CID Forecast Jun 1999

IRI NET ASSESSMENT FORECAST

Discussion In July 1999, the IRI Experimental Forecast Division prepared a Climate Outlook for July-September 1999. Of relevance in the preparation of the Outlook have been the weakening of La Nina conditions (lower than average sea-surface temperatures (SSTs) in the central equatorial Pacific Ocean). The persistence of slightly warmer than average conditions in the western Pacific have been assumed also. The SSTs of the southern tropical Indian Ocean were assumed to remain above average. The forecast of tropical Atlantic SSTs suggests a continued warming trend.

Precipitation Outlook The Outlook for July-September 1999 can also be found at http://iri.columbia.edu/forecast/net_asmt. Maps are given showing expected precipitation probabilities in tercile classes. The maps indicate probabilities that seasonal precipitation will fall into the wettest third of the years (top number), the middle third of years (middle number) or the driest third of the years (bottom). An outlook of climatology "C" indicates equal probabilities in each class i.e.; there is no basis for favoring the forecast of any particular category. Areas marked "D" experience a pronounced dry season during the forecast period, typically receiving less than 15% of their annual precipitation total during this three-month period. Boundaries between sub-regions should be considered transition zones, and their location considered to be only qualitatively correct.

July-September 1999 Precipitation Probabilities

Methods The following procedures and information were used to prepare this Climate Outlook: 1) Coupled ocean-atmosphere model predictions of tropical Pacific SST Particularly heavy weighting has been given to the NOAA /NCEP, Climate Modeling Branch coupled model , 2) statistical forecasts of Indian Ocean and Atlantic Ocean sea surface temperature, 3) Atmospheric global circulation model (GCM) predictions response to the present and predicted SST patterns, 4) Statistical analyses, 5) Appropriate Regional Climate Outlook Forum consensus guidance.

The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by National Meteorological Services in particular regions and may differ from the official forecasts issued in those areas. This Climate Outlook is dependent on the quality of of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty concerning the detailed evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook provided here.

It is stressed that the current status of seasonal-to-interannual climate forecasting allows prediction of spatial and temporal averages, and does not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal time scales and relatively large areas; local variations should be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.

July-September 1999 Temperature Probabilities

Temperature Outlook The Outlook for July-September 1999 can also be found at http://iri.columbia.edu/climate/forecast/net_asmt/. The temperature map show expected probabilities that the seasonal temperatures will fall into the warmest third of the years (top number), the middle third of the years, or the coldest third of the years (bottom number). A qualitative outlook of climatology "C" indicates equal probabilities in each class i.e.; there is no basis for favoring the forecast of any particular category. Boundaries between sub-regions should be considered transition zones, and their location considered to be only qualitatively correct.

Sources of information include ACMAD, Caribbean Meteorological Institute, CPTEC, CPC/NOAA, Department of Natural Resources (Queensland, Australia), ECMWF, Indian Meteorological Department and the South African Weather Service.