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Extratropical Highlights - July 2002

1. Northern Hemisphere
A persistent pattern of above-normal heights at 500-hPa covered large portions of the middle latitudes during July, including the United States, the North Atlantic, western Russia, and eastern Asia (Figs. E10, E12). Prominent temperature anomalies during the month (Fig. E1) included warmer than normal temperatures over most of the United States, Europe, and eastern Asia (Fig. E1). Prominent precipitation anomalies (Fig. E3) included below-average rainfall across the northeastern quadrant of the United States (Fig. E5), Mexico, and northwestern Russia, and above-average rainfall across southern Europe and southeastern China (Fig. E4).

a. North America

Above-normal 500-hPa heights covered most of the United States and southern Canada during July for a second month in association with a strengthening of the mean summertime ridge. During July this circulation was associated with exceptionally warm surface temperatures across the region, with departures exceeding the 90th percentile in the southwestern and midwestern U.S. (Fig. E1).

Below-average rainfall covered the northeastern portion of the U.S. during July, with totals in the lowest 10th percentile of occurrences observed in the Great Lakes region (Figs. E3, E5). Below-average rainfall was again observed in the Inter-Mountain, Southwest, Great Plains, Midwest, and Ohio Valley regions during July (Figs. E3, E5). Precipitation has been below normal in the Inter-Mountain and Southwest regions since June 2001, in the Great Plains region since October 2001, and in the Ohio Valley since August 2001 (Fig. E5).

b. Europe and Asia

Most of eastern Europe, western Russia, and eastern Asia also experienced well above-average surface temperatures during July (Fig. E1) in association with anomalously strong ridges at upper levels (Fig. E10). Monthly mean temperatures averaged 2-4C above average in these areas and exceeded the 90th percentile.

In southeastern China above-average rainfall continued for a third consecutive month (Fig. E4), in association with a persistent upper-level trough and well-defined jet exit region over the country throughout period (Figs. T21, T22).

2. Southern Hemisphere

The upper level circulation during July featured an amplified subtropical ridge across Australia (Fig. T22), which again contributed to warmer and drier than normal conditions in both the southwest and southeast (Figs. E1, E3). The circulation also featured cyclonic streamfunction anomalies at 200-hPa extending from west-central South America eastward to southern Africa. This circulation contributed to beneficial rainfall in southeastern Africa (Fig. E4) during the heart of that region’s dry season.

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