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Pacific ENSO Update

3rd Quarter, 2010 Vol. 16 No. 3


During the first few months of 2010, El Niño conditions matured, and by the end of June 2010, the Pacific Basin was on the verge of entering La Niña. Most forecast agencies anticipate the La Niña will commence by the end of July or by early August, and will then last for the rest of the year. The weather patterns across Micronesia are already fully those that are expected for La Niña; for example, rainfall for the first half of 2010 was below normal at most locations in Micronesia (see Figures 1a and 1b), and strong easterly surface wind prevailed. Additional alterations to the weather patterns were observed that are typical during years that follow El Niño (regardless of whether the climate state enters La Niña or returns to ENSO-neutral); these include a substantial reduction of tropical cyclones in the western North Pacific basin and a near elimination of the normal monsoon trough in Micronesia. The weather in such a circulation pattern is best described as "tranquil". Typhoon activity is shifted away from the region, the monsoon is inhibited, and extremes of heavy rainfall (e.g. more than 4 inches in 24 hours) are reduced. In American Samoa, the just-ended rainy season was wetter than normal. The dry season has now started there, and strong southeast trade winds have become established.

In the Hawaiian Islands, the recently completed October 2009 through April 2010 wet season, or "Hooilo", ranks as the driest in the past thirty years, and one of the driest in the past 55 years. This is based on average of wet season rainfall rankings from seven reporting stations across the state with continuous records. The duration of extreme drought in the Hawaiian islands was approaching one year at the beginning of June, just as the summer dry season was getting underway.

Sea-level variation in the USAPI is sensitive to the ENSO cycle, with lower (higher) sea level typically observed during El Niño (La Niña) events. As compared to May 2010, the monthly mean sea-level in June 2010 recorded a rise in most of the north Pacific stations (i.e., Guam, Palau, Yap, and Pohnpei). The forecasted values of sea level for the forthcoming seasons indicate that sea levels for most of the stations in the north Pacific are likely to continue rising. The maxima will also be higher than normal during this same time period. This will be due to prevailing stronger-than-average westerly wind anomalies in the vicinity of western and central tropical Pacific. Additionally, the Hawaiian stations are likely to be slightly elevated during this time. Forecasted values are supportive of borderline La Niña conditions, which recently appeared in mid-June.

The following comments from the latest Drought Information Statement were posted on the WFO Guam website and the PEAC website on June 24, 2010:

"Drought conditions are over for Micronesia and this is the final statement for this event..."

"Upper-level wind patterns favorable for producing rainfall across Micronesia are finally entrenched across the islands. Drought conditions are no longer expected for any of the Micronesian islands. Although some of the northern Marshall Islands and Saipan and Tinian should continue to conserve water until drinking water sources are partially recharged. Increased cloudiness and weakened trade winds should reduce the heat and dryness experienced over the last several months."

"The earlier El Niño event has transitioned into an ENSO-neutral status. The climate prediction center has issued a La Niña watch, and La Niña conditions could develop in the next few weeks. If this occurs, it will minimize monsoon and tropical cyclone activity over Micronesia. In the meantime, islands west of 155 east longitude could see a few episodes of monsoon activity and early tropical cyclone development."

"The El Niño-related drought was most severe for islands farthest north from the equator; especially the northern Marshall Islands and the Mariana Islands. Also, very dry conditions in February and early March damaged some of the subsistence food crops on many of the southern Micronesian islands, and some food shortages may still be occurring on some islands."

Pacific ENSO Applications Climate (PEAC) Center
c/o NOAA NWS - Weather Forecast Office Honolulu
2525 Correa Road, suite 250
Honolulu, HI 96822
(808) 956-2324

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Page Last Modified: August 12 2010 00:49:48 GMT


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