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Pacific ENSO Update

3rd Quarter, 2010 Vol. 16 No. 3


The PEAC tropical cyclone outlook1 for the second half of 2010 (the heart of the typhoon season) calls for below normal tropical cyclone activity in the western North Pacific basin. A weather pattern of anomalous low-level easterly winds coupled with anomalous westerly winds in the upper troposphere has become established in the western Pacific basin, which is typical during La Niña. These conditions prevent the normal development of the monsoon trough, and inhibit tropical cyclone development. These conditions (present for all of the year 2008) have returned for 2010, and should continue to suppress cyclone activity in the basin for the remainder of the year. The suppression of cyclone activity will be most prominent from Guam eastward. Normally, there are four named cyclones passing within 180 nm of Guam. This year, there may be only one or two (half of normal). The western North Pacific basin is the world's most prolific generator of tropical cyclones, with a yearly average of 31 numbered cyclones (18 typhoons, 10 tropical storms and 3 tropical depressions). The number of tropical storms and typhoons in the basin during 2010 is anticipated to be reduced by 10-15% (a net loss of 4 or 5 named storms). Cyclone activity is likely to be shifted to the west and north (as in 2008), which reduces the number of named storms in Micronesia by a more pronounced amount than in the whole basin. Islands in the western half of Micronesia (e.g., Yap and Palau) will be the only places within Micronesia with a near normal threat of gales and other hazards (e.g., high waves) associated with tropical cyclones. Islands from Pohnpei and eastward into the Republic of the Marshall Islands experience tropical storms and typhoons primarily during El Niño. The risk of a damaging tropical cyclone in these locations is considered very low during 2010.

On 06 July, 2010, the TSR group released a forecast for below-normal TC activity in the western North Pacific for the 2010 season (a net loss of 2 typhoons and 3 tropical storms). On 24 June 2010, the City University of Hong Kong group issued a forecast for a similar reduction to the number of cyclones for 2010.

Using his newly developed statistical forecast scheme, Paul Stanko at WFO Guam rated the odds of cyclone numbers for the typhoon season of 2010 as follows: Category forecasts: Record High (44 or more): 0%, Far Above Normal (38 to 44): 1%, Above Normal (33 to 37): 9%, Near Normal (29 to 32): 29%, Below Normal (27 or 28): 15%, Far Below Normal (21 to 26): 40%, Record Low (21 or less): 10%.

1The PEAC tropical cyclone forecasts for 2010 are provisional. The PEAC considers input from three seasonal outlooks for tropical cyclone activity in the western North Pacific basin: (1) The City University of Hong Kong Laboratory for Atmospheric Research, under the direction of Dr. J. C-L. Chan; (2) The Benfield Hazard Research Centre, University College London, Tropical Storm Risk (TSR) research group, UK, led by Dr Adam Lea and Professor Mark Saunders (; and, (3) an experimental typhoon outlook produced by Paul Stanko (forecaster at the Guam WFO).

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Page Last Modified: August 12 2010 00:49:54 GMT


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