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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
 
Extratropical Highlights - July 2005
 

1. Northern Hemisphere

The 500-hPa circulation pattern during July featured above-average heights over the southwestern U.S. , across the high latitudes and subtropical latitudes of the North Atlantic , and over the central North Pacific, and below-average heights over western Canada and the central North Atlantic (Figs. E9, E11). The main surface temperature departures reflected above-average temperatures across southern Alaska , the southwestern and eastern U.S. , most of Europe , both the high latitudes and subtropical latitudes of the North Atlantic , and much of China (Fig. E1). The main precipitation anomalies during July included above-average totals from Texas to the Mid-Atlantic region of the U.S. , and the western subtropical North Atlantic , and below-average totals in portions of the mid-western U.S. and across the high latitudes of the North Atlantic (Figs. E3, E5, E6).

a. North America

The main 500-hPa circulation anomalies affecting the North American sector during July were above-average heights over both the southwestern and eastern United States , and below-average heights over western Canada / eastern Alaska (Fig. E9). This anomaly pattern contributed to enhanced precipitation across southern Alaska , with totals in many regions reaching the 90th percentile of occurrences. It also contributed to well above-average temperatures in the southwestern U.S. , which was situated beneath a very persistent upper-level ridge for most of the month (Fig. E1).

The anomalous upper-level cyclonic circulation over the middle of the U.S. (Fig. E10) contributed to well above-average temperatures throughout the east (Fig. T21). Enhanced rainfall was observed downstream of the trough axis from Texas northeastward to the Mid-Atlantic region, and significantly below-average rainfall was observed in Illinois immediately upstream of the trough axis (Fig.E3).

Well above-average precipitation was also observed across the Gulf Coast region during July, in response to two landfalling tropical systems. Tropical Storm Cindy made landfall on 6 July and brought 100 150 mm of rain to the Louisiana and Mississippi coasts. Major Hurricane Dennis made landfall at Pensacola , Florida on 10 July, and brought more than 200 mm of rain to the western Florida panhandle, western Alabama , and western Georgia .

 

b. North Atlantic

The 500-hPa circulation pattern during July featured above-average heights across the high latitudes and subtropical latitudes of the North Atlantic (Fig. E11). This pattern was associated with a continuation of exceptionally warm SSTs throughout both regions, with departures exceeding the 90th percentile in the subtropics (Figs. T18, E1). This anomaly pattern reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001).

These conditions were again associated with above-average temperatures across most of Europe , with departures in most areas exceeding the 70th percentile of occurrences. In the subtropics the anomalous warmth and amplified upper-level ridge (Fig. T22) were also associated with significant Atlantic tropical storm and major hurricane activity during the month. By the end of July, a record seven tropical storms and two major hurricanes had formed in the Atlantic Basin . This tropical activity was associated with a known set of conditions that favors tropical cyclogenesis. These conditions include 1) lower surface air pressure (Fig. T19) across the tropical Atlantic and Caribbean Sea, and 2) markedly reduced vertical wind shear over the tropical North Atlantic and Caribbean Sea, in response to a combination of anomalous easterlies at 200-hPa and weaker-than-average tropical easterly trade winds at 850-hPa (Figs. T20, T21).

 

2. Southern Hemisphere

In the middle latitudes, the mean 500-hPa circulation pattern during July featured an anomalous zonal wave 3-4 pattern, with persistent above-average heights in the vicinity of the three continents, and a more variable pattern of below-average heights over portions of the three ocean basins (Fig. E15). At lower latitudes, the 200-ha subtropical ridges were stronger than average throughout the Southern Hemisphere, with the largest anticyclonic anomalies occurring between Africa and Australia (Fig. T22). This anomaly pattern was associated with enhanced upper-level westerlies and a nearly uniform South Pacific jet stream from the central Indian Ocean to southern South America (Fig. T21).

In Australia , above-average temperatures and below-average precipitation resulted from the combination of a persistent upper-level ridge across the southern part of the continent and an associated poleward shift of the mean jet stream. The most significant precipitation deficits were observed in the southwest, where totals were generally in the lowest 10th percentile of occurrences.

The persistent pattern of above-average heights over southern Africa contributed to well above-average temperatures during the month. Temperatures over much of South Africa were 1-2C above average, with many areas recording values exceeding the 90th percentile of occurrences.

 


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