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Climate Outlook

NORTH AMERICA July - December 2002

Issued: June 2002

The IRI has prepared this experimental Climate Outlook for North America for July - December 2002. Of relevance in the preparation of this outlook is the prediction of warmer than average conditions in the equatorial Pacific for the next 6 to 9 months. Currently the sea surface temperatures (SSTs) across much of the eastern and central equatorial are warmer than their long-term average (SSTs), particularly in the central and western part of the basin, where warm SST anomalies have been present for nearly a year, but now also closer to the coast of Peru and Ecuador. Weakly warm equatorial Pacific SST conditions (approximately 0.5 C) are in effect for the first seasons of the forecast, July-September 2002, August-October 2002. The central equatorial Pacific is currently observed to be warmer (SSTs over 1 degree C above normal) than these predictions, and this discrepancy has been considered in developing the climate forecasts. The predictions attain the level of a weak El Nino (SSTs between 0.5 and 1 degree C above normal) by the end of the forecast period, October-December 2002. Note that this prediction for a weak El Nino is that given by a particular ENSO prediction model. It is described here because it served as the boundary forcing for the following climate forecast, which is primarily based on dynamical global climate models. This should not be confused with the IRI's ENSO Statement September-November 2002, which while indicating an enhanced potential for an El Nino to develop in 2002 also highlights the uncertainty still present in the prediction at this time. In the other tropical oceans, warmer than average SSTs continue to dominate much of the Indian Ocean, and are not expected to decrease as rapidly as suggested by the SST predictions. However, the actual long-term evolution of the Indian Ocean SSTs is difficult to foresee at this time. The area of above-average temperature in the tropical south Atlantic Ocean is expected to persist through at least the first half of the forecast period.


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 a continuation of near-average conditions during the first forecast season. The forecast for near-neutral conditions is consistent with some, but not all, numerical and statistical forecasts of central and eastern Pacific SSTs.

B) Forecasts of the tropical Indian ocean using a statistical model developed by the IRI.

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

D) Other sources of information include CPC ; NASA's Seasonal to Interannual Prediction Project (GSFC-NASA) and also seasonal prediction research at COLA.

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 July - December 2002 is dependent on the accuracy of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty concerning the evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook provided here. Note that even if perfectly accurate SST forecasts were possible, there would still be uncertainty in the climate forecast due to chaotic internal variability of the atmosphere. These uncertainties are reflected in the probabilities given in the forecast.

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, and variations within the 3-month season should also be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.


This Outlook covers four seasons: July-September 2002, August-Octover 2002, September-November 2002 and October-December 2002. 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). The color shading indicates the probability of the most dominant tercile -- that is, the tercile having the highest forecast probability. The color bar alongside the map defines these dominant tercile probability levels. The upper side of the color bar shows the colors used for increasingly strong probabilities when the dominant tercile is the above-normal tercile, while the lower side shows likewise for the below-normal tercile. The gray color indicates an enhanced probability for the near-normal tercile (nearly always limited to 40%). As before, numbers and their associated histograms show the probabilities of the three terciles. In areas with lots of spatial detail, there may not be sufficient room on the map, to allow histograms for each region. In those cases, some idea of the probabilities may be gained from the color alone. 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 for which less than 3cm of precipitation typically occurs over the season. Otherwise, for example, in the case of July-September 2002 (Map A), there is a 25% 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 40% chance that the precipitation will be in the driest third of the years in most of southern Mexico and Central America.

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.

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. A similar map is provided in the first season indicating probabilities of extreme temperature anomalies.

Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

July-September 2002 through October-December 2002:

The following discussion briefly describes the probability anomaly forecasts:


Enhanced probabilities for below normal precipitation are forecast for parts of southern California (U.S.) and northern Baja California (Mexico) for all four forecast period, for Central America and southern Mexico for the first forecast period, and for much of the Caribbean Island vicinity for the first two forecast periods. Slightly enhanced probabilities for above normal precipitation are forecast for the southern Rocky Mountain and southern Great Basin regions of the United States during the second forecast period, for parts of the Aleutian Islands of Alaska for the third period and for southern Florida (U.S.) and northern Cuba for the fourth forecast period.


Enhanced probabilities for above normal temperature are forecast for significant parts of North America for the first two forecast periods, including all of Central America and Mexico, most of the U.S., much of the Caribbean region, and much of southern and south-central Canada. During the last two forecast periods a tendency toward warmth continues but with more limited strength and coverage. Slightly enhanced probabilities for below normal temperature are forecast for the first three forecast periods, in small pockets mainly along coastal portions of Alaska and Canada. During the first two forecast periods, a slightly enhanced probability for below normal temperature is forecast for the eastern Caribbean, from Puerto Rico eastward.

OBSERVED CLIMATOLOGY DATA for Jul-Aug-Sep, Aug-Sep-Oct, Sep-Oct-Nov and Oct-Nov-Dec


TERCILE THRESHOLDS (33%-ile & 67%-ile):

EXTREME THRESHOLDS (15%-ile & 85 %-ile):


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