Climate Outlook - Rainfall & Temperature

NORTH AMERICA January - June 1998

The IRI Climate Forecast Division has prepared this Climate Forecast Outlook for Europe for January - June 1998. Of special relevance in the preparation of this Outlook has been the exceptionally strong El Niņo episode now underway in the tropical Pacific, and the expectation that strong El Niņo conditions will persist through at least the first half of the forecast period. This present El Niņo appears to have peaked in December 1997 with the SST anomalies stronger than the ones during the major event in 1982-83 (at that 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 Niņo events over the period 1890-1989, and also the strongest 8 El Niņo events since 1950.

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 the slow decline of relatively strong El Niņo conditions, indicating that warm anomalies 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 Niņo 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 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 April-June, 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. 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 1998. Also, it has been suggested that tropical and/or mid-latitude Atlantic SSTs may play some role in modulating rainfall and temperature changes over parts of eastern North America. Thus, the uncertainties in Atlantic SST values during the forecast period may lead to some additional errors in predicted precipitation and temperature over some parts of eastern North America.

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: January-March 1998 and April-June 1998. Maps are given showing expected probabilities that the seasonal rainfall will fall into the wettest third of the years, the middle third of the years, or the driest third of the years. 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 (see Map Caption below for further details, cautions and caveats).

January-February-March 1998:

PRECIPITATION

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

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

P.B) The area indicating slight enhancement of probabilities for below-normal rainfall extending across Canada and into the northern part of the United States. Localized regions of increased likelihood for below-normal rainfall appear in western and eastern parts of this region.

P.C) Enhancement of probabilities for above-normal rainfall over northern Canada.

P.D) Slightly enhanced probabilities for below-normal precipitation over most of Central America.

TEMPERATURE

The pattern of probabilities for temperature field exhibits a distinct zonal structure, the main features are:

T.A) The region of elevated probabilities for above-normal temperatures over the latitudinal belt between 45N and 60N. Particularly high probabilities of warmer temperatures are expected over south and central Canada and over north United States.

T.B) The region of elevated probabilities for below-normal temperatures south to 35N

T.C) In addition, slightly enhanced probabilities of colder temperatures over the east coast.

April-May-June 1998:

PRECIPITATION

The pattern of probabilities somewhat resembles that for January-March. The main features of this season are as follows:

P.i) The southern boarder of the region of elevated probabilities for normal-to-above normal rainfall over the latitudinal belt between 20N and 40N (see P.A above) moves farther north. The northern half of Mexico exhibits "climate" conditions.

P.ii) The area indicating slight enhancement of probabilities for below normal rainfall extends across Canada and the northern half of the United States (see P.B above). Localized regions of increased likelihood for below-normal rainfall appear in western and central parts of this region.

P.iii) Enhancement of probabilities for above-normal rainfall over eastern parts of Canada and United States.

P.iv) The region of slightly enhanced probabilities for below-normal precipitation over most of Central America (see P.D above) now also includes the southern part of the North America.

TEMPERATURE

There is a big uncertainty in temperature anomalies distribution over most of the continent.
The are only three regions of distinct patterns of enhanced probabilities for temperature:

T.i) Slightly enhanced probabilities for colder seasonal temperatures over the northern-most regions.

T.ii) Enhanced probabilities for above-normal temperatures over the west coast of Canada and US.

T.iii) Slightly enhanced probabilities for colder seasonal temperatures over the southern and central parts of US.

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

The numbers for each region indicate the probabilities of rainfall in each of the three categories, below-, near-, and above-normal. Above-normal rainfall is defined as within the wettest third of recorded precipitation totals in each region for the given season; below-normal rainfall is defined as within the driest third of rainfall totals; near-normal is the third centered around the climatological median. The top number indicates the probability of rainfall occurring in the above-normal category, the middle number is for near-normal, and the bottom for below-normal. 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 A), 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. The numbers for each region on 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.

Cutoffs and Max/Min extremes for JFM and AMJ.