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Weather and Forecasting Challenges in the Pacific

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Weather and Forecasting Challenges in the Pacific SEPTEMBER 1998 KODAMA AND BUSINGER 523 Weather and Forecasting Challenges in the Paci®c Region of the National Weather Service KEVIN R. KODAMA* Joint Institute for Marine and Atmospheric Research, Honolulu, Hawaii STEVEN BUSINGER Department of Meteorology, University of Hawaii, Honolulu, Hawaii (Manuscript received 10 April 1997, in ®nal form 2 October 1997) ABSTRACT The large area of responsibility covered by the Paci®c Region of the National Weather Service provides a unique set of challenges to operational forecasters. Extratropical, subtropical, and tropical meteorological phe- nomena on a wide range of temporal and spatial scales must be considered on a daily basis. Compounding the problems of forecasting diverse weather for such a large area of responsibility is the fact that the Paci®c Ocean is a data-sparse region. Recent improvements in data collection platforms and the continued progress made by researchers have helped increase the understanding of weather throughout the region, ultimately resulting in improved forecast services. This article provides an overview of some of the weather phenomena encountered in the Paci®c Region and helps set the stage for the accompanying articles that focus on speci®c weather forecasting problems. Some discussion is provided on the impact of the National Weather Service's modernization program on operational forecasting in the region. 1. Introduction weather support under the auspices of the Compact of Free Association with the U.S. government. Although it is the smallest of the six administrative Because of its size, the Paci®c Region's forecasters regions, the Paci®c Region of the National Weather Ser- are frequently challenged with a wide range of mete- vice (NWS), hereafter the ``Paci®c Region,'' covers by orological phenomena that include extratropical, sub- far the largest geographic area of responsibility (AOR). tropical, and tropical systems. These systems generate Its two forecast of®ces are responsible for providing the heavy rains, high surf, and strong winds that con- weather, marine, and hydrologic forecasts, warnings, stitute the region's greatest weather-related hazards and and watches to a large portion of the Paci®c Ocean. This forecast challenges. Complicating the forecast process is an area more than twice the size of the continental is the paucity of observational data and the variability United States and includes the State of Hawaii, two U.S. introduced by the interactions between the weather sys- trust territories (Guam and American Samoa), and four tems and terrain that ranges from atolls only a few me- foreign countries. These political entities consist of is- ters above sea level to large islands with volcanoes lands and atolls from the Mariana Islands, the Caroline reaching up to 4-km elevation. Islands, the Marshall Islands, the Hawaiian Islands, and The threat to life and property from heavy rains and the Samoa Islands (Fig. 1). The foreign countries, the ¯ash ¯oods is particularly signi®cant in the Hawaiian Commonwealth of the Northern Mariana Islands, the Islands, accounting for most of its damaging weather Republic of Palau, the Federated States of Micronesia, events with an average of six heavy rain events per year, and the Republic of the Marshall Islands, are provided at least one of which has rainfall greater than 250 mm day21 (Kodama and Barnes 1997). In many of the Ha- waiian stream basins, response times to heavy rains are less than 1 h and even as fast as 15 min, making the *Current af®liation: NOAA, NWS Forecast Of®ce, Honolulu, timely detection of heavy rain extremely important. Hawaii. Adding to the forecast challenge is the fact that ¯ood- producing rains can come solely from warm-top con- Corresponding author address: Kevin Kodama, WSFO Honolulu, vection, which is not easily detected in satellite imagery. 2525 Correa Road, Suite 250, Honolulu, HI 96822. Notable examples include the Oahu New Year's Eve E-mail: [email protected] ¯ood (31 December 1987±1 January 1988; Department q 1998 American Meteorological Society 524 WEATHER AND FORECASTING VOLUME 13 FIG. 1. Map of the Paci®c Region's area of responsibility (AOR). The areas covered by the different forecast products are subsets of the entire AOR. For example, the Central Paci®c Hurricane Center (area within the bold black line) in Honolulu, Hawaii, issues forecasts, warnings, and advisories for all tropical cyclone activity within the region north of the equator between 1808 and 1408W. The area within the dashed gray line is the region covered by the High Seas Forecast, issued by the WSFO Honolulu. Other forecast products include forecasts and advisories for aviation terminals and routes, marine interests in coastal and offshore waters, and public state, zone, and local forecasts. of Land and Natural Resources 1988) and the February trade wind events. In Micronesia, most of the damaging 1979 Hilo, Hawaii, heavy