SPECIAL SECTION:
Experimental Forecasts for Pacific Island Rainfall:

In past issues of Pacific ENSO Update, this section has reported primarily on the intercomparison of two statistical models (PEAC and CPC) which generate predictions of seasonal rainfall for particular Pacific island locations. In the case of the PEAC model, these predictions are limited to the U.S. affiliated islands, while the CPC model includes predictions for several other Pacific island locations as well. In addition, other model results are now becoming available which generally apply to rainfall in tropical Pacific region. In future issues, this section of Pacific ENSO Update will include expanded coverage on these various model results. For this particular issue, the latest results from the CPC statistical model for all predicted locations is shown below. This map-like presentation applies only to two particular seasons (March-May and June-August, 1998) of the 13 three-month periods out to a year in advance that are available from the model. The full time series in graphs and tables are updated monthly and posted on the internet at: http:// nic.fb4.noaa.gov:80 / products / predictions / experimental / pacific / CCA1.html. At present the model covers the following sites, however work is underway to add more:

 
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Table 1. Names and lat/long location of the 33 Pacific Islands Stations
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    Stations                        WMO#   Latit   Longit      Years   
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 1. HILO                           91285   19.72   155.07    1955-1996 
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 2. KAHULUI                        91190   20.90   156.43    1955-1996 
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 3. HONOLULU                       91182   21.35   157.93    1955-1996 
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 4. LIHUE                          91165   21.98   159.35    1955-1996 
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 5. ANDERSEN AFB GUAM              91218   13.57  -144.92    1955-1996 
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 6. GUAM WSMO                      91217   13.55  -144.83    1957-1996 
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 7. JOHNSTON IS WSO AP             91275   16.73   169.52    1955-1996 
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 8. KOROR WSO                      91408    7.33  -134.48    1955-1996 
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 9. KWAJALEIN MISSILE RGN          91366    8.73  -167.73    1955-1996 
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10. MAJURO WSO AP                  91376    7.08  -171.38    1955-1996 
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11. POHNPEI WSO                    91348    6.97  -158.22    1955-1996 
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12. WAKE ISLAND WSO AP             91245   19.28  -166.65    1955-1996 
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13. YAP ISLAND WSO AP              91413    9.48  -138.08    1955-1996 
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14. TRUK, CAROLINE ISLANDS         91334    7.47  -151.85    1955-1996 
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15. HENDERSON, SOLOMON ISLANDS     91520   -9.42  -160.05    1955-1996 
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16. LUGANVILLE, VANUATU            91554  -15.52  -167.22    1955-1996 
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17. KOUMAC, NEW CALEDONIA          91577  -20.57  -164.28    1955-1996 
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18. NOUMEA, NEW CALEDONIA          91592  -22.27  -166.45    1955-1996 
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19. FUNAFUTI, TUVALU               91643   -8.53  -179.22    1955-1996 
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20. ROTUMA, FIJI                   91650  -12.50  -177.05    1955-1996 
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21. UNDU POINT, FIJI               91652  -16.13  -179.98    1955-1996 
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22. NADI, FIJI                     91680  -17.75  -177.45    1955-1996 
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23. HIHIFO, DETACHED ISLANDS       91753  -13.23   176.17    1955-1996 
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24. KEPPEL, TONGA                  91776  -15.95   173.77    1955-1996 
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25. VAVAU, TONGA                   91780  -18.65   173.98    1955-1996 
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26. RAROTONGA, COOK ISLANDS        91843  -21.20   159.80    1955-1996 
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27. ATNONA, FRENCH POLYNESIA       91925   -9.80   139.03    1955-1996 
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28. BORA BORA, FRENCH POLYNESIA    91930  -16.48   151.75    1955-1996 
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29. PAPEETE, FRENCH POLYNESIA      91938  -17.55   149.62    1955-1996 
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30. TAKAROA, FRENCH POLYNESIA      91943  -14.48   145.03    1955-1996 
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31. RIKITEA, FRENCH POLYNESIA      91948  -23.10   134.87    1955-1996 
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32. TUBAI, FRENCH POLYNESIA        91954  -23.35   149.48    1955-1996 
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33. RAPA, FRENCH POLYNESIA         91958  -27.62   144.33    1955-1996 
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The February 1998 CCA forecast of rainfall deviations from normal at selected tropical Pacific stations for the two coming 3-month periods (MAM98 and JJA98) are shown at right. BLUE numbers show negative deviations from normal (less rainfall than normal) while RED numbers show positive deviations from normal (more rainfall than normal). The size of the number (not the value of the number) indicates how accurate the forecast is expected to be, on the average, for the station at the given time of year and the forecast lead time. There are three sizes: the smallest size indicates low skill, the medium size indicates moderate skill, and the largest size indicates a relatively high skill. Forecasts shown in the smallest type size are considered to have little reliability, while those in the largest type size can be trusted more*. The value of the numbers tell how large a deviation from normal is expected. These values are not in millimeters, but are in standardized units that indicate how typical (or nontypical) the rainfall conditions are expected to be relative to the station's normal climate for the season in question. For example, numbers from 0 to 25 (or 0 to -25) are small deviations, indicating conditions that would usually be considered typical of the climate for the station and the time of year. Deviations from 25 to 60 (or -25 to -60) are moderate deviations, indicating somewhat wetter (or dryer) conditions than would be expected for the station and the time of year. Deviations of over 60 (or less than -60) are large deviations, indicating much wetter (or dryer) conditions than normal for that location and time of year.

*NOTE - Lower skill forecasts also tend to be for smaller deviations in standardized units - meaning the model tends toward "near-normal" predictions for seasons where skill is low - however, climatological or "near normal" conditions are not necessarily more likely for those seasons...especially when higher skill forecasts indicate larger deviations for adjacent seasons, which may indicate a prevailing trend. ENSO is generally the dominant influence on tropical Pacific climate. This is the case most strongly during the months of November through May, when wetter-than-average conditions are expected for near-equatorial locations in the central and eastern Pacific (e.g. Atuona, Rikitea, Takaroa, and Funafuti), and drier-than-average conditions are expected for the western Pacific and for locations farther from the equator (e.g. nearly every other location in the model). This general pattern is well reflected in these latest results from the model.


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