NHC 2012 Atlantic Hurricane Season Predictions
2012 Atlantic Hurricane Season Predictions from the NHC:
9-15 Named Storms,
1-3 Major Hurricanes
An Accumulated Cyclone Energy (ACE) range of 65%-140% of the median.
The complete text is below:
NOAA 2012 Atlantic Hurricane Season Outlook
Issued: 24 May 2012
The 2012 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 a general guide to the expected overall activity during the upcoming hurricane season. It is not a seasonal hurricane landfall forecast, and it does not predict levels of activity for any particular region.
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), the National Hurricane Center (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 7 out of 10 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 known to influence seasonal hurricane activity, and 2) climate models that directly predict seasonal hurricane activity.
Sources of uncertainty in this seasonal outlook
1. Predicting El Niño and La Niña (also called the El Niño-Southern Oscillation- ENSO) impacts is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have limited skill.
2. 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.
3. Model predictions of sea-surface temperatures, vertical wind shear, moisture, and stability have limited skill this far in advance of the peak months (August-October) of the hurricane season.
4. Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.
2012 Atlantic Hurricane Season Outlook: Summary
NOAA’s 2012 Atlantic Hurricane Season Outlook indicates that a near-normal season is most likely. The outlook calls for a 50% chance of a near-normal season, a 25% chance of an above normal season, and a 25% chance of a below-normal season. See NOAA definitions of above-, near-, and below-normal seasons. The Atlantic hurricane region includes the North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico.
This outlook reflects the possibility of competing climate factors, combined with several circulation and sea surface temperature (SST) features that suggest a less active season compared to many in recent years. Favoring an above-normal season is the ongoing conditions that have been associated withincreased Atlantic hurricane activity since 1995, combined with expected near-average SSTs across much of the tropical Atlantic Ocean and Caribbean Sea (called the Main Development Region, or MDR).
A potentially competing climate factor is the possible development of El Niño during the season. If El Niño develops, it could make conditions less conducive for hurricane formation and intensification during the peak months (August-October) of the season, thus shifting the activity toward the lower end of the predicted range.
If they persist, two other factors that are now present could also compete with conditions associated with the high-activity era. These are: 1) Enhanced vertical wind shear across the MDR, and 2) Cooler-than-average SSTs in the far eastern tropical Atlantic.
Given the current and expected conditions, combined with model forecasts and possible competing factors, we estimate a 70% probability for each of the following ranges of activity during 2012:
- 9-15 Named Storms,
- 4-8 Hurricanes
- 1-3 Major Hurricanes
- An Accumulated Cyclone Energy (ACE) range of 65%-140% of the median.
The seasonal activity is expected to fall within these ranges in 70% of seasons with similar climate conditions and uncertainties to those expected this year. These ranges do not represent the total possible ranges of activity seen in past similar years.
Note that the above ranges are centered near the official NHC 1981-2010 seasonal averages of 12 named storms, 6 hurricanes, and 3 major hurricanes.
This Atlantic hurricane season outlook will be updated in early August, which coincides with the onset of the peak months of the hurricane season.
It only takes one storm hitting your area to cause a disaster, regardless of the overall activity predicted in the seasonal outlook. Therefore, residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare every hurricane season regardless of this, or any other, seasonal outlook.
Predicting where and when hurricanes will strike is related to daily weather patterns, which are not reliably 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 given locality will be impacted by a hurricane this season.
1. Expected 2012 activity
Climate signals and evolving oceanic and atmospheric conditions, combined with dynamical model forecasts, indicate that a near-normal 2012 Atlantic hurricane season is most likely. This outlook calls for a 50% chance of a near-normal season, a 25% chance of an above-normal season, and a 25% chance of a below normal season.
An important measure of the total overall seasonal activity is NOAA’sAccumulated Cyclone Energy (ACE) index, which accounts for the intensity and duration of named storms and hurricanes during the season. This outlook indicates a 70% chance that the 2012 seasonal ACE range will be 65%-140% of the median. According to NOAA’s hurricane season classifications, an ACE value above 111% of the 1981-2010 median reflects an above-normal season. An ACE value below 71.4% of the median reflects a below-normal season.
