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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - July 2006

1. Northern Hemisphere

During July the 500-hPa circulation pattern featured persistent positive height anomalies over the central North Pacific, western North America , the northwestern North Atlantic , and Europe , and negative height anomalies over central Russia (Figs. E9, E11). The main surface temperature departures during the month reflected significantly warmer-than-average conditions throughout the western United States and much of Europe (Fig. E1). In the United States , the main precipitation departures included above-average totals in the Great Lakes and Northeast regions, and below-average totals in the northwestern, north-central and southeastern regions (Figs. E3, E6). Significantly below-average precipitation was also recorded in Northern Europe and Scandinavia (Figs. E3, E4).


a. North America

In the United States , an anomalous upper-level ridge-trough pattern, characterized by above-average heights in the west and an amplified trough in the east, was evident during much of July. This pattern contributed to exceptionally warm surface temperatures in the western U.S. and southwestern Canada , with departures in many areas exceeding the 90th percentile of occurrences (Fig, E1). It also contributed to below-average precipitation between the ridge and trough axes, with area-averaged totals across Pacific Northwest and Northern Plains regions in the lowest 10th percentile of occurrences.

Monthly precipitation totals were generally above-average within and downstream of the mean trough axis, in the Gulf Coast region, and in the Southwest U.S. monsoon region. Area-averaged totals in the Northeast exceeded the 90th percentile of occurrences for the second consecutive month (Fig. E5), while totals in the Gulf Coast region were above average for the first time since September 2005. In the Southwest, above-average monsoonal rains were associated with the persistent upper-level ridge over the western part of the country.


b. North Atlantic / Eurasia

A persistent upper-level ridge was also centered over Europe during July (Fig. E9). As a result, surface temperatures were generally 2-3C above average throughout the continent. This persistent ridge also led to a continuation of significantly below average precipitation in northern Europe , where area-averaged totals were in the lowest 10th percentile of occurrences for the second consecutive month.


2. Southern Hemisphere

      The mean 500-hPa circulation pattern during July featured generally above-average heights in the middle latitudes, and below-average heights across the eastern South Pacific, the central South Atlantic , and Antarctica (Fig. E15). Over South America a pattern of above-average heights in the subtropics and below-average heights at higher latitudes was evident. This pattern was associated with a poleward shift in the mean upper-level westerlies (Fig. T22) and a more poleward position of mean surface cold-frontal boundaries. These conditions contributed to exceptionally warm surface temperatures exceeding +2C across central South America (Fig. E3).  



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