Atlantic Hurricane Season Outlook 20XX
NWS Climate Prediction Center College Park MD
1100 AM EDT Day Month DY 20XX
 
 
Note: figures mentioned in the discussion are available
on the internet at http://www.cpc.ncep.noaa.gov
 
NOAA 20XX Eastern Pacific Hurricane Season Outlook Issued DY Mon 20xx


The 20XX eastern Pacific hurricane season outlook is an official
product of the National Oceanic and Atmospheric Administration (NOAA)
Climate Prediction Center (CPC), and is produced in collaboration
with scientists from the NOAA National Hurricane Center (NHC). The
eastern Pacific hurricane region covers the eastern North Pacific
Ocean east of 140deg W north of the equator.

Interpretation of NOAA?s eastern Pacific hurricane season outlook
This outlook is general guide to the expected overall activity
during the upcoming hurricane season. It is not a seasonal hurricane
landfall forecast, and it does not imply 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 even a tropical storm) to cause a disaster. Therefore,
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 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 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 predictions of large-scale climate factors
and conditions known to strongly influence seasonal eastern Pacific
hurricane activity, along with climate model forecasts. The outlook
also takes into account uncertainties inherent in such climate
outlooks.

Sources of uncertainty in this seasonal outlook 1. Predicting El
Nino and La Nina (also called 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. Weather patterns that are unpredictable on seasonal time scales
can sometimes develop and last for weeks or months, possibly
affecting seasonal hurricane activity.


20XX Eastern Pacific Hurricane Outlook Summary

NOAA?s 20XX eastern Pacific Hurricane Season Outlook [FIG. 1]
indicates a near- or above-normal season is likely, with a 50% chance
of an above-normal season, a 40% chance of a near-normal season,
and a 10% chance of an below normal season. See NOAA definitions
(Joe; Link here to the Background Information) of above, near-, and
below- normal seasons. The eastern Pacific hurricane region covers
the eastern North Pacific Ocean east of 140deg W north of the equator.

The main climate factor guiding the outlook is the likely development
of El Nino (Fig. 2) during the summer.  El Nino decreases the
vertical wind shear over the eastern tropical Pacific, favoring more
and stronger tropical storms and hurricanes. The eastern Pacific
has been in an era of lower hurricane activity since 1995 (Fig. 3),
but this pattern will be offset in 20XX by El Nino.

Historically, this combination of factors produces a near- or
above-normal eastern Pacific hurricane season. We estimate a 70%
chance of occurrence for each of the following ranges of activity
this season: * 14-20 named storms, * 6-11 hurricanes, * 3-6 major
hurricanes, * An ACE range 95%-160% of the median.

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.

The predicted ranges are centered near or above the official NHC
1981-2010 seasonal averages of 15 named storms, 8 hurricanes,
and 4 major hurricanes.

There will be no further updates to this outlook.


DISCUSSION

1. Expected 20XX Activity

Evolving climate signals, combined with dynamical and statistical
model forecasts, indicate that a near- or above-normal eastern
Pacific hurricane season is likely during 20XX. This outlook calls
for a 50% chance of an above-normal season, a 40% chance of a
near-normal season, and a 10% chance of a below-normal season.

Predictions from NOAA's Climate Forecast System (CFS), NOAA
Geophysical Fluid Dynamics Lab (GFDL) model FLOR-FA, the European
Centre for Medium Range Weather Forecasting (ECMWF), and the EUROpean
Seasonal to Inter-annual Prediction (EUROSIP) ensemble are all
suggesting near- or above-normal tropical cyclone activity in the
eastern Pacific this season, though their forecast skill for the
region is limited at this lead time.

An important measure of the total seasonal activity is NOAA?s
Accumulated Cyclone Energy (ACE) index [Fig. 3], which accounts for
the combined strength and duration of tropical storms and hurricanes
during the season. We estimate a 70% chance that the 20XX seasonal
ACE index will be in the range of 95%-160% of the median. According
to NOAA?s definitions of season strength (Joe; Link here to the
Background Information), an ACE value above 115% of the 1981-2010
median indicates an above-normal season, and a value of 80%-115%
of the median indicates a near-normal season. Consistent with the
expected ACE range, other likely (70% chance) ranges of activity
for 20XX [Fig. 1] are 14-20 named storms, 6-11 hurricanes, and 3-6
major hurricanes.

2. Science behind the outlook a. El Nino

The 20XX seasonal hurricane outlook reflects the likely development
of El Nino (Fig. 2). El Nino reduces the vertical wind shear in the
eastern Pacific hurricane basin, making atmospheric conditions more
conducive to hurricane activity.

At present, equatorial Pacific SSTs are above average (Fig. 4),
with the largest departures (exceeding 1oC) centered on the date
line. SST anomalies in all of the Nino regions are also increasing,
and anomalies in the Nino 3.4 region, which spans the central and
east-central equatorial Pacific between 120oW-170oW, are currently
0.4oC. This value is approaching the CPC?s lower threshold for El
Nino (+0.5oC).

Observations show that the atmosphere is also trending to an El Nino
state. For example, a time-longitude section of 200-hPa velocity
potential anomalies shows anomalous upper-level divergence since
January over the central equatorial Pacific (Fig. 5). This signal
is consistent with enhanced convection near the date line (Fig. 6),
a key feature of El Nino.

Anomalous westerly trade winds across the western equatorial Pacific
(Fig. 7), along with several westerly wind bursts, have also been
present since January. A westerly wind burst triggered a strong
downwelling equatorial oceanic Kelvin wave in February (Fig. 8),
and this wave subsequently reached the west coast of South America in
late April. This Kelvin wave acted to shift the oceanic thermocline
deeper into the ocean, resulting in above-average temperatures and
increased heat content between the thermocline and the ocean surface.

