Extended Range Forecast Of Atlantic Seasonal Hurricane Activity And U.S. Landfall Strike Probability For 2009
By Philip J. Klotzbach and William M. Gray
We foresee average activity for the 2009 Atlantic hurricane season. We have decreased our seasonal forecast from our initial early December prediction. We anticipate an average probability of United States major hurricane landfall (as of 7 April 2009). This forecast as well as past forecasts and verifications are available via the World Wide Web at http://hurricane.atmos.colostate.edu/Forecasts.
Atlantic Basin Seasonal Hurricane Forecast For 2009
Forecast Parameter and 1950-2000
Climatology (in parentheses) 7 Dec 2008 9 April 2009
Named Storms (NS) (9.6) 14 12
Named Storm Days (NSD) (49.1) 70 55
Hurricanes (H) (5.9) 7 6
Hurricane Days (HD) (24.5) 30 25
Intense Hurricanes (IH) (2.3) 3 2
Intense Hurricane Days (IHD) (5.0) 7 5
Accumulated Cyclone Energy (ACE) (96.1) 125 100
Net Tropical Cyclone Activity (NTC) (100%) 135 105
Probabilities For At Least One Major (Category 3-4-5) Hurricane Landfall On Each Of The Following Coastal Areas:
1) Entire U.S. coastline - 54% (average for last century is 52%)
2) U.S. East Coast Including Peninsula Florida - 32% (average for last century is 31%)
3) Gulf Coast from the Florida Panhandle westward to Brownsville - 31% (average for last century is 30%)
4) Average major hurricane landfall risk in the Caribbean
Information obtained through March 2009 indicates that the 2009 Atlantic hurricane season will have about as much activity as the average 1950-2000 season. We estimate that 2009 will have about 6 hurricanes (average is 5.9), 12 named storms (average is 9.6), 55 named storm days (average is 49.1), 25 hurricane days (average is 24.5), 2 intense (Category 3-4-5) hurricanes (average is 2.3) and 5 intense hurricane days (average is 5.0). The probability of U.S. major hurricane landfall is estimated to be about 105 percent of the long-period average. We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2009 to be approximately 105 percent of the long-term average. We have decreased our seasonal forecast from early December.
This forecast is based on an extended-range early April statistical prediction scheme that utilizes 58 years of past data. Analog predictors are also utilized. The influence of El Niño conditions is implicit in these predictor fields, and therefore we do not utilize a specific ENSO forecast as a predictor.
We expect current weak La Niña conditions to transition to neutral and perhaps weak El Niño conditions by this year’s hurricane season. If El Niño conditions develop for this year’s hurricane season, it would tend to increase levels of vertical wind shear and decrease levels of Atlantic hurricane activity. Another reason for our forecast reduction is due to anomalous cooling of sea surface temperatures in the tropical Atlantic. Cooler waters are associated with dynamic and thermodynamic factors that are less conducive for an active Atlantic hurricane season.
... Weak La Niña conditions occurred during the winter of 2008-2009. This event has weakened somewhat over the past few weeks. SSTs are generally slightly below average across the eastern and central tropical Pacific. Table 6 displays January and March SST anomalies for several Nino regions. Note that all four regions have experienced warming since January, with more warming occurring in the central Pacific. This anomalous warming is unlike the warming that occurred last year in that the early springtime warming that occurred last year was concentrated in the eastern Pacific.
...As was the situation last year, the big question is whether this current observed warming will continue through this year’s hurricane season. The spring months are known as the ENSO predictability barrier time period, as this is when both statistical and dynamical models show their least amount of skill. This is likely due to the fact that from a climatological perspective, trade winds across the Pacific are weakest during the late spring and early summer, and therefore, changes in phase of ENSO are often observed to occur during the April-June period. Unlike March 2008 when none of the available statistical or dynamical models called for a warm ENSO event during August-October, several models are predicting a warm ENSO event this year (Figure 6). The dynamical model consensus calls for a weak El Niño event this August-October (August-October averaged Nino 3.4 anomaly of +0.6°C). By contrast, the statistical models tend to predict less warming.
ENSO forecasts from various statistical and dynamical models. Figure courtesy of the International Research Institute (IRI). Currently, three dynamical models (1) POAMA, (2) COLA CCSM3, and (3) NASA GMAO are calling for a significant warm ENSO event. All other models call for neutral or cool conditions for the August-October period.
Based on this information, we believe that the current weak La Niña will likely continue to moderate over the next couple of months. At this point, we believe there is an approximately 50% chance of a weak El Niño developing during this summer/fall. The potential for a weak El Niño is one of the reasons that we have reduced our forecast from early December. El Niños typically increase levels of vertical wind shear in thetropical Atlantic, causing detrimental conditions for Atlantic tropical cyclone formation and intensification. We should know more about the potential for an El Niño by the time of our next forecast on June 2.
... Conditions in the Atlantic are less favorable for an active season than they were in November 2008...Current Tropical North Atlantic index (defined as 5.5-23.5°N, 57.5-15°W) SST anomaly values of approximately -0.4°C are the lowest that have been observed since June-July 1994. This strong anomalous cooling is another reason for the reduction in our Atlantic basin hurricane forecast. Cooler-than-normal waters provide less latent and sensible heat flux for developing tropical cyclones. In addition, an anomalously cool tropical Atlantic is typically associated with higher sea level pressure values and stronger-than-normal trade winds, indicating a more stable atmosphere with increased levels of vertical wind shear.
