North America Discussion for July 2001

Climate Outlook

NORTH AMERICA August 2001 - January 2002

Issued: July 2001

The IRI has prepared this experimental Climate Outlook for North America for August 2001 - January 2002. Of relevance in the preparation of this outlook is the prediction of near-average to slightly warmer than average conditions in the eastern equatorial Pacific for the next 6 to 9 months. Currently the sea surface temperatures (SSTs) across much of the eastern and central equatorial are near their long-term average (SSTs) , although slightly lower than average SSTs have developed along the immediate western coast of South America and warmer than average SSTs persist in the extreme western part of the basin. Near neutral equatorial Pacific SST conditions are in effect for the first season of the forecast, August-September-October 2001, September-October-November 2001, October-November-December 2001, while during the second season, November-December 2001-January 2002, they are expected to be slightly above average. The warmer than average SSTs that continue to dominate much of the Indian Ocean are expected to decrease slowly through the forecast period. 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.

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 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).

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 2001 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.

OUTLOOK -

This Outlook covers two seasons: July - September 2001 and October - December 2001. 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 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 August-September-October 2001 (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 much of the state of California in the U.S.

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.

August-September-October 2001:

Precipitation

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

Slightly enhanced probabilities for above normal precipitation (30%-30%-40%) are forecast for southern Baffin Island, in Canada.

Slightly enhanced probabilities for above normal precipitation (30%-30%-40%) are forecast for the southwestern coast and the Aleutian islands of Alaska.

Slightly enhanced probabilities for above normal precipitation (25%-35%-40%) are forecast for the Alaska Panhandle and adjacent portion of western Canada.

Slightly enhanced probabilities for below normal precipitation (40%-35%-25%) are forecast for northern Canada.

Slightly enhanced probabilities for below normal precipitation (40%-35%-25%) are forecast for the southern Rocky mountain region of the United States, and part of northwestern Mexico and Baja Calif.

Slightly enhanced probabilities for below normal precipitation (40%-35%-25%) are forecast for a portion of the southeastern United States

An enhanced probability for below normal precipitation (45%-35%-20%) is forecast for most of Haiti and the Dominican Republic, and eastern Jamaica. Weaker probabilities for below normal precipitation (40%-35%-25%) surround this region, including much of Cuba the extreme southern Bahamas, and Puerto Rico.

Slightly enhanced probabilities for below normal precipitation (40%-35%-25%) are forecast for southern Mexico, Guatemala, and parts of the countries of Belize, ElSalvador, Honduras, and Nicaragua.

Slightly enhanced probabilities for below normal precipitation (40%-35%-25%) are forecast for the southern Hawaiian Islands (the Big Island and Maui).

Temperature

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

Slightly enhanced probabilities for near normal temperature (30%-40%-30%) are forecast for a small area in the southern United States (state of Texas).

An enhanced probability for above normal temperature (20%-35%-45%) is forecast for extreme western Alaska and the Bering Strait region. Weaker probabilities for above normal temperature (25%-35%-40%) flank this region on the east side, including more area in western Alaska.

An enhanced probability for above normal temperature (20%-35%-45% and 15%-40%-45%) is forecast for southeastern Canada. Weaker probabilities for above normal temperature (30%-30%-40%, 20%-40%-40% and 25%-35%-40%) surround this region, including additional portions of eastern and central Canada and the northeastern and north-central United States.

A greatly enhanced probability for above normal temperature (20%-30%-50%) is forecast for Haiti, the Dominican Republic, the southern Bahamas, and Puerto Rico, as well as a separate but associated region in southwestern Mexico (20%-30%-50%). Weaker probabilities for above normal temperature (20%-35%-45%, 25%-35%-40% and 20%-40%-40%) surround and connect these two regions, including the remainder of the Bahamas, southeastern Florida of the U.S., the Lesser Antilles, most of Cuba, much of Mexico, part of Belise, most of Guatemala, and most or all of Honduras, El Salvador, Nicaragua, Costa Rica and Panama, extending southeward into South America.

Slightly enhanced probabilities for below normal temperature (40%-30%-30%) are forecast for mainly the northern portion of Alaska.

Slightly enhanced probabilities for below normal temperature (40%-30%-30% and 25%-35%-40%) are forecast for southwestern Canada and the extreme northwestern United States.

Slightly enhanced probabilities for below normal and near normal temperature (40%-40%-20%) are forecast for the Aleutian Islands of Alaska.

Slightly enhanced probabilities for below normal and near normal temperature (40%-40%-20%) are forecast for the Hawaiian Islands of the United States.

September-October-November 2001: