Extratropical Highlights –May 2020

 

1. Northern Hemisphere

The 500-hPa circulation during May featured above-average heights across the subtropical central and eastern North Pacific, much of western North America, Europe, and north-central Asia (Fig. E9). Below-average heights were observed over the high latitudes of the North Pacific, the high latitudes of the western North Atlantic, and Scandinavia.

The main land-surface temperature signals during May included above-average temperatures in the western U.S., southern Alaska, western Europe, and north-central Asia, and below-average temperatures in north-central Canada, the mid-Atlantic region of the U.S., and portions of eastern Europe (Fig. E1). The main precipitation signals included above-average totals in the northwestern and south-eastern/ mid-Atlantic region of the U.S., and western Russia, and below-average totals in Great Britain (Fig. E3).

 

a. North America

The 500-hPa circulation during May featured above-average heights over the southwestern U.S., western Canada and most of Alaska, and below-average heights over the Gulf of Alaska (Fig. E9). This pattern was associated with anomalous southwesterly jet stream winds into the western U.S. and with anomalous southeasterly winds across southern Alaska (Fig. E10).

These conditions contributed to exceptionally warm surface temperatures in the western U.S. and southern Alaska, with large areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). They also contributed to above-average precipitation in the northwestern U.S. (Fig. E3), with the Pacific Northwest region recording its first monthly precipitation surplus since February (Fig. E5).

 

b. Eurasia

The 500-hPa circulation during May featured an amplified wave pattern across Europe and northern Asia, with ridges over Europe and north-central Asia and a deep trough extending from Scandinavia southward to the Black Sea (Fig. E9). Well above-average surface temperatures were observed in the vicinity of the ridge axes, with western Europe and central Siberia recording departures in the upper 90^th percentile of occurrences (Fig. E1). For both areas, this represents a continuation of exceptional warmth that has persisted for several months.

 

2. Southern Hemisphere

The 500-hPa height field during May featured above-average heights near New Zealand and southwestern South America, and over the eastern South Atlantic Ocean (Fig. E15). Below-average heights were observed in the area south of Australia and over the high latitudes of the eastern South Pacific. At 200-hPa, the circulation featured an amplified trough over eastern Australia (Fig. T22). This overall pattern contributed to anomalously cool (Fig. E1) and dry (Fig. E3) conditions across the eastern half of Australia, and to anomalously dry conditions across southern Australia.