The IRI Experimental Forecast Division has prepared this Climate Outlook for North America for January - June 1999. Of relevance in the preparation of this Outlook have been the continuation of cooler than average sea-surface temperatures (SSTs) in the eastern and central equatorial Pacific Ocean (La Niña), and the persistence of warmer than average conditions in the western Pacific. The SSTs of the central tropical Indian Ocean have cooled from record high temperatures in 1998. The SSTs there are currently below average, and negative anomalies are expected to persist. It has been assumed that the northern and tropical Atlantic Ocean will remain warmer than normal, and that sea-surface temperatures in the South Atlantic will increase slightly 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, slowly decaying by the end of the forecast period, with the persistence of positive sea-surface temperature anomalies in the region of Indonesia. This sea-surface temperature forecast 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) 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 January - June 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: January - March 1999 and April - June 1999. Maps are given showing expected tercile probabilities of precipitation and temperature. The maps of precipitation indicate 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 probabilities 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 that are marked by "D" represent regions experiencing their dry season (less than 15% of annual precipitation) and for which less than 5cm of precipitation typically occurs over the season. Otherwise, for example, in the case of California in January - March 1999 (Map A), there is a 15% 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 50% 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.

January - March 1999:

PRECIPITATION

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

P.A) slightly enhanced probabilities of above-normal precipitation over the northwest of the United States and western parts of Canada;

P.B) enhanced probabilities of near-normal precipitation over the western coast of Canada and over the Alaska;

P.C) slightly enhanced probabilities of above-normal precipitation over most of the Ontario and western half of Quebec;

P.D) slightly enhanced probabilities of near-normal precipitation over central and northeast of the United States;

P.E) enhanced probabilities of below-normal precipitation over the southeast of the United states and Cuba (there is an indication of slightly enhanced risk of extremely dry conditions there);

P.F) slightly enhanced probabilities of below-normal precipitation over southwest of the United states and northern half of Mexico;

P.G) enhanced probabilities of below-normal precipitation over southern half of Mexico and western parts of Guatemala (there is also an indication of slightly enhanced risk of extremely dry conditions there).

TEMPERATURE

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

T.A) slightly enhanced probabilities of below-normal temperatures over much of Canada (except for the southcentral parts) and Alaska;

T.B) enhanced probabilities of above-normal temperatures over most of the United States south of 45N (except for the western-most parts) and over most of Mexico; probabilities are especially high over the southern part of this region;

April - May 1999:

PRECIPITATION

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

P.i) slightly enhanced probabilities of below-normal precipitation over southern California, Arizona, western half of New Mexico and north-western parts of Mexico;

P.ii) slightly enhanced probabilities of below-normal precipitation over Hawaii;

TEMPERATURE

The following region of enhanced probabilities of temperature anomalies is indicated:

T.i) slightly enhanced probabilities of above-normal temperatures over most of California, parts of Nevada and Arizona;

Cutoffs and Max/Min extremes for JFM and AMJ.

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