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
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 90th 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 +3oC
and in the upper 90th 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,
A similar feature was also evident in the Northern Hemisphere.