rain event (Cram and Tatum wind events are due to tropical cyclone activity (K. 1979). Both cases exhibit warm-top convection with Waters 1997, personal communication). maximum 24-h rainfall in excess of 585 mm. Cram and This paper provides an overview of some of the me- Tatum note cloud tops of less than 4 km in the February teorological phenomena that Paci®c Region forecasters 1979 case. consider in developing forecasts and to discuss some of High surf accounts for most of the Paci®c Region's the impacts of the ongoing NWS modernization and weather-related fatalities. In the Hawaiian Islands, ex- associated restructuring (MAR) program on the region's tratropical cyclones are the most frequent cause of high operations. The articles accompanying this paper ex- surf events. Strong, slow-moving storm systems that amine a few of these issues in greater detail. These develop far to the northwest of the islands can generate include papers on a Hawaiian severe weather event and large swells that produce surf heights in excess of 6 m on the implementation of the National Centers for En- along north-facing shores. Southern Hemisphere extra- vironmental Prediction (NCEP) Regional Spectral Mod- tropical cyclones that form during the austral winter can el (RSM) for operational forecasting in Hawaii. also generate Hawaiian south shore surf heights over 3 m. Due to their location, the islands and atolls of Mi- 2. The large-scale environment cronesia experience a much higher percentage of high surf events from tropical cyclone activity than is ex- The main components of the general circulation af- perienced in Hawaii (D. Mundell and J. Pangelinan fecting the tropical Paci®c are the Hadley circulation 1997, personal communication). Low-lying atolls are and the Walker circulation. The Walker circulation, a also vulnerable to inundation from tropical cyclone term ®rst coined by Bjerknes (1969), is the zonal com- storm surges and high surf from westerly gales and dis- ponent and includes several large-scale, thermally direct tant extratropical cyclones. For example, portions of circulation cells within the equatorial regions of the Majuro Atoll, with a maximum elevation of ;4.5 m, globe. Normally, the cell over the Paci®c contains an have been inundated twice in the last ®ve years. ascending branch in the equatorial western Paci®c and While not as frequent as heavy rain or high surf a descending branch in the eastern Paci®c. In the ther- events, strong wind events are responsible for most of mally direct Hadley circulation, air rises near the equator the Paci®c region's weather-related monetary losses. In and ¯ows poleward aloft. This poleward-moving air is the Hawaiian Islands, most of the damaging wind events generally subsident and results in a semipermanent an- are due to strong extratropical and subtropical cyclones, ticyclone at the surface in the subtropical latitudes of though strong anticyclones also produce destructive the northeastern and southeastern Paci®c. The anticy- SEPTEMBER 1998 KODAMA AND BUSINGER 525 clones go through an annual migration. For example, 1954). Around the Hawaiian Islands, the average trade the mean January position of the northeast Paci®c an- wind inversion base is near the 2300-m level. Although ticyclone is near 308N, 1308W, with a subtropical ridge the height of the inversion near Hawaii can vary con- extending southwestward to near 258N (Fig. 2). By July, siderably from day to day, it usually remains between the anticyclone has expanded considerably with its cen- 1500 and 3000 m above sea level (Grindinger 1992). ter shifted northward to near 358N, 1558W and the sub- Data suggest that the increase in trade wind inversion tropical ridge axis between 308 and 358N. The ¯ow height does not continue all the way to the NETWC. In around these anticyclones results in northeasterly separate experiments, Ramage et al. (1981) and Kloesel (southeasterly) low-level winds over the tropical North and Albrecht (1989) show little change in the height of (South) Paci®c. Known as the trade winds, this persis- the trade wind inversion (near 2000 m) between 158N tent low-level ¯ow regime plays a major role in de®ning and the NETWC. Instead, variability presents itself in the climatology of the region. Over the Hawaiian Is- the frequency of inversion occurrence, with inversions lands, trade winds occur ;70% of the year. A seasonal being less frequent near the NETWC. strati®cation shows that they are most persistent during Another large-scale convergence feature affecting the the summer months, when trade winds blow on ;90% Paci®c region is the South Paci®c convergence zone of the days. During the Hawaiian cool season (October± (SPCZ). The SPCZ extends southeastward from New April), equatorward excursions of extratropical storms Guinea past the Samoa Islands to approximately 308S, may cause a temporary loss of trade wind ¯ow. As a 1408W and is one of the most persistent and expansive result, the trade wind frequency for Hawaii drops down convective cloud bands on earth (Vincent 1994).
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