The 2017 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 hurricane experts
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 location.
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),
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 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 the seasonal outlooks:
- The possible changing phase of the Atlantic Multi-Decadal Oscillation (AMO)
- Predicting El Niño and La Niña (also
called the El Niño-Southern Oscillation, or ENSO) impacts is an ongoing
scientific challenge facing climate scientists today. Such forecasts
made during the spring generally have limited skill.
- 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.
- Model predictions of sea surface
temperatures (SSTs), vertical wind shear, moisture, and stability have
limited skill this far in advance of the peak months (August-October) of
the hurricane season.
- Weather patterns that are unpredictable
on seasonal time scales can sometimes develop and last for weeks or
months, possibly affecting seasonal hurricane activity.
2017 Atlantic Hurricane Season Outlook: Summary
a. Predicted Activity
NOAA's 2017 Atlantic Hurricane Season Outlook
indicates that an above-normal or near-normal hurricane season is most
likely. The outlook indicates a 45% chance for an above-normal season, a
35% chance for a near-normal season, and a 20% chance for a
below-normal season. See NOAA definitions
of above-, near-, and below-normal seasons. The Atlantic hurricane
region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of
Mexico.
The outlook calls for a 70% probability
for each of the following ranges of activity during the 2017 hurricane
season, which runs from June 1st through November 30th:
- 11-17 Named Storms, which includes Tropical Storm Arlene in April
- 5-9 Hurricanes
- 2-4 Major Hurricanes
- Accumulated Cyclone Energy (ACE) range of 75%-155% of the median, which includes Arlene in April
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. These expected ranges are centered near or above the 1981-2010
seasonal averages of 12 named storms, 6 hurricanes, and 3 major
hurricanes. Most of the predicted activity is likely to occur during the
peak months (August-October, ASO) of the hurricane season.
b. Reasoning behind the outlook
NOAA's 2017 Atlantic hurricane season outlook reflects three main factors during August-October:
- (1) Either ENSO-neutral or weak El Niño conditions are expected over the tropical Pacific Ocean [ENSO refers to El Niño/ Southern Oscillation, which has three phases: El Niño, Neutral, and La Niña.],
- (2) Near- or above-average sea-surface
temperatures (SSTs) across much of the Atlantic hurricane Main
Development Region (MDR, which includes the tropical North Atlantic
Ocean and Caribbean Sea between 9.5°N and 21.5°N latitude), and
- (3) Near-average or weaker-than-average vertical wind shear in the MDR.
There is currently large model forecast
uncertainty regarding both oceanic and atmospheric predictions for
August-October 2017. This uncertainty is reflected in the forecasts for ENSO,
and for forecasts of SSTs and vertical wind shear in the MDR. The
combination of ENSO-neutral, a warmer MDR, and weaker vertical wind
shear within the MDR would likely yield levels of activity near the
higher ends of the predicted ranges. The combination of El Niño, a
cooler MDR, and near- or above-average vertical wind shear in the MDR,
would likely yield levels of activity toward the lower ends of the
predicted ranges.
Preparedness for Tropical Storm and Hurricane Landfalls:
It only takes one storm hitting an area to
cause a disaster, regardless of the overall activity. 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.
DISCUSSION
1. Expected 2017 activity
NOAA's 2017 Atlantic Hurricane Season Outlook
indicates that an above-normal or near-normal hurricane season is most
likely. The outlook indicates a 45% chance for an above-normal season, a
35% chance for a near-normal season, and a 20% chance for a
below-normal season. See NOAA definitions of above-, near-, and below-normal seasons.
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 named storms
and hurricanes during the season. This outlook indicates a 75% chance
that the 2017 seasonal ACE range will be 75%-155% of the median.
According to NOAA's hurricane season classifications,
an ACE value between 71.4% and 120% of the 1981-2010 median reflects a
near-normal season. Values above this range reflect an above-normal
season and values below this range reflect a below-normal season.
The 2017 Atlantic hurricane season is predicted to produce
(with 70% probability for each range) 11-17 named storms (which
includes TS Arlene in April), of which 5-9 are expected to become
hurricanes, and 2-4 of those are expected to become major hurricanes.
These ranges are centered near or above the 1981-2010 period averages of
about 12 named storms, 6 hurricanes and 3 major hurricanes.
Predicting the location, number, timing,
and strength of hurricanes landfalls is ultimately related to the daily
weather patterns including genesis locations and steering 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 2017 Outlook
NOAA's Atlantic hurricane season outlooks
are based on predictions of the main climate factors and their
associated conditions known to influence seasonal Atlantic hurricane
activity. The predictions are based on extensive monitoring, analysis,
and research activities, a suite of statistical prediction tools, and
dynamical models. The dynamical model predictions come from the NOAA
Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab
(GFDL) models FLOR-FA and HI-FLOR, the United Kingdom Met Office (UKMET)
GloSea5 model, and the European Centre for Medium Range Weather
Forecasting (ECMWF) model. ENSO forecasts are also provided from a
suite of statistical and other dynamical models contained in the suite of Niño 3.4 SST forecasts,
which is compiled by the IRI (International Research Institute for
Climate and Society) and the NOAA Climate Prediction Center.
