2009 East Pacific Hurricane Season

posted Jul 11, 2012, 3:09 PM by Tom Kachelman
On the 15th of May was the start of the 2009 East Pacific hurricane season, about 2 weeks earlier than the Atlantic Basin hurricane season began (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:

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 these 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!!

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.

Published on: Jul 21, 2009 @ 7:04