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Extratropical Highlights - August 1999

1. Northern Hemisphere

A strong inter-hemispheric symmetry of upper-level circulation features was again evident during August across the lower latitudes of the Pacific and Atlantic basins (Fig. T22). This pattern featured anomalous anticyclonic circulations across the eastern Pacific eastward to Africa, and anomalous cyclonic circulation anomalies across the low latitudes of the western Pacific. Anticyclonic circulation anomalies also dominated the middle latitudes of the Northern Hemisphere during the month. These conditions are consistent with the ongoing pattern of tropical rainfall and tropical convective activity, which featured suppressed rainfall across the central equatorial Pacific, an amplified Southeast Asian monsoon system (Fig. T25) and above-normal rainfall across the Sahel region of northern Africa (Fig. E3).

a. North America

During August the far-western United States and Canada were again situated downstream of an upper-level trough, while most of central North America remained under the influence of an amplified upper-level ridge (Fig. E9). This circulation contributed to above-normal precipitation in the Pacific Northwest (Figs. E3, E5) and to below-normal rainfall across the eastern one-third of the United States (Fig. E3) in the region downstream of the mean ridge axis. This dryness contributed to a continuation of drought conditions over large portions of the Southeast and Northeast. Large portions of the mid-Atlantic states also experienced a continuation of drought conditions during the month, with some relief occurring at the very end of the month in association with hurricane Dennis.

Anomalously warm temperatures were confined to the Gulf Coast states during August (Fig. E1), with near-normal temperatures observed across most of eastern North America. These conditions are in marked contrast to the extreme heat that dominated most of eastern North America during July.

b. Europe/ Asia

An anomalous anticyclonic circulation again prevailed across southern Europe, southwestern Russia and northern Africa during August (Fig. T22b). This circulation has persisted for several months, and has resulted in a continuation of extremely warm temperatures (exceeding the 90th percentile) throughout the region during the period (Fig. E1). In contrast, western Europe and England were situated downstream of a mean mid-latitude trough during the month, which contributed to above-normal rainfall in these areas. Farther north, a large-scale pattern of below-normal heights covered all of Siberia and the high latitudes of the North Pacific (Fig. E9), which contributed to below-normal temperatures in these regions (Fig. E1).

2. Southern Hemisphere

The Southern Hemisphere circulation during August (Fig. E15) featured above-normal heights across southeastern Australia and New Zealand, over extreme southern South America, and in the region poleward of Africa. Accompanying these conditions, abnormally warm temperatures were observed across the southern regions of all three continents during the month (Fig. E1). In contrast below-normal heights were observed over the three ocean basins.

During most of the month there was a general progression of the mid-latitude wave features (Fig. E18). However, late in the month a large amplitude blocking pattern developed over the high latitudes of the eastern South Pacific. This contributed to a strengthening of the mid-latitude trough and an equatorward shift of the wintertime jet stream (Fig. T21) over the eastern South Pacific. These circulation features contributed to a dipole pattern of well above-normal precipitation across extreme southern South America (Fig. E3), and below-normal rainfall over southern Brazil, Uruguay and northern Argentina.

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