Extratropical Highlights –May 2017

 

1. Northern Hemisphere

The 500-hPa circulation during May featured above-average heights over the western U.S., western Canada, Europe, and central Asia, and below-average heights over the north-central U.S., the central North Atlantic, and northwestern Russia (Fig. E9). Over the North Atlantic, the anomaly pattern projected onto the strong negative phase of the North Atlantic Oscillation (NAO, -1.7) (Table E1, Fig. E7).

The main land-surface temperature signals during May included above-average temperatures across Europe and central Asia, and below-average temperatures in northwestern Russia (Fig. E1). The main precipitation signals included above-average totals across the eastern half of the U.S. (Fig. E3).

 

a. North America

The 500-hPa circulation during May featured an amplified ridge in the western U.S. and Canada, and a broad tough across most of the U.S. (Fig. E9). Overall, these conditions were associated with above-average precipitation across the eastern half of the U.S. (Fig. E3), with many locations recording totals of more than 150% above normal (Fig. E6). Area-averaged totals exceeded the 80^th percentile of occurrences in the Ohio Valley region, the Great Lakes region, and the Northeast region (Fig. E5). According to the U.S. Drought Monitor, drought ended in large portions of the southeast. However, severe drought persisted in southern Georgia, and extreme drought persisted in central Florida

 

b. North Atlantic/ Eurasia

The 500-hPa circulation during May featured above-average heights across the high latitudes of the North Atlantic and Europe, and below-average heights across the central North Atlantic (Fig. E9). This pattern projected strongly onto the negative phase of the North Atlantic Oscillation (NAO, -1.7) (Table E1, Fig. E7).

These conditions contributed to well above-average temperatures across Europe, with departures exceeding the 90^th percentile of occurrences in western Europe (Fig. E1). They also contributed to below-average temperatures in northwestern Russia.

 

2. Southern Hemisphere

The mean 500-hPa circulation during May featured generally above-average heights in the middle latitudes and below-average heights at high latitudes (Fig. E15). It also featured a trough upstream of South America and another trough over central Australia. These conditions contributed, respectively, to well above-average precipitation across central South America and to well below-average precipitation across southern Australia (Fig. E3).