A depth-longitude section of sub-surface temperature anomalies and a
time series of the upper-ocean heat content highlight the substantial
sub-surface warmth associated with this Kelvin wave (Fig. 4). The
persistence of the westerly wind anomalies has helped to lock in
this anomalous warmth, further setting the stage for El Nino.

The average forecast of the dynamical prediction models (closed
markers) contained in the suite of IRI/ CPC Nino 3.4 SST forecasts
(Fig. 9) (yellow line) predicts El Nino to form during the May-July
(MJJ) season and to reach moderate strength (SST values of 1oC -
1.5oC) during ASO. The statistical model forecasts (open markers)
are generally cooler than the dynamical model predictions, and show
a weak El Nino during the ASO season.

There are also differences in the predicted timing and strength of
El Nino within models having a similar formulation. For example,
NOAA's high-resolution CFS model (T-382) is forecasting a delay
in the development of El Nino, while the lower resolution model
(T-126) is predicting El Nino to develop during JJA (Fig. 10). These
results produce some uncertainty as to exactly when El Nino will
develop and how strong it will become.  Despite these differences,
both CFS models predict upper-level westerly wind anomalies and
reduced vertical wind shear across the eastern Pacific hurricane
region during JJA and ASO (Fig. 11).

b. Low-activity era for eastern Pacific hurricanes

Eastern Pacific hurricane seasons have been less active since 1995,
while the Atlantic Basin has been more active. During 1995-2013,
42% of eastern Pacific hurricane seasons were below normal, 37%
were near normal, and only 21% were above normal. Seasons during
this period averaged about 13 named storms, 7 hurricanes, and 3
major hurricanes, with an ACE value of 88% of the median.

These levels of activity contrast sharply with the more active period
1982-1994. For the eastern Pacific, 62% of hurricane seasons during
1982-1994 were above normal, 23% were near normal, and only 15%
were below normal. Seasons during this period averaged about 18
named storms, 10.5 hurricanes, and 5 major hurricanes, with an ACE
value of 150% of the median. This ACE value falls within NOAA?s
definition for an above-normal season.

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. Christopher Landsea, Meteorologist;
Chris.Landsea@noaa.gov

BACKGROUND INFORMATION: EASTERN PACIFIC HURRICANE SEASON

The eastern Pacific hurricane region covers the eastern Pacific Ocean
east of 140oW north of the equator. This area is one of the most
prolific tropical storm formation regions in the world. Eastern
Pacific tropical storms most often track westward over open
waters, sometimes reaching Hawaii and beyond. However, some storms
occasionally head toward the northeast, bringing rainfall to the
arid southwestern United States during the summer months. Also,
during any given season, tropical storms can affect western Mexico
or Central America, especially early and late in the season

The official eastern Pacific hurricane season runs from 15 May
through 30 November. The peak activity typically occurs during July
through September. During the period 1981-2010, the eastern Pacific
seasonal averages were 15.4 named storms (maximum 1-minute surface
winds between 39-73 mph), with 8.4 of those becoming hurricanes
(maximum 1-minute surface winds of at least 74 mph) and 3.9 becoming
major hurricanes (maximum 1-minute surface winds exceeding 111 mph,
categories 3-5 on the Saffir-Simpson hurricane scale).

Eastern Pacific hurricane seasons exhibit long periods of above-
normal and below-normal activity in response to large-scale
climate patterns. Seasons also exhibit year-to-year variability
in response to ENSO.  El Nino contributes to decreased easterly
vertical wind shear and favors above-normal hurricane activity
in this region.  Historically, El Nino is not associated with
below-normal seasons. Conversely, La Nina contributes to increased
vertical shear and less overall activity. Historically, 60% of
La Nina episodes have been associated with below-normal hurricane
seasons, and only 28% have produced an above-normal season. However,
the ENSO impacts can be strongly influenced by the background climate
patterns. As a result, NOAA accounts for the combined influences
of both climate factors when making its seasonal hurricane outlooks.

Measuring overall activity: The Accumulated Cyclone Energy (ACE)
Index

The phrase "total seasonal activity" refers to the collective
intensity and duration of eastern Pacific named storms and
hurricanes occurring during a given season. The measure of total
seasonal activity used by NOAA is called the Accumulated Cyclone
Energy (ACE) index [Fig 3]. The ACE index is a wind energy index,
defined as the sum of the squares of the maximum sustained surface
wind speed (knots) measured every six hours for all eastern Pacific
named systems while they are at least tropical storm strength.

NOAA?s eastern Pacific hurricane season classifications

Reliable tropical storm and hurricane data for the eastern Pacific
began in 1971. The 1981-2010 mean value of the ACE index is 113.3
x 104 kt2, and the median value is 100.4 x 104 kt2. The following
season classifications are based on an approximate 3-way partitioning
of seasons based on the ACE value, combined with the seasonal number
of tropical storms, hurricanes and major hurricanes.

Above-normal season: An ACE index above 115 x 104 kt2 (115% of
the median) and at least two of the following three conditions:
17 or more named storms, 9 or more hurricanes, and 5 or more major
hurricanes.


Near-normal season: An ACE index in the range 80-115 x 104 kt2
(80%-115% of the median), or an ACE value higher than 115 x 104 kt2
but with less than two of the following three conditions being met:
17 or more named storms, 9 or more hurricanes, and 5 or more major
hurricanes.

Below-normal season: An ACE index below 80 x 104 kt2 (80% of
the median.

 
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