Consistent with the expected ACE range, the 2012 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 9-15 named storms, of which 4-8 are expected to become hurricanes, and 1-3 are expected to become major hurricanes. These ranges are centered near the 1981-2010 period averages of 12 named storms, 6 hurricanes and 3 major hurricanes.
2. Science behind the 2012 Outlook
The 2012 seasonal hurricane outlook reflects the possibility of competing climate factors, combined with current conditions in the MDR (which includes the Caribbean Sea and tropical Atlantic ocean between 9oN-21.5oN; Goldenberg et al. 2001) which are less-conducive compared to those seen in many past active seasons.
The three main climate factors for this outlook are the tropical multi-decadal signal (which reflects conditions associated with the ongoing high-activity era), the El Niño/ La Niña cycle (ENSO), and tropical Atlantic SSTs. Favoring an above-normal season is the expected continuation of conditions associated with the Atlantic high-activity era that began in 1995, combined with the expectation of continued near-average SSTs in the Main Development Region.
The main competing climate factor is the possible development of El Niño later in the summer or early fall. If El Niño develops, it could make conditions less conducive to hurricane formation and intensification during August-October (the peak months of the season), perhaps shifting the seasonal activity toward the lower end of the predicted range.
The outlook also takes into account dynamical model predictions from the NOAA Climate Forecast System (CFS), NOAA’s Geophysical Fluid Dynamics Lab (GFDL) model CM2.1, the European Centre for Medium Range Weather Forecasting (ECMWF), the United Kingdom Meteorology (UKMET) office, and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble.
These models show large spreads in the ENSO forecasts for ASO, ranging from ENSO-Neutral to a moderate-strength El Niño episode. As a result, their forecasts for the Atlantic hurricane season also show a considerable spread, ranging from slightly above normal to slightly below normal.
Two other factors, which are now present, also suggest that the MDR could be less conducive to an active hurricane season. These are: 1) Enhanced wind shear resulting from strong upper-level westerly winds across the MDR, and 2)Below-average SSTs in the far eastern tropical Atlantic in association with the recent La Niña episode. Because of uncertainties in the upcoming ENSO evolution (i.e., ENSO-Neutral or El Niño), it is unclear whether these factors will persist throughout the hurricane season.
The CFS model is predicting the vertical wind shear within the MDR to become below-average in the next few months as the upper-level westerlies weaken. The CFS is also predicting the SSTs in the far eastern tropical Atlantic to return to near-average, which reflects the demise of La Niña and no development of El Niño (i.e. ENSO-Neutral). Other models predict El Niño to develop, and consequently they call for the vertical wind shear to remain strong.
a. Expected continuation of tropical multi-decadal signal
One factor guiding this outlook is the expected continuation of the tropical multi-decadal signal (Bell and Chelliah 2006), which has contributed to the current high-activity era in the Atlantic basin that began in 1995. This signal incorporates the warm phase of the Atlantic Multi-decadal Oscillation (AMO) and an enhanced west African monsoon system. It is associated with inter-related atmospheric conditions that are conducive to increased Atlantic hurricane activity.
During 1995-2011, some key aspects of the tropical multi-decadal signal within the MDR have included reduced vertical wind shear and weaker easterly trade winds, below-average sea-level pressure, a configuration of the African easterly jet that is more conducive to hurricane development from tropical cloud systems (aka Easterly waves) moving off the African coast, and warmer than average SSTs.
Two key aspects of the tropical multi-decadal signal are now present: the warm phase of the AMO, along with weaker mid-level easterly winds across the eastern tropical Pacific and tropical Atlantic Ocean. During ASO, this wind pattern typically produces a more conducive configuration of the African Easterly Jet.
However, other features of the tropical multi-decadal signal are not yet present, as indicated by strong northeasterly trade winds extending southward into the northeastern MDR, and by enhanced upper-level westerly winds and strong vertical wind shear across the MDR.
