Summer 2008 forecast for North America - update

Summary: The temperature and precipitation anomaly patterns in this update have not changed much from the previous forecast issued March 17, 2008. Our confidence in hot and dry summer over much of the western United States is relatively high. However, due to a northward shifted monsoon anticyclone, the North American monsoon is now predicted to be early and wet, with above normal precipitation over Arizona and New Mexico in July-August.

Fig. 1. Forecast of surface air temperature anomalies in North America for the summer (JJA) of 2008.  The blue (red) contours and the letter C (W) indicate colder (warmer) than normal temperatures. CL means near normal. The base period for calculating anomalies is 1971-2000. The numbers at the contours are confidence factors of the forecast.

Fig. 2. Same as Fig. 1, except for precipitation. The orange (green) contours and the letter D (W) indicate drier (wetter) than normal conditions.

Discussion

Temperature

The rules triggered since the previous forecast (March 17, 2008) did not cause much change in the temperature anomaly pattern expected for the summer of 2008. Our confidence in hot and dry summer over much of the western United States remains relatively high. Anomalously cold waters along the West Coast will cool air temperature at the coastal stations, particularly in May-June. The effect, however, will be limited to a narrow strip along the coast and will not be felt more than 100 km or so inland.

Near or slightly below normal temperatures are expected over the eastern United States. This means a decrease in temperature in many locations compared to the summer of 2007. The cooling is more likely in the Great Lakes region and the Southeast.

The primary difference with the previous forecast is the lower confidence factor for anomalously high temperature in the Southwestern United States, partly due to changes in the monsoon forecast (see below). Anomalously cold temperature in Tucson (relatively to the average for the past 10 years) is suggested by rule 422 and rule 424. Overall, summer temperatures are expected to be slightly above the 1971-2000 average near the U.S. - Mexican border, with the magnitude of  positive SAT anomalies increasing farther north over the western United States. 

Precipitation

The major change from the forecast issued on March 17, 2008 was related to the North American monsoon. Previously it was suggested that the monsoon will be late and dry in the summer of 2008. Now, with more data available, the weight of evidence shifted toward the early and wet monsoon, although the confidence factor of the latter is relatively low. Due to dry conditions in the Southwest in March 2008, the entire winter (JFM) precipitation pattern in the western US does not longer support a forecast of a dry monsoon (rule 283). Colder than normal SST in the Niño 3 region (rule 277) also suggests a possibility of a wet monsoon. According to this rule, a low Niño-3 SST index is related to a north displacement of the monsoon ridge from its climatological position. In the ridge north configuration, a strong upper atmospheric ridge is located over the Great Plains. Upper-level easterlies carry moisture from the Gulf of Mexico south of the ridge. The monsoon arrives early and is wet in the Southwest. The Great Plains are dry on the subsiding branch of the ridge.

The only rule that indicates a possibility of a dry monsoon is rule 268, which links the PDO and AMO phases with the frequency of drought in the United States. This rule, however, refers to the decadal, rather than interannual, time scale.