Climate Outlook - Rainfall & Temperature

NORTH AMERICA April - September 1998

The IRI Experimental Forecast Division has prepared this Climate Outlook for North America for April - September 1998. Of relevance in the preparation of this Outlook has been the strong El Niņo episode now subsiding in the tropical Pacific, and the expectation that warm El Niņo conditions will persist for the next few months. Also relevant to this Outlook are the warm sea surface temperature (SST) anomalies in the Indian Ocean basin and in the tropical Atlantic Ocean.

METHODS -

This Outlook was prepared using the following procedures and information:

A) 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 moderate, but weakening, El Niņo conditions in the tropical Pacific through the forecast period.

B) Statistical predictions of the tropical Atlantic SST's.

C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted SST patterns.

D) Statistical analyses of the observed climate over North America.

E) Other sources of information include NCEP, CPC-HawaiianIslands Canada, and Central America.

The procedures, models, and data used to derive this Climate Outlook may be 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-September is dependent of the quality of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is considerable spread in coupled model predictions concerning the evolution of SSTs. This spread in predictions is a primary source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SSTs will continue to decline at a modest rate. Also, it is also known that tropical and/or mid-latitude Atlantic SSTs may play some role in modulating rainfall changes over parts of eastern North America. Thus, the uncertainties in Atlantic SST values during the forecast period may lead to additional uncertainty over some parts of eastern North America.

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: April-June 1998 and July-September 1998.
Maps are given showing expected probabilities of precipitation. The maps 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). An additional 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. 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. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Otherwise, for example, in the case of Northern California in April-June 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 tem- peratures 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.

April-June 1998:

PRECIPITATION

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

P.A) Elevated probabilities for normal to above-normal rainfall across the southern tier and central United States, and along and east of the Rocky Mountains. Greater confidence is given to probabilities that suggest an increased likelihood for above-normal precipitation along the west coast. There is also a perceived enhancement of risk for extreme above-normal precipitation (precipitation values reaching values only seen in the wettest 15% of years in the observed record) along the west coast of the United States, extending northward into British Columbia. See map.

P.B) Probabilities slightly favor near-normal to above-normal conditions over central Canada.

P.C) Enhancement of probabilities for above-normal rainfall along the eastern seaboard of the United States and up into eastern Canada. This region along the eastern seaboard carries a slight enhancement of risk for extreme above-normal precipitation. See map.

P.D) Slightly enhanced probabilities for below-normal precipitation over most of Central America. An enhanced risk of extreme below-normal precipitation (precipitation values reaching values only seen in the driest 15% of years in the observed record) is also seen for this region. See map.

TEMPERATURE

The main features of the pattern of probabilities for temperature field are:

T.A) Enhancement of probabilities for above-normal temperatures along the eastern seaboard of the United States and up into eastern Canada.

T.B) Enhanced probabilities for warmer than normal temperatures over the Canada, especially over the region along the western coast.

T.C) Enhanced probabilities for the below-normal temperatures over the south-western part of the US and in Mexico.

July-September 1998:

PRECIPITATION:

The main features of this season are as follows:

P.i) Slightly elevated probabilities for above-normal precipitation over the eastern Canada.

P.ii) Enhanced probabilist for dry conditions over the western part of Canada.

P.iii) Enhanced probabilities for above-normal rainfall over the south-western part of Canada and across the western tier of the United States.

P.iv) Slightly elevated chances for below-normal rainfall over the south-eastern tier of the United States.

P.v) Normal conditions over Florida and Cuba.

TEMPERATURE

The main features of the pattern of probabilities for temperature field are:

T.i) Slightly enhanced probabilities for warm temperatures over the west coast of Canada.

T.ii) Elevated probabilities for the above-normal temperature across the Central America.

Cutoffs and Max/Min extremes for AMJ and JAS.

To top   Back