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2009 East Pacific Hurricane Season Starts

May 15, 2009

Today, May 15th, is the start of the 2009 East Pacific hurricane season, about 2 weeks earlier than the Atlantic Basin hurricane season begins (June 1). Both end November 30th. These dates have been set to encapsulate well over 90% of tropical cyclones that form in each basin. On rare occasions we find development before each season "officially" begins and/or after the season "officially" ends. The last time we had an early start to the East Pacific hurricane season was in 1996 when a tropical depression formed on May 13th, two days before the official start dated; the following day it became the first tropical storm that year, and to this day remains nameless, the second tropical storm to form that year was named "Alma".

It turns out that on-average the East Pacific is more active than the Atlantic Basin, and this is the case in most but not all years (2005 being the clearest recent example). Since the satellite era began in 1965 tropical cyclone annual averages are 16 tropical storms, 8 of those becoming hurricanes. Of particular interest is the fact that prior to being able to see tropical cyclones in this area of the world with weather satellites, many tropical cyclones were missed as this is not a typical high ship traffic area, hence weather reports from them have been historically sparse.


A large majority of tropical cyclone tracks are toward west of west-northwest and out to sea away from land. However each year is unique, some years have many more tracks that turn north and/or northeast toward land than other years. These track differences are related to changes in steering from one year to the next. El Nino years are more likely to see landfalls in Mexico and remnant tropical cyclones affecting California and the southwest U.S. Locations for formation of tropical cyclones in the East Pacific are the narrowest in the world, with a local maximum located near 15N latitude, 110W longitude.

So what is the forecast for this 2009 season? The NOAA provides the following predictions:

1. Expected 2009 Activity

This Outlook is a general guide to the expected overall activity for the 2009 eastern Pacific hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular area.
The climate factors expected to guide the 2009 hurricane season are

  • The continuation of conditions that have been suppressing activity since 1995.

  • Either ENSO-neutral (no El Niño or La Niña) conditions or El Niño during the peak (July-September) of the season.

Historically, seasons with climate patterns similar to those expected this year have produced a wide range of activity. This outlook considers the historical distribution of activity for these climate factors, uncertainties in whether El Niño will develop, and the possibility of other unpredictable factors also influencing the season. Based on these factors, we estimate an 80% chance of a near-normal to below-normal hurricane season (40% chance of each), and a 20% chance of an above-normal season.

An important measure of total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the collective strength and duration of tropical storms and hurricanes during the season. The ACE index is also used to define the season type. A value of 92%-138% of the median (Median value is 109) defines a near-normal season.

Based on the above factors, we estimate a 70% chance that the 2009 seasonal ACE range will be 70%-130% of the median. This range can be satisfied even if the numbers of named storms, hurricanes, or major hurricanes fall outside their likely ranges.

The likely (70% chance) ranges of activity for 2009 are: 13-18 named storms, 6-10 hurricanes, and 2-5 major hurricanes.

2. The ongoing low-activity hurricane era in the eastern Pacific

The eastern Pacific has seen generally suppressed hurricane activity since 1995. During 1995-2008, 64% of seasons were below normal, 29% were near normal, only one was above normal. These seasons averaged about 14 named storms, 7 hurricanes, and 3 major hurricanes, with an ACE value of 80% of the median. This value falls within NOAA’s definition for a below-normal season. During this low-activity era, only the strong El Niño of 1997 produced an above-normal season. This ongoing low-activity era is the main reason we expect the 2009 hurricane season to be near- or below- normal.

These conditions contrast with the previous high-activity era (1982-1994) in the eastern Pacific, in which 62% of hurricane seasons were above normal, 31% were near normal, and only one was below normal. These seasons averaged 18.2 named storms, 10.8 hurricanes, and 5.6 major hurricanes, with an ACE value of 157% of the median. This value falls within NOAA’s definition for an above-normal season. During this period, 5 of 8 El Niño episodes led to an above-normal season, and only 1988 was categorized as below-normal, due to a strong La Niña.


The El Niño/ Southern Oscillation (ENSO) is an important climate predictor for eastern Pacific hurricane activity. El Niño generally produces conditions that are more conducive to hurricane activity, while La Niña suppresses the activity. However, these typical impacts can be strongly modulated by conditions associated with a low- or high-activity era.

La Niña dissipated during April 2009, and current sea-surface temperature anomalies in the east-central equatorial Pacific are near average. NOAA defines these conditions as ENSO-neutral.

There is considerable spread and uncertainty in the climate models regarding the evolution of ENSO during the next several months, and most of the models have historically shown little-to-no skill at this time of the year. The most recent dynamical model forecasts suggest El Niño will develop during the summer, although there is considerable disagreement as to its strength. In contrast, all of the recent statistical model forecasts suggest ENSO-neutral conditions will prevail.

The seasonal hurricane outlook reflects the possibility of El Niño developing and becoming strong enough to impact the season. However, because of the ongoing low-activity era, the historical data suggests these impacts would most likely raise the activity into the near-normal range. However, some dynamical model simulations predict significant El Niño-related circulation anomalies to develop this summer, would leaves open the possibility (20% chance) of an above-normal season. A below-normal season is more likely if ENSO-neutral conditions persist, and a near-normal season is more likely if El Niño develops.

So before you head on that vacation to Baja, Mexico or Central America, check with The Weather Channel for any tropical cyclone threats that might affect you vacation destination. Oh and those of you in Southern California, you might win a party bet with that 1939 Long Beach landfall, just remember you are routinely vulnerable at the beach in high surf that can come from distant or not so distant East Pacific hurricanes.


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