The IRI Experimental Forecast Division has prepared this Climate Outlook for North America for April - September 1999. Of relevance in the preparation of this Outlook have been the continuation of lower than average sea-surface temperatures (SSTs) in the central equatorial Pacific Ocean (La Niña), and the persistence of warmer than average conditions in the western Pacific. The SSTs of the western Indian Ocean are above average. It has been assumed that the tropical Atlantic Ocean will remain colder than normal, and that sea-surface temperatures in the North and South Atlantic will remain above average during the forecast period.

METHODS -

This Outlook was prepared using the following procedures and information:

A) Coupled ocean-atmosphere model predictions of tropical Pacific SST covering the forecast period. Particularly heavy weighting has been given to predictions from the coupled model operated by the NOAA National Centers for Environmental Prediction, Climate Modeling Branch. This model suggests the continuation of moderate La Niña conditions, decaying by the end of the forecast period, with the persistence of positive sea-surface temperature anomalies in the region of the Philippines. The persistence of colder than average conditions in the equatorial Pacific is consistent with most other numerical and statistical forecasts.

B) Forecasts of the tropical Indian and Atlantic oceans using statistical models developed by the IRI and CPTEC respectively.

C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted sea-surface temperature patterns.

D) Statistical analyses of the observed climate over North America.

E) Statement From the Second Climate Outlook Forum for Mesoamerica

F) Other sources of information include (ECMWF ; UKMO ; CPC ; PEAC ).

The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by the national meteorological services in North America. Thus, this product may differ from the official forecasts issued in those areas. The Climate Outlook for April - September 1999 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 between coupled model predictions concerning the detailed evolution of SSTs. This 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.

OUTLOOK -

This Outlook covers two seasons: April - June 1999 and July - September 1999. Maps are given showing tercile probabilities of precipitation and temperature. The maps for precipitation indicate the probabilities that the seasonal precipitation will fall into the wettest third of the years (top number), the middle third of the years (middle number), or the driest third of the years (bottom number). An additional precipitation map is provided for the first season indicating risk for extreme precipitation anomalies. Extremes are defined as anomalies that fall within the top and bottom 15th percentile of the observed records. Maps of temperature show expected probabilities that the seasonal temperatures will fall into the warmest third of the years, the middle third of the years, or the coldest third of the years. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas marked "D" represent regions experiencing their dry season (less than 15% of annual precipitation) and for which less than 5 cm of precipitation typically occurs over the season. Otherwise, for example, in the case of northwestern Mexico in April - June 1999 (Map A), there is a 20% probability that the precipitation will be in the wettest third of the years, a 35% chance it will be in the near-normal third of the years, and a 45% chance that the precipitation will be in the driest third of the years. The numbers for each region on the temperature maps indicate the probabilities of temperatures to fall in each of the three categories, above-, near-, and below-normal. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

April - June 1999:

PRECIPITATION

The following regions of enhanced probabilities of precipitation anomalies are indicated:

P.A) enhanced probabilities of near-normal to above-normal precipitation over western Alaska, and the western coast of Canada/Pacific Northwest of the US;

P.B) enhanced probabilities of near-normal to below-normal precipitation over parts of western Canada,extending into eastern Alaska;

P.C) slightly enhanced probabilities of above-normal rainfall the central portion of southern Canada;

P.D) enhanced probabilities of below-normal precipitation over northwestern Mexico and the southwestern US,extending into the central plains states of the US, with a slightly enhanced risk of extreme below-normal precipitation over the southern Rocky Mountains;

P.E) slightly enhanced probabilities of below-normal precipitation over the southeastern US and parts of central Mexico;

P.F) enhanced probabilities of below-normal precipitation over the Caribbean islands;

P.G) enhanced probabilities for above-normal precipitation over western Nicaragua and Costa Rica;

P.H) enhanced probabilities of near-normal to below-normal precipitation for the remainder of Central America, extending south over the northern countries of South America.

P.I) probabilities suggesting near-normal to below-normal precipitation over Hawaii.

TEMPERATURE

The following regions of enhanced probabilities of temperature anomalies are indicated:

T.A) enhanced probabilities of below-normal temperatures over the Pacific Northwest region, extending into western Canada, and over Alaska;

T.B) greatly enhanced probabilities of above-normal temperatures over most of the central and eastern US;

T.C) enhanced probabilities of below-normal temperatures over the region encompassing Baffin;

T.D) enhanced probabilities of normal to above-normal temperatures over the southern countries of Central America and much of the Caribbean.;

July - September 1999:

PRECIPITATION

The following regions of enhanced probabilities of precipitation anomalies are indicated:

P.i) slightly enhanced probabilities of normal to below-normal precipitation much of Central America, the northern coastal regions of Colombia and Venezuela, and the Caribbean Islands.

TEMPERATURE

The following regions of enhanced probabilities of temperature anomalies are indicated:

T.i) enhanced probabilities of above-normal temperatures over western Canada, the western US, and western and central Mexico;

T.ii) slightly enhanced probabilities of near-normal to above-normal temperatures over eastern Canada;

T.iii) enhanced probabilities of near-normal temperatures over southern Central America, the Caribbean, and all of northern South America.

Cutoffs and Max/Min extremes for AMJ and JAS.

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