North America Guide for Oct. 1997

Climate Outlook - Rainfall

NORTH AMERICA October 1997 - March 1998

The IRI Climate Forecast Division has prepared this Climate Forecast Outlook for North America for October 1997 - March 1998. Of special relevance in the preparation of this Outlook has been the exceptionally strong El Niño episode now underway in the tropical Pacific, and the expectation that strong El Niño conditions will persist through much of the forecast period. This present El Niño is comparable, if not stronger than the major event in 1982-83 (at the time the strongest of the century) which was associated with severe climate anomalies around the globe that caused major socio-economic losses in many regions.

METHODS -

This Outlook was prepared using the following procedures and information:

A) Statistical analyses of the observed historical response of the climate system over North America during the strongest 12 El Niño events over the period 1890-1989.

B) Coupled ocean-atmosphere model predictions of tropical Pacific sea surface temperatures (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 moderating, but still relatively strong, El Niño conditions will persist in the tropical Pacific through the forecast period.

C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted SST patterns in the tropical Pacific (especially) and in other oceans as well. These models have been shown to reproduce important aspects of the climatic response to El Niño in simulations covering such significant events as the 1982-83 event described above.

The procedures, models, and data used to derive this Climate Outlook are 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 in the time available, using all information that was reasonably accessible. Inclusion of other climate information and guidance requires further arrangements. The IRI is engaged in estab- lishing such collaborative arrangements with the goal of improving its capability to provide the best and most complete global climate guidance. The Climate Outlook for January-March, especially, are dependent of the quality of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information. However, there is considerable spread in coupled model predictions concerning the rate at which tropical Pacific SSTs will decline in early 1998 (assuming the El Niño peaks near the end of 1997). This spread in predictions is a primary source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SSTs will decline at a modest rate during early 1998. Also, it has been suggested that tropical and/or mid-latitude Atlantic SSTs may play some role in modulating rainfall changes over parts of eastern North America.The impacts of possible changes in Atlantic SSTs during the forecast period have not been considered in this report. It is also 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 1997 and January-March 1998. Maps are given showing expected probabilities that the seasonal rainfall will fall into the driest third of the years, the middle third of the years, or the wettest third of the year (see Map Caption below for further details, cautions and caveats).

October-November-December 1997:

The map is dominated by the following main regions of enhanced probabilities of rainfall anomalies:

A) Enhanced probabilities for normal to above normal rainfall over much of the United States and northern and central Mexico. The maximum probabilities for above normal precipitation are found over the west coast of the United States, extending across the southern half of the United States with the greatest probabilities over the southwestern and extreme southeastern United States.

B) Slight enhancement of probabilities for below normal rainfall across Canada from the west coast of Alaska to the east coast of Canada, with the greatest probabilities for below normal rainfall over western Canada.

C) Slight enhancement of probabilities for above normal rainfall over areas to the north and east of Hudson Bay.

D) Enhanced probability of below normal precipitation over Central America with the probabilities of below normal precipitation increasing towards the south.

January-February-March 1998:

The pattern of probabilities shows some similarities to that for October - December 1997. The main features of this season are as follows:

i) The region of elevated probabilities for normal-to-above normal rainfall over the latitudinal belt between 20N and 40N (see A above). Particularly high probabilities of enhanced precipitation are depicted over the southern tier of the United States and northern-most Mexico.

ii) The area indicating slight enhancement of probabilities for below normal rainfall across Canada is shifted southward and now extends into the northern half of the United States. Localized regions of increased likelihood for below normal rainfall appear in western and eastern parts of this region.

iii) Enhancement of probabilities for above normal rainfall over northern Canada.

iv) Slightly enhanced probabilities for below normal precipitation over most of Central America.

The probabilities given here are based primarily on statistical analyses of historical data. Further confidence is added from the atmospheric GCMs forced with a variety of predicted SST scenarios including persistence, statistical estimates, and dynamical forecasts. These atmospheric GCM climate predictions are in agreement with each other and with the statistical analysis for the regions of enhanced probabilities for above normal precipitation over western and south-eastern United States as well as the area of enhanced probabilities for below normal precipitation over northern North America. In the western United States probabilities for above normal precipitation have been increased and expanded northward in agreement with GCM results.

Map Caption

For each sub-region, during the 1997-98 season, the likelihood associated with this Outlook is represented by the probabilities of that sub-region receiving below-normal, near-normal or above normal rainfall (see keys on Maps), based on model results and expert interpretation. Over many years, rainfall will fall equally frequently into three categories. For example, between 1900 and 1989 30 years will fall into the below-normal category, 30 years will be nor- mal and 30 will fall into the above-normal category.

If all three outcomes were equally likely this season, the chance for each category would be 33%. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Otherwise, for example, in the case of Southern California in January-March 1998 Map B), there is a 50% probability that the rainfall will be in the wettest third of the years, a 30% chance it will be in the near-normal third of the years, and a 20% chance that the rainfall will be in the driest third of the years. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

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