Extratropical Highlights - September 2026

 

1. Northern Hemisphere

The 500-hPa circulation during September featured a wavetrain pattern with above average heights across most of North America and Europe (Fig. E9). The main land-surface temperature signals include above average temperatures for most of North America and Europe (Fig. E1). The main precipitation signals include drier than average conditions across Europe and above average precipitation across central North America (Fig. E3).

 

a. North America

The 500-hPa circulation over North America featured predominantly above average height anomalies with a maximum located over the Great Lakes, the Hudson Bay, and the North Pacific Ocean (Fig. E9). Below average heights were observed over the Pacific Northwest (Fig. E9). Above average temperatures were recorded across most of North America with many areas reaching the highest 90th percentile of occurrences (Fig. E1). Above average precipitation was recorded in the Pacific Northwest, Inter-Mountain West, Southwest, Great Plains, and Alaska's North Slope, with many areas reaching the highest 90th percentile of occurrences (Figs. E3, E5, E6). Drier than average conditions were recorded along the Gulf Coast, Southeast, Ohio Valley, Great Lakes, Mid-Atlantic, and southern Alaska with many areas reaching the lowest 10th percentile of occurrences (Figs. E3, E5, E6).

 

b. Europe and Asia

The 500-hPa height anomalies were above normal across most of Eurasia (Fig. E9). Above average temperatures were recorded across Europe and Asia with many areas reaching the highest 90th percentile of occurrences (Fig. E1). Drier than average conditions were recorded in Northern and Southern Europe and above average precipitation was recorded across Scandinavia (Figs. E3, E4).

 

c. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. After below normal August rainfall, September rainfall in the Sahel reached the 70th percentile of occurrences (Figs. E3, E4). With the exception of August, the 2026 west African monsoon season was generally enhanced (Figs. E3, E4).

 

2. Southern Hemisphere and Ozone Hole

The 500-hPa height field during September featured a wavetrain pattern of alternating above and below average height anomalies across Antarctica and adjacent oceans (Fig. E15). Southern South America, southern Africa, and southern Australia recorded above average temperatures with many areas in southern Australia reaching the highest 90th percentile of occurrences (Fig. E1). Above average rainfall was recorded in Southern Africa and drier than average conditions were recorded in Northeast Australia and southeast South America (Figs. E3, E4).

The Antarctic ozone hole typically develops during August and reaches peak size in September. The ozone hole then gradually decreases during October and November, and dissipates on average in early December. By the end of September 2026, the size of the ozone hole approached 24 million square kilometers, which is near the maximum range of mean ozone hole sizes during 2015-2024 (Fig. S6). Associated with the ozone hole during September was an above average polar vortex area and above normal polar stratospheric cloud coverage area (Fig. S6).