NOAA's 2017 Atlantic hurricane season
outlook reflects three main factors during August-October: (1) Either
ENSO-neutral or weak El Niño conditions most likely over the tropical
Pacific Ocean, (2) Near- or above-average SSTs across much of the
Atlantic hurricane MDR, (3) Near-average or weaker-than-average vertical
wind shear in the MDR, and (4) Near-average sea level pressures in the
MDR.
a. ENSO-neutral or weak El Niño
La Niña dissipated in February, and ENSO-neutral conditions have been present since that time. Recently, above-average SSTs
have emerged in the east-central and eastern equatorial Pacific, and
near-average SSTs are evident in the central equatorial Pacific. The SST index for the Niño 3.4 region,
which spans the east-central equatorial Pacific between 120°W-170°W, is
currently near +0.5°C. ENSO-neutral conditions have Niño 3.4 index
values between -0.5°C and +0.5°C. El Niño is classified as a sustained
Niño 3.4 index value at or above +0.5°C for 5-consecutive months, along
with consistent atmospheric impacts. A weak El Niño is defined by a Niño
3.4 index between +0.5° and +1.0°C, and a moderate-strength El Niño is
defined by a Niño 3.4 index between +1° and +1.5°C.
El Niño causes increased vertical wind shear in the MDR,
along with anomalous sinking motion, and increased atmospheric
stability. These conditions are not conducive to hurricane formation and
intensification. Conversely, La Niña causes decreased vertical wind
shear in the MDR, along with anomalous rising motion and decreased
atmospheric stability. These conditions are conducive to a more active
Atlantic hurricane season.
Sub-surface ocean temperatures
are warmer than average in the central and eastern equatorial Pacific,
and near average across the east-central equatorial Pacific. This
pattern indicates that the anomalous warmth in the Niño 3.4 region is
currently confined to the near-surface, a condition that does not favor a
significant short-term evolution toward El Niño.
A time longitude diagram of the equatorial Pacific oceanic heat content anomalies
(i.e. average temperature anomalies in the upper 300 m of the ocean)
highlights the recent sub-surface temperature evolution and also shows
mixed conditions across the central and east-central equatorial Pacific.
Following the disappearance of negative heat content anomalies in
February, positive anomalies developed in both the central and eastern
Pacific. However, the anomalies across the east-central equatorial
Pacific have been weak, fluctuating between negative and positive
values. Also, the positive anomalies in the eastern Pacific have
recently decreased in strength. This variability is linked to a series
of equatorial oceanic Kelvin waves, whose downwelling phase (dashed
line) produces warming and whose upwelling phase (dotted line) produces
cooling. Such intra-seasonal variability can cause large fluctuations in
model predictions from one month to the next, and is also making it
difficult to predict confidently whether El Niño will develop in time
and of sufficient strength to suppress the 2017 Atlantic hurricane
season.
Large uncertainties for predictions of El
Niño and La Niña are typical at this time of the year, because this is
when the model forecast skill is at its lowest. Currently, there is considerable model spread
in the SST predictions for the Niño 3.4 region during ASO 2017,
generally ranging from ENSO-neutral to a weak El Niño. The average of
the dynamical model predictions (thick orange line) indicates a weak El
Niño during ASO, perhaps with sufficient strength to have a suppressing
influence on the Atlantic hurricane season. The average of the
statistical model predictions (thick green line) indicates borderline
neutral to weak El Niño conditions during ASO 2017.
There is even considerable spread in the SST forecasts coming from different resolutions of the same model. For example the CFS high-resolution (T-382) model
is predicting below-average SSTs in the Niño 3.4 region during ASO,
while the lower-resolution CFS T-126 model is predicting a weak El Niño
but with no suggestion that El Niño will be strong enough to increase
the vertical wind shear in the MDR.
Based on current conditions, the recent oceanic evolution, and the large spread in model forecasts, the latest ENSO outlook
issued by the CPC and IRI indicates approximately equal probabilities
of ENSO-neutral and El Niño throughout the hurricane season. That
outlook also states that "while the Niño-3.4 index may be near or
greater than +0.5°C for several months, the warmth may not last long
enough to qualify as an El Niño episode and/or may not impact the
atmospheric circulation."
b. Near-average or above-average sea surface temperatures in the MDR
Currently, SSTs are above average across the MDR, with the largest departures of between +0.5° and +1°C observed in the Caribbean Sea. For the ASO season, SSTs have been above average in the MDR since 1995.
