The IRI Experimental Forecast Division has prepared this Climate Outlook for North America for October 1999 - March 2000. Of relevance in the preparation of this Outlook has been the current weak La Niņa conditions (lower than average sea-surface temperatures (SSTs) in the central equatorial Pacific Ocean) that are expected to amplify slightly through the end of 1999 and then decline slowly in early 2000. The persistence of slightly warmer than average conditions in the western Pacific has been assumed also. The SSTs of the tropical Indian Ocean were assumed to return to near normal conditions with the period October-December 1999. The forecast of tropical South Atlantic SSTs suggests a warming trend through December 1999.

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 resurgence of weak to moderate La Niņa conditions, with the persistence of slightly positive sea-surface temperature anomalies in the western Pacific. The return of colder that average conditions in the central and eastern 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) Other sources of information include ( CPC ).

The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by the national meteorological services in the region. Thus, this product may differ from the official forecasts issued in those areas. The Climate Outlook for October 1999 - March 2000 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.

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 1999 and January - March 2000. 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). 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 northern Mexico in October - December 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.

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. A priori, there is a 15% probability of being within the extremely wet category, and a 15% probability of being within the extremely dry category, leaving a 70% probability that the precipitation will not be extreme. The maps indicate areas of increased risk of extreme precipitation totals. Three levels of increased risk are defined: slightly enhanced risk, enhanced risk, and greatly enhanced risk. For slightly enhanced risk, there is a 25-40% probability that precipitation will be within the indicated extreme, i.e. wet or dry. This represents an approximate doubling of the climatological risk. For enhanced risk, there is a 40-50% probability that precipitation will be within the indicated extreme. This represents an approximate tripling of the climatological risk. For greatly enhanced risk, the probability that precipitation will be within the indicated extreme exceeds 50%, i.e. the indicated extreme is the most likely outcome.

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(Map A). 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 1999:

PRECIPITATION

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

P.A) slightly enhanced probabilities of above-normal precipitation over the Pacific Northwest corner of the USA, extending into Canada over British Columbia and much of Alberta;

P.B) slightly enhanced probabilities of below-normal precipitation over the south-west USA, and northern Mexico, between about 20N-36N, and 115W-95W, another region of similar, but less confident, probabilities extends to the east covering eastern Texas to the western edge of Georgia, and northward to Kentucky;

P.C) slight enhanced probabilities of above-normal precipitation over Panama, and the northern coasts of Colombia and western Venezuela;

P.D) slightly enhanced probabilities of near-normal to above-normal precipitation over eastern Colombia and western Venezuela;

P.E) enhanced probabilities of above-normal precipitation over eastern Venezuela, extending eastward along the northern coast into Brazil;

TEMPERATURE

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

T.A) enhanced probabilities of below-normal temperatures along the west coast of USA and Canada;

T.B) slightly enhanced probabilities of below-normal to near-normal temperatures over the southern interior of western Canada reaching westward along the southern coast of Alaska;

T.C) enhanced probabilities of near-normal temperatures over central Canada, west of Hudson Bay;

T.D) slightly enhanced probabilities of below-normal temperatures over northern Quebec and Newfoundland;

T.E) slightly enhanced probabilities of above-normal temperatures over central and northern Mexico, southern Baja, extending into Arizona, New Mexico, and Texas;

T.F) greatly enhanced probabilities of above-normal temperatures over central America, from southern Mexico into the northern areas of South America;

January - March 2000:

PRECIPITATION

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

P.i) slightly enhanced probabilities of above-normal precipitation over western interior Canada;

P.ii) slightly enhanced probabilities of near-normal precipitation over the Great Lakes region;

P.iii) slightly enhanced probabilities of near-normal to below-normal precipitation over Florida region;

P.iv) slightly enhanced probabilities of near-normal precipitation over California;

P.v) enhanced probabilities of near-normal to below-normal precipitation over north-western Mexico and southern Baja;

TEMPERATURE

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

T.i) slightly enhanced probabilities of above-normal temperatures over the eastern part of northern Canada, surrounding Hudson Bay; there is another region with similar, but less confident, probabilities to the west extending as far as Alaska;

T.ii) slightly enhanced probabilities of below-normal temperatures over western-most USA;

T.iii) slightly enhanced probabilities of near-normal to above-normal temperatures over the mid-Atlantic and New England coastal states;

T.iv) slightly enhanced probabilities of above-normal to near-normal temperatures over Florida, and Central America from southern Mexico, to the northwest region of South America;

Cutoffs and Max/Min extremes for OND and JFM.

To top   Back