How Active Will this Year's Western Pacific Typhoon Season Be?

June 26, 2014

Earlier this year there was a lot of hype about how the Western Pacific Typhoon Season was off to a jackrabbit start. First, let's get one thing straight: there is no such thing as a season in the Western Pacific Typhoon Basin. Typhoons can-and do-form in any month of the year.

Granted they are not as intense in February as they are in September, but in 2002 Mitag did get down to 930 mb in early March-and it was already the second typhoon of the year. You might then assume that 2002 must have been an active year, but it was not. In fact it was just barely average. Take away the two storms that formed before March and the three storms that formed in November and it would have been sub-par.

The fact of the matter is that fast starts have not historically meant active years. The first typhoon of 2010 formed in March but this proved to be the least active year in the JMA record since 1951. The table below shows that there is really no greater chance for an active season (>29 named storms) when there have been three or more early season storms (before April 30) than when there have been none, one, or two.

Early Active Seasons Inactive Seasons Rel. Probability
0 3 9 0.250
1 9 12 0.429
2 11 5 0.688
3 4 6 0.400
4 2 2 0.500
5 1 0 1.000

Probability for an active season by number of named storms before April 30 (1951-2013)

This brings us to 2014. By the end of April, there were five named storms according to the JMA. But only one, Faxai, achieved hurricane strength (with a minimum central pressure less than 985mb) and none lasted even six days. The little bursts of activity in February and April were likely the result of a favorable Madden-Julien Oscillation. The MJO, as it is often called, is a reference to an oscillation of winds in the stratosphere that circle the globe every 30-60 days. When the winds are divergent at upper levels, that allows deep (organized) convection to develop and that is conducive to the formation of typhoons.

So, now its late June and the JMA typhoon count is up to seven-possibly with some more help from the next wave of the MJO. Still there has not been anything more intense than Faxai (975 mb). And still there has not been anything longer lived than 5.25 days.

Of course the "season" is still young. There are a few more MJO waves that will cross the basin before November, when some of the most intense storms can still form-as Super Typhoon Haiyan did last year.

A favorable MJO, coupled with other favorable conditions with respect to shear, sea surface temperatures, and pre-existing monsoon troughs can rack up typhoon counts in a hurry. And, it is the number of storms that develop during the prime time(June-September) much more than the starting numbers that determine how active a season it will really be-in the Western Pacific Basin, or in any other basin for that matter.

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