However, there is typically low skill and considerable spread in model
predictions of Atlantic SSTs this far ahead of the ASO season.
Complicating this situation is the possible continuance of a strong cold
bias in forecasts from NOAA's CFS high-resolution model that was
evident during past two hurricane seasons. This model is again
predicting well below-average SSTs in the MDR, while the
lower-resolution CFS runs are predicting overall above-average SSTs in that region.
Uncertainty in the state of the Atlantic Multi-decadal Oscillation (AMO)
is another factor complicating the prediction of SST anomalies within
the MDR, and their impacts on the Atlantic hurricane season. The AMO
is a main climate factor that influences the Atlantic hurricane season,
and it sets the backdrop upon which other climate phenomena such as El
Niño and La Niña overlay. The AMO results in Atlantic hurricane seasons
historically exhibiting 25-40 year periods
of generally above-normal activity (called a high-activity era)
followed by 25-40 years of generally below-normal activity (called a
low-activity era).
At present, there is uncertainty as to
whether or not the warm (i.e. positive) phase of the AMO, which has been
associated with the high-activity era for Atlantic hurricanes which
began in 1995, has ended. There is also uncertainty as to whether a cool
(i.e., negative) phase of the AMO and low-activity era has begun
similar to that observed during 1971-1994. Two different measures of the strength and phase of the AMO
are the Kaplan AMO index and the Klotzbach-Gray AMO index. For
Jun.-Nov. (Red line), which is the Atlantic hurricane season, both
indices have generally shown strong positive values in association with
the recent high-activity era for Atlantic hurricanes. However, in recent
years, the Kaplan AMO index has remained positive, while the
Klotzbach-Gray index has fluctuated near zero. Similar results apply to
the ASO season (Green line). SSTs in the Atlantic MDR
have remained well above average since 1995, which is consistent with
the warm AMO during that season. The 2017 outlook reflects the
likelihood of either the neutral or warm phase of the AMO during the
2017 Atlantic hurricane season.
Conversely, for the cool season
January-May (Blue line), both indices have been much cooler during the
past few years compared to their Jun.-Nov. and ASO counterparts. Also,
the Kaplan AMO index has remained generally positive during the Jan.-May
season, whereas the Klotzbach-Gray index has recently turned strongly
negative. Thus, the AMO index in the months prior to the Atlantic
hurricane season has recently been a poor indicator of the upcoming AMO
index during the hurricane season.
c. Near-average or below-average vertical wind shear in the MDR
The strength and spatial distribution of
the vertical wind shear is critical for predicting the strength of the
hurricane season. Weaker vertical wind shear is conducive to stronger
and longer-lasting hurricanes, while strong shear can either prevent a
storm from forming or significantly weaken an existing storm. An El Niño
of sufficient strength acts to increase the vertical wind shear within
the MDR, thereby suppressing the Atlantic hurricane season.
At present, the model forecasts of
vertical wind shear vary considerably from one model to the next, are
generally dependent upon the model's predicted strength of El Niño. The
CFS model is predicting anomalously weak shear in the MDR during ASO
2017. At present, there is no indication that the shear will be
excessively weak so as to support an extremely active hurricane season,
and also no confident indication that the shear will be exceptionally
strong. The MDR will most likely experience near-average or
below-average vertical wind shear during ASO 2017.
d. Near-average sea level pressures in the MDR
Relatively small changes in sea level
pressures within the MDR can also affect the Atlantic hurricane
activity. Higher pressures on the order of 0.5-1.0 mb above average are
associated with more sinking and drying of the tropical atmosphere,
which is not conducive for hurricane activity. The opposite is true for
lower pressures than usual. Currently, pressures across the MDR are
near average and forecasts from the various global models do not
indicate large anomalies for either higher or lower pressure during ASO
2017.
NOAA FORECASTERS:
Climate Prediction Center
National Hurricane Center
Hurricane Research Division
REFERENCES
- Bell, G. D., and M. Chelliah, 2006: Leading tropical modes
associated with interannual and multi-decadal fluctuations in North
Atlantic hurricane activity. J. Climate. 19, 590-612.
- 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.
- Goldenberg, S. B. and L. J. Shapiro, 1996: Physical mechanisms for
the association of El Niño and west African rainfall with Atlantic major
hurricane activityJ. Climate, 9, 1169-1187.
- Klotzbach, P.J., and W. M. Gray, 2008: Multi-decadal Variability in North Atlantic Tropical Cyclone Activity, J. Climate, 21, 3929-3935.
- Landsea, C. W., G. A. Vecchi, L. Bengtsson, and T. R. Knutson, 2010:
Impact of Duration Thresholds on Atlantic Tropical Cyclone Counts. J. Climate, 23, 2508-2519.
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