |
174 |
|
Mean |
13.0 |
75.1 |
7.4 |
40.4 |
4.0 |
11.80 |
161 |
179 |
|
2007 Forecast |
17 |
85 |
9 |
40 |
5 |
11 |
170 |
185 |
4
ENSO
The weak to moderate El Niño event that rapidly developed during August to October 2006 has now dissipated. Current observed SST anomalies in the various regions range from approximately +0.5°C in the Nino 4 region (5°S-5°N, 160°E-150°W) to approximately -0.5°C in the Nino 1+2 regions (10°S-0°, 80-90°W), indicating that we currently have neutral conditions in the tropical Pacific. The likelihood of redevelopment of warm ENSO conditions this summer/fall is quite unlikely, as the western Pacific warm pool has been largely depleted, and enhanced trade winds have been blowing across the eastern and central Pacific for the past couple of months. In addition, it is very rare to have warm ENSO conditions, followed by dissipation of the event, and then by redevelopment of another warm event within a one-year period.
Most of the ENSO forecast models indicate that neutral or cool ENSO conditions are likely for this upcoming summer/fall. Based on the latest prediction plume figure from the International Research Institute (IRI) (Figure 2), only one of 16 models is calling for El Niño conditions (SST anomaly greater than 0.5°C) in the Nino 3.4 region (5°S-5°N, 120-170°W) during the August-October period. Ten models are calling for neutral conditions, while the remaining five models are calling for La Niña conditions (SST anomaly less than -0.5°C).
Based on the latest ENSO
predictions as well as currently observed conditions in the tropical Pacific,
we expect either neutral or cool ENSO conditions to be in place in the tropical
Pacific during the upcoming hurricane season.
Since SSTs in the tropical and northern
The rapid dissipation of the weak to moderate El Niño event during the latter part of this winter has been quite impressive. Table 3 displays the five most significant cooling episodes of SST anomalies in the Nino 3.4 region from October-November to the following year’s February-March time period. Based on this metric, using data since 1950, the observed cooling during this time period in 2006-2007 is the strongest cooling on record.
Figure 2: Latest prediction of Nino 3.4 anomalies from a variety of dynamical and statistical ENSO prediction models. Figure courtesy of the International Research Institute (IRI).
Table 3: Largest observed cooling anomalies in the Nino 3.4 region between November-December of the previous year and the current year’s February-March period. The cooling observed in the 2006-2007 winter period is also provided.
|
Year |
Previous Year’s
Nov-Dec Nino 3.4 Anomaly (°C) |
Following Year’s
Feb-Mar Nino 3.4 Anomaly (°C) |
(Feb-Mar) – (Nov-Dec) |
|
1972-1973 |
2.1 |
1.0 |
-1.1 |
|
2002-2003 |
1.7 |
0.7 |
-1.0 |
|
1987-1988 |
1.7 |
0.7 |
-1.0 |
|
1997-1998 |
2.8 |
1.9 |
-0.9 |
|
1963-1964 |
1.0 |
0.1 |
-0.9 |
|
|
|
|
|
|
2006-2007 |
1.3 |
0.1 |
-1.2 |
5
Atlantic Meridional Mode
A new predictor that we are
evaluating for the first time this year is a prediction of the July-November
Atlantic Meridional Mode (AMM), calculated by Dan Vimont and Jim Kossin at the
University of Wisconsin-Madison. The
Atlantic meridional mode evaluates the strength of the SST gradient between the
northern tropical and southern tropical
6
Adjusted 2007 Forecast
Table 4 shows our final adjusted early April forecast for the 2007 season which is a combination of our new statistical forecast, our analog forecast and qualitative adjustments for other factors not explicitly contained in any of these schemes. Both our statistical forecast and our analog forecast indicate activity at well above-average levels. We anticipate that the current neutral ENSO conditions will either remain neutral or will transition to cool ENSO conditions by this summer/fall. Warm sea surface temperatures are likely to continue being present in the tropical and North Atlantic during 2007, due to the fact that we are in a positive phase of the Atlantic Multidecadal Oscillation (AMO) (e.g., a strong phase of the Atlantic thermohaline circulation).
Table 4:
Summary of our early April statistical forecast, our analog forecast and
our adjusted final forecast for the 2007 hurricane season.