The 2010 Atlantic hurricane season outlook is an official product of the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC). The outlook is produced in collaboration with scientists from the National Hurricane Center (NHC), and the Hurricane Research Division (HRD). The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

Interpretation of NOAA’s Atlantic hurricane season outlook
This outlook is general guide to the expected overall activity during the hurricane season. It is not a seasonal hurricane landfall forecast, and it does not predict levels of activity for any particular region.

Preparedness
Hurricane disasters can occur whether the season is active or relatively quiet. It only takes one hurricane (or tropical storm) to cause a disaster. Residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare for every hurricane season regardless of this, or any other, seasonal outlook. NOAA, the Federal Emergency Management Agency (FEMA), NHC, the Small Business Administration, and the American Red Cross all provide important hurricane preparedness information on their web sites.

NOAA does not make seasonal hurricane landfall predictions
NOAA does not make seasonal hurricane landfall predictions. Hurricane landfalls are largely determined by the weather patterns in place as the hurricane approaches, which are only predictable when the storm is within several days of making landfall.

Nature of this Outlook and the “likely” ranges of activity
This outlook is probabilistic, meaning the stated “likely” ranges of activity have a certain likelihood of occurring. The seasonal activity is expected to fall within these ranges in about 70% of seasons with similar climate conditions and uncertainties to those expected this year. They do not represent the total possible ranges of activity seen in past similar years.

This outlook is based on 1) predictions of large-scale climate factors and conditions known to be strong indicators of seasonal Atlantic hurricane activity, and 2) promising new climate models that are now beginning to directly predict seasonal hurricane activity.

Sources of uncertainty in this seasonal outlook
1. Many combinations of named storms and hurricanes can occur for the same general set of climate conditions. For example, one cannot know with certainty whether a given climate signal will be associated with several short-lived storms or fewer longer-lived storms with greater intensity.

2. Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.

Updated 2010 Atlantic Hurricane Season Outlook: Summary

NOAA’s updated 2010 Atlantic Hurricane Season Outlook calls for a 90% chance of an above normal season. There is only a 10% chance of a near-normal season, and no expectation the season will be below normal. Therefore, 2010 is expected to become the eleventh above-normal season since 1995. See NOAA definitions of above-, near-, and below-normal seasons. The Atlantic hurricane region (or basin) includes the North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico.

The atmospheric and oceanic conditions now in place over the tropical Atlantic Ocean and Caribbean Sea are very conducive to hurricane formation, as was predicted in NOAA’s pre-season outlook issued in May. These conditions are expected to persist throughout the peak months (August-October) of the Atlantic hurricane season, in association with three climate factors; the tropical multi-decadal signal, La Niña, and very warm temperatures in the tropical Atlantic Ocean and Caribbean Sea. In addition, dynamical model forecasts tropical cyclone activity continue to predict a very active season.

We estimate a 70% probability for each of the following seasonal (June-November) ranges of activity during 2010. These ranges are consistent with NOAA’s May outlook, and reiterate a high likelihood of a very active season (i.e. hyperactive, defined by ACE ? 175% of median), perhaps one of the more active on record.

• 14-20 Named Storms,
• 8-12 Hurricanes
• 4-6 Major Hurricanes
• An ACE range of 170%-260% of the median.

The activity is expected to fall within these ranges in about 70% of seasons with similar climate conditions and uncertainties to those expected this year. They do not represent the total possible ranges of activity seen in past similar years.

These ranges include the two tropical storms and one hurricane seen to date. During June – July 2010, two named storms (Hurricane Alex and Tropical Storm Bonnie) formed in the Atlantic basin. The pre-season outlook issued in late May reflected the possibility of even more early-season activity. As a result, the upper ends of the predicted ranges have been reduced.

Nonetheless, significant activity is predicted for the remainder of the season, with an additional 12-17 named storms, of which 7-11 are expected to become hurricanes with 4-6 reaching major hurricane status.

Any region or community can experience a devastating hurricane regardless of the overall seasonal activity. However, during hyperactive seasons, the historical probability for multiple U.S. hurricane strikes, and for multiple hurricane strikes in the region around the Caribbean Sea, increases sharply. Therefore, it is even more imperative for this season that residents and government officials in hurricane-vulnerable communities have an effective hurricane preparedness plan in place.

NOAA does not make an official seasonal landfall outlook. Predicting where and when hurricanes will strike is related to daily weather patterns, which are not predictable weeks or months in advance. Therefore, it is currently not possible to reliably predict the number or intensity of landfalling hurricanes at these extended ranges, or whether a specific locality will be impacted by a hurricane this season.

DISCUSSION

1. Expected 2010 activity

Current oceanic and atmospheric conditions, known climate factors, and dynamical model forecasts, all point to an above normal Atlantic hurricane season during 2010. NOAA’s updated seasonal outlook calls for a 90% chance of an above-normal season, and a 10% chance of a near-normal season. This updated outlook is consistent with NOAA’s pre-season outlook issued in May, which indicated an 85% chance of an above normal season.

