Extratropical Highlights –July 2017

 

1. Northern Hemisphere

The 500-hPa circulation during July featured above-average heights over the high latitudes of the North Pacific, the western U.S., and central Asia, and below-average heights throughout the polar region, over the Gulf of Alaska and over Scandinavia (Fig. E9).

The main land-surface temperature signals during July included above-average temperatures in the northwestern U.S., western Alaska, Europe and eastern Asia (Fig. E1). The main precipitation signals included above-average totals in the southwestern U.S., the African Sahel region and southeastern Asia, and below-average totals in the northwestern U.S. and most of Canada (Fig. E3).

 

a. North America

The 500-hPa circulation during July featured an amplified wave pattern across North America, with above-average heights in the west and an amplified tough in the east (Fig. E9). This pattern contributed to anomalously warm (Fig. E1) and dry (Fig. E3) conditions in the northwestern and north-central U.S., and to below-average precipitation across most of Canada.

Area-averaged rainfall totals in northwestern U.S. were the lowest 10^th percentile of occurrences (Fig. E5), with most locations recording totals of less than 25% of normal (Fig. E6). Area-average totals in the U.S. northern Plains states were in the lowest 30^th percentile of occurrences (Fig. E5). According to the U.S. Drought Monitor, extreme or exceptional drought expanded in North Dakota and eastern Montana, and severe drought expanded across the western half of South Dakota.

 

b. North Atlantic/ Eurasia

The 500-hPa circulation during July featured above-average heights across central Asia (Fig. E9). This pattern was associated with a northward shift of the mean upper-level westerly winds (Fig. T21), and with well above-average surface temperatures across the eastern half of China (Fig. E1).

 

c. West African monsoon

 

The west African monsoon extends from June through September, with a peak during July-September. During July 2017, the west African monsoon system was enhanced (Fig. E3, Fig. T24) with area-average totals again exceeding the 90^th percentile of occurrences (see Sahel region, Fig. E4). This region recorded well above-average precipitation during May-July.

 

2. Southern Hemisphere

The mean 500-hPa circulation during July featured an anomalous zonal wave-3 pattern, with above-average heights over the three central ocean basins, and below-average heights poleward of the three continents (Fig. E15). This pattern contributed to warmer and drier than average conditions in eastern Australia, with the northeast recording temperatures in the upper 90^th percentile of occurrences (Fig. E1).