Extratropical Highlights – May 2012

 

1. Northern Hemisphere

The 500-hPa circulation during April featured above-average heights over the central North Pacific, the eastern U.S., the high latitudes of the North Atlantic, and central Russia, and below-average heights over Alaska and the eastern North Atlantic (Figs. E9, E11). Over the subtropical Pacific, the 200-hPa circulation featured cyclonic streamfunction anomalies in both hemispheres near the Date Line (Fig. T22). This pattern likely reflects a lingering feature of La Niña (Fig. T25).

The main land-surface temperature signals during May included well above-average temperatures across the central United States and western/ central Russia (Fig. E1). The main precipitation signals (Figs. E3, E6) included above-average totals along the U.S. eastern seaboard and in south-central Canada, and below-average totals in the central U.S.

 

a. North America

The mean 500-hPa circulation during May featured an amplified trough over Alaska and above-average heights across the north-central and northeastern U.S. (Fig. E9). These conditions were associated with below-average surface temperatures in portions of southern Alaska and southwestern Canada. They were also associated with exceptionally warm surface temperatures were observed across the eastern half of the U.S., with many areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). Above-average precipitation was also recorded along the U.S. eastern seaboard (Figs. E3, E6), with the Southeast, Mid-Atlantic, and Northeast regions receiving above-average totals for the first time since November 2011(Fig. E5).

By the end of May, extreme or exceptional drought was present across Georgia, southeastern Alabama, southern South Carolina, and much of northern Florida. Extreme drought conditions persisted in isolated portions of western Texas and most of southeastern New Mexico. Moderate drought persisted across the Delmarva Peninsula and in portions of New England.

 

b. Western Russia

The 500-hPa circulation during May featured strong and persistent ridge over western/ central Russia (Figs. E9, E11). This pattern contributed to a continuation of exceptionally warm surface temperature across the region, with many areas recording departures exceeding +3^oC and in the upper 90^th percentile of occurrences.

 

2. Southern Hemisphere

In the extratropics, the mean 500-hPa circulation during May was fairly nondescript, with above-average heights over the South Atlantic Ocean and below-average heights across the high latitudes of the South Pacific Ocean (Fig. E15). At 200-hPa, the subtropical circulation featured an enhanced mid-Pacific trough east of the date line, which likely reflects a lingering response to La Niña (Figs. T22, T25).  A similar feature was also evident in the Northern Hemisphere.