An important measure of the total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the combined intensity and duration of all named storms and hurricanes during the season. According to NOAA’s hurricane season classifications, an ACE value at or above 175% of the median reflects a very active (or hyperactive) season. For 2010, we estimate a 70% chance that the seasonal ACE range will be 170%-260% of the median. This range indicates a high likelihood of a hyperactive season.

The 2010 Atlantic hurricane season is expected (with 70% chance) to produce a total of 14-20 named storms, of which 8-12 are expected to become hurricanes with 4-6 becoming major hurricanes. If the activity reaches the upper end of our predicted ranges, the season will be one of the more active on record.

These ranges include the two tropical storms and one hurricane seen to date. During June – July 2010, two named storms (Hurricane Alex and Tropical Storm Bonnie) formed in the Atlantic basin. The pre-season outlook issued in late May reflected the possibility of even more early-season activity. As a result, the upper ends of the predicted ranges have been reduced.

Nonetheless, significant activity is predicted for the remainder of the season, with an additional 12-17 named storms, of which 7-11 are expected to become hurricanes with 4-6 reaching major hurricane status.

Because of the high threat of a very active season, we are including some historical U.S. hurricane landfall statistics based on past similar seasons. These statistics do not represent an explicit hurricane landfall forecast, as it is not currently possible to reliably predict such activity so far in advance.

Very active seasons feature more hurricanes and major hurricanes, as well as systematic changes in where these systems develop and track. In particular, they feature significantly increased tropical cyclone activity in the Main Development Region (MDR, which includes the Caribbean Sea and tropical Atlantic Ocean between 9°N-21.5°N; Goldenberg et al. 2001). Such storms typically have more time to strengthen, and often track farther westward than their counterparts that develop to the north. As a result, there is a systematic increase in tropical cyclone activity in the western portion of the Atlantic basin, which results in an increased threat to the continental U.S., the Gulf of Mexico, and the region around the Caribbean Sea.

The historical probabilities for multiple hurricane strikes increase markedly during very active seasons for both the Gulf Coast and Atlantic Coast of the United States. The region around the Caribbean Sea also sees a sharp increase in hurricane activity during these seasons.

Historically, all above normal seasons have produced at least one named storm in the Gulf of Mexico, and 95% have produced at least two named storms in the Gulf. Most of this activity (80%) occurs during August-October, with seventy percent of above normal seasons seeing at least three named storms in the Gulf of Mexico during this period.

While the climate patterns are indicating an increased threat of hurricane landfalls this year, predicting the location, number, timing, and strength, of those events is ultimately related to the daily weather patterns, which are not predictable weeks or months in advance. As a result, it is currently not possible to reliably predict the number or intensity of landfalling hurricanes at these extended ranges, or whether a given locality will be impacted by a hurricane this season. Therefore, NOAA does not make an official seasonal hurricane landfall outlook.

2. Science behind the updated 2010 Outlook

The updated 2010 Atlantic hurricane season outlook primarily reflects a set of atmospheric and oceanic conditions now in place, which are very conducive to hurricane formation. These conditions are expected to persist through the peak months of the Atlantic hurricane season, and are linked to three climate factors: 1) the tropical multi-decadal signal, 2) La Niña, and 3) a continuation of exceptionally warm sea surface temperatures (SSTs) in the Main Development Region. These conditions are consistent with NOAA’s pre-season outlook issued in May.

This outlook also takes into account dynamical model predictions from new models such as the NOAA Climate Forecast System (CFS), the European Centre for Medium Range Weather Forecasting (ECMWF), the United Kingdom Meteorology (UKMET) office model, and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble. All of these models continue to predict a high likelihood of a very active season.

a. Expected continuation of tropical multi-decadal signal
One factor guiding this outlook is the continuation of the tropical multi-decadal signal, which has contributed to the high-activity era in the Atlantic basin that began in 1995. Key components of this signal include an enhanced west African monsoon circulation and above average SSTs in both the lower and higher latitudes of the North Atlantic Ocean.

Within the MDR, atmospheric aspects of the tropical multi-decadal signal seen since 1995 include reduced vertical wind shear, weaker easterly trade winds, and an extensive area of cyclonic shear at 700-hPa along the equatorward flank of the African Easterly Jet. These conditions are now in place, along with other related circulation features in the upper atmosphere that are also conducive to hurricane formation, including 1) an extensive area of anomalous easterly winds extending westward from Africa and 2) anticyclonic circulation (i.e. streamfunction) anomalies in the upper atmosphere over subtropical Atlantic in both hemispheres.

b. La Niña
Another climate factor known to impact Atlantic hurricane activity is the El Niño/ Southern Oscillation (ENSO). The three phases of ENSO are El Niño, La Niña, and Neutral. La Niña refers to a periodic anomalous cooling of SSTs in the central and eastern equatorial Pacific Ocean. This cooling affects rainfall patterns across the tropical Pacific which, in turn, alters wind patterns so as to reduce the vertical wind shear in the MDR. Consequently, La Niña is typically more conducive to increased Atlantic hurricane activity (Gray 1984).

