Extratropical Highlights –August 2017

 

1. Northern Hemisphere

The 500-hPa circulation during August featured above-average heights over the central North Pacific, the western U.S., and western Russia, and below-average heights over the Gulf of Alaska, the central U.S., western Europe, and the polar region (Fig. E9).

The main land-surface temperature signals during August included above-average temperatures in the northwestern U.S., Europe, western Russia and the Middle East (Fig. E1). Surface temperatures were below average across the central U.S.

The main precipitation signals included above-average totals in the central and southeastern U.S., Alaska, and the African Sahel region, and below-average totals in the northwestern U.S., most of Canada, and the region around the Caspian Sea (Fig. E3).

In the U.S., Major Hurricane (MH) Harvey made landfall in southeastern Texas with maximum sustained winds of 140 mph. Harvey produced more than 40 inches of rain in the Houston area, causing widespread and massive flooding. Harvey was the first MH to make landfall in the continental U.S. since 2005.

 

a. North America

The 500-hPa circulation during August featured an amplified wave pattern across North America, with above-average heights over the western U.S. and an amplified tough over the central U.S. (Fig. E9). This pattern contributed to anomalously warm (Fig. E1) and dry (Fig. E3) conditions in the northwestern U.S., and to below-average precipitation across most of Canada. It also contributed to cooler and wetter than average conditions in the central U.S.

Area-averaged rainfall totals in northwestern U.S. were in the lowest 20^th percentile of occurrences (Fig. E5), with many locations recording totals of less than 25% of normal (Fig. E6). In contrast, area-averaged totals in the U.S. Great Plains were in the upper 90^th percentile of occurrences (Fig. E5). According to the U.S. Drought Monitor, extreme or exceptional short-term drought expanded in eastern Montana, and severe short-term drought persisted across the western half of both North and South Dakota.

 

b. Europe/ western Russia

The 500-hPa circulation during August featured above-average heights across southern Europe and extending eastward to the Caspian Sea, along with below-average heights in western Europe (Fig. E9). This pattern was associated above-average surface temperatures throughout Europe, western Russia and the Middle East (Fig. E1).

 

c. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. During August 2017, the west African monsoon system was enhanced (Fig. E3, Fig. T24) with area-average rainfall totals near the 80^th percentile of occurrences (see Sahel region, Fig. E4). This region has recorded well above-average precipitation during May-August. An enhanced west African monsoon system, along with above-average SSTs across the tropical Atlantic and Caribbean Sea (Fig. T18), typify the warm phase of the Atlantic Multi-Decadal Oscillation (AMO). These conditions are also typical features of an active Atlantic hurricane season.

 

2. Southern Hemisphere

The mean 500-hPa circulation during August featured above-average heights over the three central ocean basins, and below-average heights over southern South America (Fig. E15). This pattern contributed exceptionally warm (Fig. E1) and wet conditions (Fig. E3) in southeastern South America, where surface temperatures and area-averaged rainfall totals were both being the upper 90^th percentile of occurrences (Fig. E5).

The Antarctic ozone hole typically develops rapidly during August and reaches its peak size in September. The ozone hole then gradually decreases during October and November, and dissipates in early December (Fig. S8). During August 2017, the ozone hole intensified and reached 15 million square kilometers by the end of the month. This size is roughly the average for the 2007-2016 period.