Extratropical Highlights –May 2019

 

1. Northern Hemisphere

The 500-hPa circulation during May featured an anomalous wave pattern from the central North Pacific Ocean to eastern Europe (Fig. E9). Aspects of this pattern included above-average heights over western Canada, the eastern U.S., the polar region, and southwestern Europe, and below-average heights over the central North Pacific, the western U.S., the central North Atlantic, and eastern Europe (Fig. E9). Over the Atlantic basin, this anomaly pattern reflected a strong negative phase (-2.4 std. dev.) of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7).

The main land-surface temperature signals included above-average temperatures in Alaska and the southeastern U.S., and below-average temperatures across the central U.S. and eastern Europe (Fig. E1). The main precipitation signals included above-average totals in the southwestern and central U.S., and portions of eastern Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during May featured below-average heights over the southwestern U.S. and above-average heights over the eastern U.S. (Fig. E9). This pattern contributed to a pronounced east-west dipole of surface temperature anomalies, with well below average temperatures in the west and well above-average temperatures in the southeast and mid-Atlantic regions (Fig. E1). It also produced excessive rainfall and flooding across the central U.S., with many locations recording totals above the 90^th percentile of occurrences (Fig. E3). Area-averaged totals exceeded the 90^th percentile of occurrences in the following regions: Southern California, Inter-Mountain, Great Plains, and Midwest (Fig. E5).

 

b. North Atlantic and Europe

The 500-hPa height pattern during May featured above-average heights in the polar region and below-average heights across the central North Atlantic (Fig. E9). Farther downstream, an anomalous ridge-trough pattern resulted in below-average precipitation in southwestern Europe and above-average precipitation in eastern Europe (Fig. E3).

 

2. Southern Hemisphere

The 500-hPa height field during May featured above-average heights over both western and eastern Australia and across the central South Pacific Ocean, along with below-average heights across the high latitudes of the South Pacific (Fig. E15). This pattern resulted in well below-average precipitation in both western and eastern Australia, with many regions recording totals in the lowest 10^th percentile of occurrences (Fig. E3).