La Niña developed during July, according to the latest ENSO Diagnostic Discussion issued by NOAA’s Climate Prediction Center. Nearly all ENSO forecast models now predict La Niña to continue through the Atlantic hurricane season.

The upper-level atmospheric circulation is now in a La Niña state, as indicated by cyclonic streamfunction anomalies in the central subtropical Pacific of both hemispheres (blue shading in NH, red shading in SH). Over the eastern subtropical Pacific and Atlantic basins in both hemispheres, the upper-level circulation is also consistent with La Niña, as indicated by extensive easterly wind anomalies and anticyclonic streamfunction anomalies (Red in NH, blue in SH).

Over the Atlantic basin, these conditions act to extend westward and accentuate the circulation anomalies associated with the tropical multi-decadal signal. This combination accounts for the basin-wide patterns of upper-level easterly wind anomalies and reduced vertical wind shear now present across the MDR, and greatly increases the probability of a very active Atlantic hurricane season during 2010.

c. Above average SSTs in the Main Development Region
Another factor guiding the outlook is the expectation of much above-average to near-record SSTs in the MDR during August-October. This prediction is based on climate model forecasts such as NOAA’s Climate Forecast System (CFS). and on observations over the last several months. Since March, monthly SST departures averaged over the entire MDR have been at record levels. The most recent SST analysis indicates departures exceeding +1.0^oC across most of the region. This warmth is much larger than anywhere else in the global tropics, and is a further indication that climate conditions are conducive for hurricane development in the Atlantic basin.

3. Further analysis of the Ongoing High Activity Era in the Atlantic Basin

Atlantic hurricane seasons exhibit extended periods lasting decades of generally above-normal or below-normal activity. These fluctuations in hurricane activity result almost entirely from differences in the number of hurricanes and major hurricanes forming from tropical storms first named in the MDR.

The current high-activity era has been in place since 1995. Hurricane seasons during 1995-2009 have averaged about 14.5 named storms, 8 hurricanes, and 4 major hurricanes, with an average ACE index of 160% of the median. NOAA classifies ten of the fifteen seasons since 1995 as above normal, with seven being hyperactive (ACE > 175% of median). Only five seasons since 1995 have not been above normal, which include four El Niño years (1997, 2002, 2006, and 2009) and the 2007 season.

This high level of activity since 1995 contrasts sharply to the low-activity era of 1971-1994 (Goldenberg et al. 2001), which averaged only 8.5 named storms, 5 hurricanes, and 1.5 major hurricanes, and had an average ACE index of only 75% of the median. One-half of the seasons during this low-activity era were below normal, only three were above normal (1980, 1988, 1989), and none were hyperactive.

Within the MDR, the atmospheric circulation anomalies that contribute to these long-period fluctuations in hurricane activity are strongly linked to the tropical multi-decadal signal (Bell and Chelliah 2006). A change in the phase of the multi-decadal signal coincides with the transition in 1995 from a low-activity era to the current high-activity era.

NOAA FORECASTERS

Climate Prediction Center
Dr. Gerry Bell, Lead Forecaster, Meteorologist; Gerry.Bell@noaa.gov
Dr. Jae Schemm, Meteorologist; Jae.Schemm@noaa.gov

National Hurricane Center
Eric Blake, Hurricane Specialist; Eric.S.Blake@noaa.gov
Todd Kimberlain, Hurricane Specialist; Todd Kimberlain@noaa.gov
Dr. Chris Landsea, Meteorologist; Chris.Landsea@noaa.gov
Dr. Richard Pasch, Hurricane Specialist; Richard.J.Pasch@noaa.gov

Hurricane Research Division
Stanley Goldenberg, Meteorologist; Stanley.Goldenberg@noaa.gov

REFERENCES

Bell, G. D., and M. Chelliah, 2006: Leading tropical modes associated with interannual and multi-decadal fluctuations in North Atlantic hurricane activity. J. of Climate. 19, 590-612.

M. Chelliah and Bell, G. D., 2004: Tropical multi-decadal and interannual climate variations in the NCEP/ NCAR Reanalysis. J. of Climate. 17, 1777-1803.

Goldenberg, S. B., C. W. Landsea, A. M. Mestas-Nuñez, and W. M. Gray, 2001: The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293, 474-479.

Gray, W. M., 1984: Atlantic seasonal hurricane frequency: Part I: El Niño and 30-mb quasi-bienniel oscillation influences. Mon. Wea. Rev., 112, 1649-1668.

Knaff, J. A., 1997: Predicting summertime Caribbean pressure in early April. Wea. and Forecasting, 13, 740-752.