Note that it is not possible to know with certainty whether the tropical multi-decadal signal is indeed continuing during 2012, and current climate models cannot skillfully forecast the multi-decadal variability of the Atlantic climate system. However, given that the warm AMO phase now present has also been present for the past 17 years, and that this SST pattern has been previously linked to the tropical multi-decadal signal, it is reasonable to expect that this signal is again present in 2012. If so, this would reflect a continuation during the 2012 Atlantic hurricane season of the active Atlantic phase of the tropical multi-decadal signal that began in 1995.
b. Expected near-average SSTs in the Main Development Region
The second factor guiding this outlook is the expectation of near-average SSTs across most of the MDR during ASO, and the possibility of continued below-average SSTs in the far eastern tropical Atlantic. These expectations are based on current observations, the high-activity era, the dissipation of La Niña, and model forecasts for the possibility of El Niño.
Sea surface temperatures measured over the entire MDR are currently near-average, with below-average SSTs in the far eastern tropical Atlantic and above-average SSTs in the central and western MDR. These departures are considerably less than was observed last year at this time, and are consistent with a springtime cooling and enhanced northeasterly trade winds typical of La Niña. Given that La Niña has dissipated.
A concern for this outlook is that seasonal Atlantic hurricane activity can be especially sensitive to SST departures in the eastern tropical Atlantic. Below average SSTs in that region during ASO are typically associated with stronger northeasterly trade winds and less conducive thermodynamics, both of which can suppress hurricane activity. Another current feature of interest is that the average SST departures in the MDR are comparable to those of the remainder of the global tropics. If this condition persists, it is also suggestive of a less active season.
There is considerable uncertainty in forecasting ASO SSTs for the tropical Atlantic so far in advance, partly because of forecast uncertainties in the strength of the west African monsoon system and partly because of uncertainties in predicting ENSO. The CFS high-resolution model is predicting the SSTs in the far eastern tropical Atlantic to return to near-average, which reflects the demise of La Niña and no development of El Niño (i.e. ENSO-Neutral). Conversely, the CFS low-resolution model is predicting SSTs in the eastern tropical Atlantic to remain below average.
c. ENSO-Neutral or El Niño conditions likely
Another climate factor known to significantly impact Atlantic hurricane activity is ENSO. The three phases of ENSO are El Niño, La Niña, and ENSO-Neutral. El Niño events tend to suppress Atlantic hurricane activity, while La Niña events tend to enhance it (Gray 1984). These typical impacts can be strongly modulated by conditions associated with a low- or high-activity era.
The 2011-12 La Niña episode dissipated in April, as equatorial SSTs across the central and east-central Pacific returned to near-average and SSTs in the eastern Pacific became above average. Based on observations and ENSO forecast models, the official CPC/IRI ENSO forecast issued in early Mayindicates that ENSO-Neutral conditions are likely through the summer with approximately equal chances of Neutral and El Niño conditions during the fall.
Predicting the various phases of ENSO and their impacts is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have limited skill. The present uncertainty in the model forecasts is indicated by the large spread in predicted SST departures in the east-central equatorial Pacific during ASO 2012. Approximately one-half of the models predict ENSO-Neutral conditions (defined by the CPC as SST departures in the Niño 3.4 region between -0.5oC and +0.5oC) during the hurricane season. Also, approximately one-half of the models predict El Niño conditions (Niño 3.4 SST departures above +0.5oC).
This spread in the model forecasts, combined with the limited predictive skill exhibited by all such models at this time of the year, is a main reason why we are presently indicating only a 25% chance of an above-normal season. If El Niño fails to develop, the probability of an above- normal Atlantic hurricane season will be higher and the actual seasonal activity will likely be toward the upper end of our predicted ranges. ENSO and other climate conditions will be re-evaluated for the update of this outlook to be issued in early August.
3. Multi-decadal fluctuations in Atlantic hurricane activity
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 that first develop in the MDR.
The current high-activity era has been in place since 1995. Hurricane seasons during 1995-2011 have averaged about 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE index of 153% of the median. NOAA classifies 12 of the 17 seasons since 1995 as above normal, with eight being extremely active (i.e., hyperactive defined by ACE > 165% of median). Only five seasons since 1995 have not been above normal, which include the 2007 season and four El Niño years (1997, 2002, 2006, and 2009).
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 74% 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 Tropics-wide multi-decadal signal (Bell and Chelliah 2006), which incorporates the warm phase of the AMO and an enhanced west African monsoon system.. A change in the phase of the tropical multi-decadal signal coincides with the transition in 1995 from a low-activity era to the current high-activity era.
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