Summer 2008 in North America - post-mortem analysis
Precipitation
For June through August, precipitation across the contiguous United States averaged 9.05 inches, which was 0.8 inch above the 1901-2000 average and ranked as the 15th wettest summer since 1895 (Fig. 7). As predicted (see our forecast issued in March 2008 and updated in April 2008), much of the western United States was drier than normal (Fig. 8). Wetter than normal conditions observed in many parts east of the Continental Divide and along the U.S.-Mexican border.
Typically, there is an out-of-phase relationship between rainfall in the southwestern United States and over the Great Plains (see rules 280 and 281). This summer, however, was unusual in that there was both a flooding in the Midwest and above normal monsoonal rains.
As discussed here, the atmospheric circulation during the preceding winter was characterized by an anomalous troughing over the West and ridging over the East. The storms following this pattern brought record or near record snow to the Colorado Rockies and the Midwest. This storm track persisted practically unchanged through the entire spring. In the early summer, a high pressure center started forming over the western U.S. and pushing the storm track northward. At the same time, a deep trough formed over the Great Lakes region. The hydrological conditions in the Midwest, characterized by excessive amount of soil moisture were favorable in maintaining this trough. Thus, a positive feedback loop was created between the hydrological conditions and the atmospheric circulation aloft that continued to bring more storms and, hence precipitation, to the Midwest. As a result, Iowa, Ohio, and Missouri had the wettest January-June on record.
Another contributor to the Midwest flooding was the moisture from the Gulf of Mexico. During spring and summer, large amounts of heat and moisture are transported northward from the Gulf of Mexico into the central United States by the Great Plains Low Level Jet (GPLLJ). These fluxes and their convergence exert profound influence on the hydroclimate of the Great Plains by providing both moisture and the necessary thermodynamic environment for precipitation formation.
During the summer of 2008, the GPLLJ index was the highest since the Great Flood of 1993. According to rule 299, the anomalously high GPLLJ index was, at least partly, a result of a strongly negative NAO index (Fig. 5). More information on this summer flood in the Midwestern U.S. is available from NCDC.
In July, the high pressure center over the western U.S. strengthened and expanded eastward. This expansion was enough to create the mid- and upper-level moisture transport on its southern periphery from the Gulf of Mexico to the Southwest and start the monsoon season. First, the monsoon rains were limited to the southern parts of Arizona and New Mexico, with extremely dry conditions in the more northern areas.
By the end of July, the high pressure center shifted father east and north, bringing to an end the long rainy period in the Midwest. In August, many Midwestern states were much drier than average. The monsoonal rains in the Southwest intensified and spread northward. Heavy rains fell in Colorado ending an extremely dry spell.
Overall, the monsoon season of 2008 is qualified as “wet,” right in line with the forecast. Some parts of the Southwest received near record amount of precipitation. For example, Phoenix received 5.72 inches of precipitation in July-August, the highest since 1984 and the fourth highest on record since 1948 (Fig. 10).