A Climate Outlook Forum was convened on March 29-30, 1999 to analyze the current state of the global and regional climate and formulate a consensus precipitation forecast for Mesoamerica for the period of May - June - July 1999. The Forum consisted of researchers and representatives of meteorological and hydrological services (NMHSs) from Mexico, Belize, El Salvador, Guatemala, Honduras, Nicaragua, Costa Rica, Panama, Columbia and Venezuela, as well as, Universidad Nacional de Columbia Sed, Ministerio de Agricultura y Ganaderia, Instituto Mexicano de Tecnologia del Agua (IMTA), CICESE, ETESA, Panama Canal Commission, Centro de Investigationes Hidraulicas e Hidrotecnicas (UTP), MIDA, Universidad de Panama, Universidad Santa Maria la Antigua, EDELCA, International Research Institute for Climate Prediction (IRI), Autoridad Nacional del Ambiente (ANAM), Centro del Agua del Tropico Humedo para America Latina y el Caribe (CATHALAC), National Oceanic and Atmospheric Administration - Office of Global Programs (NOAA-OGP), and the U.S. Agency for International Development - Office of Foreign Disaster Assistance (USAID-OFDA). The event was co-sponsored by ANAM, CATHALAC, NOAA-OGP, and the USAID-OFDA.

Sea-surface temperatures (SSTs) in the tropical Pacific Ocean are a significant factor influencing regional atmospheric dynamics and rainfall in the Mesoamerican region. Historical climate records, present climatic conditions, and SST predictions for the central and eastern equatorial Pacific from a variety of coupled ocean-atmosphere models were used to produce a climate outlook for Mesoamerica. In addition, results from several atmospheric global circulation models were considered. After examining several SST forecasts, the Forum agreed that the current La Niña episode is likely to decay over the next several months. Consistent with this, it was noted that slight anomalous warming of equatorial Pacific sea surface temperatures off the coast of South America had already been observed.

The outlook applies to precipitation for the period of May - June - July 1999, as a longer time frame would increase uncertainty in the outlook. In addition, May represents the onset of the rainy season for most of Mesoamerica, except for Panama, Colombia, and Venezuela, which generally experience the beginning of the rainy season in April. The activity of the Inter-Tropical Convergence Zone (ITCZ), easterly waves, and hurricane and storm activity in the Pacific and the Caribbean will likely influence the amount of rainfall in the Mesoamerican region.

Projected precipitation anomalies are shown using a probabilistic approach. The predictions consider the likelihood that the total amount of rainfall will be near normal, above normal or below normal (fig. 1). The map indicates the probability that precipitation during the period May - June - July will be amongst the wettest (upper block), amongst the normal (middle block) or the driest (lower block) years in the historical record.

The projected slow decay of current La Niña conditions played an important role in the outlook.

Important climatic factors related to these conditions include the behavior of the Inter-Tropical Convergence Zone (ITCZ) and tropical cyclone activity. The ITCZ is the band of clouds and heavy tropical rains that is typically migrating northward through the region during June - August, but is influenced by the fluctuations in El Niño conditions. The expected weak La Niña conditions suggest that the ITCZ will be located somewhat north of its climatological position during the forecast period. In addition, the weak La Niña conditions also favor normal- to slightly above-normal Atlantic hurricane activity. Mid-season updates to this outlook will consider the affect of the hurricane season in more detail.

The precipitation outlook for the period May - June - July 1999 for the Mesoamerican region is given in Figure 1. The boundaries between the sub-regions should be considered as transition zones rather than sharp boundaries among projected climatic conditions.

There was a general consensus that the rainy season will begin in mid- to late-May for most of this region. It should be noted that, in many regions of Mesoamerica, the rainy season may be preceded by one or two "false starts". In summary, elevated probabilities above normal rainfall are projected along the Pacific coasts of Nicaragua, Costa Rica and extreme southwest Panama, while normal- to above-normal rainfall is most likely in an area of extreme northwest Panama, the eastern half of Costa Rica, and the extreme southeastern Honduras. Near-normal rainfall is the most probable outcome in most of El Salvador, and central and western Honduras, while climatological probabilities (each rainfall category having approximately the same probability) are given for southern Belize. Near normal rainfall is the most likely outcome in eastern Panama, Columbia and Venezuela.

The forecast is a general vision of the most probable behavior of the regional climate. Due to the large-scale nature of the forecast, there are likely to be rainfall patterns that vary significantly from the outlook. Decision-making at the national or local level should take this into account. Users of climate data are invited to contact their national organizations that are tasked with making national climate predictions to receive more specific guidance on how to interpret the available information.

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