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 continued development of moderate La Niņa conditions, 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 CPC http://nic.noaa.gov/products/predictions/multi_season/, and NCEP/EMC http://nic.fb4.noaa.gov:8000/research/climate.html.

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. This Outlook has been prepared using all information that was reasonably accessible. Inclusion of other climate information and guidance requires further arrangements. The IRI is engaged in establishing such collaborative arrangements with the goal of improving its capability to provide the best and most complete global climate guidance. The Climate Outlook for October 1998 - March 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 spread in coupled model predictions concerning the evolution of SSTs. This spread in predictions is a source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SST's will continue to decline at a modest rate, resulting in moderate La Niņa conditions by the end of the year. Also, it is known that Atlantic Ocean SSTs may play some role in modulating precipitation changes over parts of North America. Thus, the uncertainties in Atlantic Ocean SST values during the forecast period may lead to additional uncertainty over some parts of the region.

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: October - December 1998 and January - March 1999. Maps are given showing expected tercile probabilities of precipitation. The maps 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 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 marked "D" experience their dry season during the forecast period, typically receiving less than 15% of their annual precipitation total during the three months. Otherwise, for example, in the case of Southern California in October - December 1998 (Map A), there is a 25% probability that the precipitation will be in the wettest third of the years, a 40% chance it will be in the near-normal third of the years, and a 35% 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.

October - December 1998:

PRECIPITATION

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

P.A) Enhanced probabilities of above-normal precipitation over Alaska, south-western parts of Canada and Pacific North-west;

P.B) Slightly enhanced probabilities for wetter than normal conditions in Ontario province;

P.C) Slightly enhanced probabilities of non-dry conditions over Oregon and Northern California;

P.D) Slightly enhanced probabilities of normal to dry conditions over Central and Southern California;

P.E) Enhanced probabilities of below-normal precipitation over the southern part of the United states;

P.F) Slightly enhanced probabilities of below-normal precipitation over the eastern part of the United States and over the Caribbean;

P.G) Enhanced probabilities of above-normal precipitation over Central America;

TEMPERATURE

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

T.A) Enhanced probabilities of below-normal temperatures over Alaska, and most western half of Canada with the probabilities for colder temperatures being higher in the eastern part of Alaska and western Canada;

T.B) Enhanced probabilities of above-normal temperatures over most part of the United States south of Oregon and west of Missouri and in Mexico;

T.C) Enhanced probabilities of near-normal temperatures over the south-eastern states of the US;

T.D) Enhanced probabilities of near-normal temperatures over the eastern part of Canada;

T.E) Highly Enhanced probabilities of above-normal temperatures over Central America;

January - March 1999:

PRECIPITATION

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

P.i) Slightly enhanced probabilities of above-normal precipitation over the Pacific Northwest;

P.ii) Enhanced probabilities of below-normal precipitation over central and southern California, Arizona and southern part of Nevada;

P.iii) Enhanced probabilities of near-normal precipitation over Montana, Idaho, parts of Wyoming, Utah, Colorado and Nevada;

P.iv) Enhanced probabilities of below-normal rainfall over southern-east states of the US and Texas;

P.v) Slightly enhanced probabilities of above-normal precipitation over mid-west, central part of the southern states of the US, most of the eastern states and in southern part of Quebec.

TEMPERATURE

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

T.i) Slightly enhanced probabilities of below-normal temperatures over south-eastern half of Canada and north-western part of United States;

T.ii) Slightly enhanced probabilities of above-normal temperatures over eastern half of Quebec, Nova Scotia and Newfoundland;

T.iii) Enhanced probabilities for warmer than normal conditions over most of the Central America

Cutoffs and Max/Min extremes for OND and JFM.

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