Estimated Probability of US Landfall For 2009
Coastal Region Tropical Cat 1-2 Cat 3-4-5 All Named
Storms Hurricane Hurricane Hurricanes Storms
Entire US 81% (79%) 69% (68%) 54% (52%) 86% (84%) 97%(97%)
Gulf Coast 60% (59%) 44% (42%) 31% (30%) 62% (60%) 85%(83%)
Florida & 52% (50%) 46% (44%) 32% (31%) 63% (61%) 82% (81%)
East Coast
Has Global Warming Been Responsible for the Recent Large Upswing (Since 1995) in Atlantic Basin Major Hurricanes and U.S. Landfall?
The U.S. landfall of major hurricanes Dennis, Katrina, Rita and Wilma in 2005 and the four Southeast landfalling hurricanes of 2004 (Charley, Frances, Ivan and Jeanne) raised questions about the possible role that global warming played in these two unusually destructive seasons. In addition, three Category 2 hurricanes (Dolly, Gustav and Ike) pummeled the Gulf Coast last year causing considerable devastation.
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 that has taken place over the last three decades, the global numbers of hurricanes and their intensity have not shown increases in recent years except for the Atlantic (Klotzbach 2006).
The Atlantic has seen a very large increase in major hurricanes during the 14-year period of 1995-2008 (average 3.9 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 the Atlantic Ocean thermohaline circulation (THC) that is not directly related to global sea surface temperatures or CO2 increases. Changes in ocean salinity are believed to be the driving mechanism. These multi-decadal changes have also been termed the Atlantic Multidecadal Oscillation (AMO).
Although global surface temperatures have increased over the last century and over the last 30 years, there is no reliable data available to indicate increased hurricane frequency or intensity in any of the globe’s other tropical cyclone basins.
In a global warming or global cooling world, the atmosphere’s upper air temperatures will warm or cool in unison with the sea surface temperatures. Vertical lapse rates will not be significantly altered. We have no plausible physical reasons for believing that Atlantic hurricane frequency or intensity will change significantly if global ocean temperatures were to continue to rise. For instance, in the quarter-century period from 1945-1969 when the globe was undergoing a weak cooling trend, the Atlantic basin experienced 80 major (Cat 3-4-5) hurricanes and 201 major hurricane days. By contrast, in a similar 25-year period from 1970-1994 when the globe was undergoing a general warming trend, there were only 38 major hurricanes (48% as many) and 63 major hurricane days (31% as many) (Figure 8). Atlantic sea surface temperatures and hurricane activity do not necessarily follow global mean temperature trends.
Anticipated Large Increase in US Hurricane Destruction
The large increase in the hurricane-spawned destruction that occurred in 2004, 2005 and 2008 has not surprised us. We have been anticipating a great upsurge in hurricane destruction for many years as illustrated by the statements we have made in previous seasonal forecast reports such as:
“…major increases in hurricane-spawned coastal destruction are inevitable.” (April 1989)
“A new era of major hurricane activity appears to have begun…. As a consequence of the exploding U.S. and Caribbean coastal populations during the last 25-30 years, we will begin to see a large upturn in hurricane-spawned destruction – likely higher than anything previous experienced.” (June 1997)
“We must expect a great increase in landfalling major hurricanes in the coming decades. With exploding southeast coastal populations, we must also prepare for levels of hurricane damage never before experienced.” (April 2001)
“If the future is like the past, it is highly likely that very active hurricane seasons will again emerge during the next few years, and the prospects for very large U.S. and Caribbean increases in hurricane damage over the next few decades remains high. We should indeed see future hurricane damage much greater than anything in the past.” (May 2002)
“Regardless of whether a major hurricane makes landfall this year, it is inevitable that we will see hurricane-spawned destruction in coming years on a scale many, many times greater than what we have seen in the past.” (May 2003)
These projections of increased U.S. hurricane destruction were made with our anticipation that the Atlantic thermohaline circulation (THC) (which had been very weak from the late-1960s to the mid-1990s) would be changing to a stronger mode making for a large increase in Atlantic basin major hurricane activity. The THC has become much stronger since about 1995. These projections were made with no consideration given to rising levels of atmospheric CO2.
We were very fortunate during the early part of this strong THC period in that only 3 of 32 major hurricanes that formed in the Atlantic between 1995-2003 made U.S. landfall. The long-term average is that approximately 1 in 3.5 major hurricanes that forms in the Atlantic makes U.S. landfall. This luck failed to hold beginning with the 2004 hurricane season.
Forthcoming Updated Forecasts of 2009 Hurricane Activity
We will be issuing seasonal updates of our 2009 Atlantic basin hurricane forecasts on Tuesday 2 June, Tuesday 4 August, Wednesday 2 September and Thursday 1 October 2009. The 4 August, 2 September and 1 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 2009 forecasts will be issued in late November 2009. Our first seasonal hurricane forecast for the 2010 hurricane season will be issued in early December 2009. All of these forecasts will be available on the web at: http://hurricane.atmos.colostate.edu/Forecasts.
Editor’s note: We have taken our information from the Tropical Meteorology Project at Colorado State University. Dr. Philip Klotzbach and Dr. William Gray are highly respected in their field of study. Their predictions are used by government and media experts including Federal Emergency Management Agency (FEMA) and National Oceanic and Atmospheric Administration. Please note that all of the following information can be found on their website as a PDF file (http://hurricane.atmos.colostate.edu/Forecasts/).
We have taken their predictions from that file and are presenting them to you here. We have taken liberties to share portions of their detailed, scientific report in this article. To read this report in its entirety please visit http://hurricane.atmos.colostate.edu/Forecasts. This information was released on April 7, 2009
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