Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 1:51 PM GMT on May 21, 2007
It's going to be a below-average typhoon season in the Western Pacific, according to the April 23, 2007 forecast issued by Dr. Johnny Chan of the City University of Hong Hong. Dr. Chan is calling for 25 named storms and 14 typhoons in 2007, slightly below the average of 27 named storms and 17 typhoons. The forecast for a below-average typhoon season was based on three main factors:
1) The Western Pacific has been in an inactive period since 1998, and this inactivity is expected to persist.
2) This year should have neutral El Niño conditions, or a La Niña event. These conditions have led to below-average typhoon activity since the current inactive typhoon period began in 1998 (Figure 1).
3) A stronger-than-normal high pressure system has been in place over the subtropics in the Western Pacific in February and March of 2007. Such higher pressures are associated with reduced typhoon activity later in the year.
Figure 1. Time series of the annual number of named tropical storms in the Western Pacific. Red circles and blue squares indicate El Niño and La Niña years, respectively. The green triangle is this year's forecast. An average year has 27 named storms. Image credit: City University of Hong Kong.
How good are these forecasts?
Dr. Chan has been making seasonal typhoon forecasts since 2000, and his forecasts have been skillful. The 2000-2006 forecasts issued in April for number of named storms and typhoons have a Mean Square Skill Score of about 30 and 50, respectively, according to some quick calculations I did. This is considerably higher than the late May seasonal forecasts for Atlantic hurricane activity issued by both Dr. Bill Gray/Phil Klotzbach's team at Colorado State University and Tropical Storm Risk, Inc. However, hurricane activity in the Atlantic has varied much more from year-to-year in the past decade than typhoon activity in the Western Pacific. This makes the Atlantic forecast problem more challenging.
Typhoon Yutu, the season's first major typhoon, intensified to a Category 4 storm with 145 mph winds yesterday. The typhoon is over open water and weakening, but could pass through the islands near Iwo Jima as a Category 2 typhoon on Tuesday. Yutu is the second named storm of the year in the Western Pacific, which usually sees three named storms by the end of May.
Figure 2. Typhoon Yutu at 00 GMT May 20, 2007. Image credit: Navy/NRL.
Tuesday (tomorrow), I'll report on NOAA's 2007 Atlantic hurricane season forecast, which will be released at 11am EDT.
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