Date and Period of Forecast

In March 2000, the IRI Experimental Forecast Division prepared a long-lead Climate Outlook for July-September 2000. Here we provide a subset of the March Net Assessment Forecast. The complete forecast, which includes regional forecasts for April-June 2000, can be found at http://iri.ldeo.columbia.edu/climate/forecasts/net_asmt/.

Uncertainties

This Climate Outlook is dependent on the quality 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. 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.

Regional Influences

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.

Precipitation Outlook

The Outlook for July-September 2000 can also be found at http://iri.ldeo.columbia.edu/climate/forecasts/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" (light grey) indicates equal probabilities in each class; i.e., there is no basis for favoring the forecast of any particular category. Areas marked "D" (dark grey) experience a pronounced dry season during the forecast period, typically receiving less than 15% of their annual precipitation or less than 5 cm total during this three-month period. Boundaries between sub-regions should be considered transition zones, and their location considered to be only qualitatively correct.

Temperature Outlook

The Outlook for July-September 2000 can also be found at http://iri.ldeo.columbia.edu/climate/forecasts/net_asmt/.

The temperature map shows 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" (light grey) 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.
July-September 2000 Temperature Probabilities

Ocean Conditions

Of relevance in the preparation of this Outlook has been the current La Niņa conditions (lower than average sea-surface temperatures (SSTs) in the central equatorial Pacific Ocean) that had been amplifying through the end of 1999. Thus, weakening La Niņa conditions are in effect for the first sseason of the forecast, April-May-June 2000, and a return to near-normal conditions is expected for the second July-August-September 2000. The SSTs of the tropical Indian Ocean were assumed to remain near-normal. The forecast of tropical North Atlantic SSTs suggests a cooling trend through June 2000.

Methods

The following procedures and information were used to prepare this Climate Outlook:

• Coupled ocean-atmosphere model predictions of tropical Pacific SST -- particularly heavy weighting has been given to the NOAA /NCEP, Climate Modeling Branch coupled model
• Statistical forecasts of Indian Ocean and Atlantic Ocean sea surface temperature
• The response of Atmospheric global circulation model (GCM) predictions to the present and predicted SST patterns
• Statistical analyses
• Appropriate Regional Climate Outlook Forum consensus guidance.