Summer 2009 forecast for North America

Summary. Warmer than normal temperatures are expected over much of the interior United States. A colder than normal summer is forecast for the Atlantic coastal areas of the United States, particularly the Northeast. Most of the contiguous United States is also expected to be dryer than normal. A slightly increased likelihood of wetter than normal conditions exists for the area west of the Great Lakes and the Southwest. Although the monsoon is expected to be wet, it may start later than normal this summer.

Discussion

Temperature

Fig. 1. Forecast of surface air temperature anomalies for the summer (JJA) of 2009. The numbers at the contours are confidence factors for anomalously warm and cold temperature categories. EC means equal chances. The base period for calculating anomalies is 1971-2000.

Average temperature over the contiguous United States during the summer of 2009 is expected to be above normal due to a continuing expansion of the tropical zone and a northward shift of the atmospheric circulation belts (rule 363). This is also suggested by a positive mode of the circum-global teleconnection (CGT) pattern during this spring (rule 298).

Warmer than normal temperatures are likely over much of the interior United States. This conclusion is supported by a precipitation anomaly pattern during the past winter (rule 501) and the current soil moisture deficit in this area (rule 242). If the PDO pattern remains negative through this summer, it will increase the chances of warmer than normal temperatures over the western United States.

A colder than normal summer is forecast for the Atlantic coastal areas of the United States, particularly the Northeast. This forecast is based primarily on the effects of the QBO and solar activity (rules 412, 413, and 421).

The saturated soils across the upper Midwest may lead to colder than normal summer temperatures in this region (due to soil moisture – circulation feedback mechanisms). However, the distribution of SST anomalies in the North Pacific suggests that the large-scale atmospheric circulation is likely to be more favorable for above normal temperatures in the Midwest. Therefore, the forecast is for equal chances of above and below normal categories.

Publicly available forecasts by climate models vary significantly from model to model. For example, the CCA forecast calls for above normal temperature over most of the United States and below normal temperatures from the Pacific Northwest to Alaska. The forecast temperature anomaly patterns produced by the NCEP and CFS models are almost opposite to that. The official forecast from the CPC calls for above normal temperatures over the western half of the United States and below normal temperatures or equal chances over much of the eastern half, except the Atlantic coast. Due to inconsistency between the climate model forecasts they have no effect of the CFs of our forecast.

Precipitation

Fig. 2. Same as Fig. 1, except for precipitation and confidence factors for wetter and dryer categories.

A number of rules link summer precipitation over North America to concurrent or preceding SST anomaly patterns in the Pacific Ocean. Thus, according to rule 265, a positive mode of the North Pacific (NP) SST pattern (Barlow et al. 2001), which is expected during the summer of 2009, is primarily related to negative precipitation anomaly values in the Pacific Northwest and in the area extending diagonally from Texas through the Northeast. The NP pattern is similar to the SVD2 pattern for North Pacific SST and North American precipitation obtained by Lau and Weng (2002). Their conclusion regarding the North Pacific effect on North American precipitation is consistent with that of Barlow et al. (2001). The probability of a wetter than normal summer in the Pacific Northwest and dryer than normal conditions in the southern states will somewhat increase if the negative PDO mode will continue through the summer.

Rules 266, 285 and 399 suggest an increased probability for dryer than normal conditions in the Great Plains and Midwest. However, the saturated soils across the upper Midwest could lower this probability. Anomalously wet summer conditions in this area (and most of the United States) are forecast by the CFS model.

Fig. 3. SST and atmospheric circulation patterns conducive to a wet monsoon.

Precipitation variability in the Great Plains tends to be out-of-phase with that in the Southwest. This is usually explained through the monsoon ridge position in the troposphere (rule 276) and Great Plains low-level jet (rule 4), both of which affecting the direction and strength of moisture transport. The distribution of SST anomalies in the North Pacific appears to be conducive to a wet monsoon this summer (Fig. 3). The official CPC forecast also calls for wet summer conditions in the Southwest.

One rule, however, decreases the CF for a wet summer in the Southwest. Rule 316 links the winter (JFM) NP SST index with the timing of the North American monsoon. This year the NP index was the highest since 1950, suggesting the late start of the monsoon. And the late monsoon is usually a dry monsoon (rule 318).