Extratropical Highlights –April 2021

 

1. Northern Hemisphere

The 500-hPa circulation during April featured above-average heights over northern Siberia, Greenland and surrounding seas and The North Pacific Ocean, and below-average heights over the North Atlantic Ocean, the Arctic Ocean, Scandinavia, and central Canada (Fig. E9).

The main land-surface temperature signals included above-average temperatures in eastern Canada, southwestern and west-coastal states of the continental U.S., northwestern portions of Russia, the Middle East, and western portions of North Africa (Fig. E1). The main precipitation signals included above-average totals in the southeastern U.S., eastern portions of Canada, western, central, and eastern portions of Russia, and below-average totals in the northwest and midwest regions of the U.S., the British Isles, and western Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during April featured above-average heights over eastern portions of Canada, The Gulf of Alaska, and along the west coast of the continental U.S. (Fig. E9). Despite these circulation anomalies, much of the continental U.S. and Alaska had near normal temperatures on average for April.  The west coast states, southwest states, Great Lakes region, and New England had moderately above-average temperatures, with percentiles between the 70th and 90th percent of occurrence (Fig. E1).  The circulation pattern off the Gulf of Alaska contributed to below-average precipitation in the northwest states of the U.S., resulting in dry conditions at the 10th percentile of occurrence (Fig. E3).  The U.S. Drought Monitor for May also indicates no change in the “abnormal” to “extreme” dry conditions there, with some degradation in status for eastern portions of Oregon.  Departures from normal precipitation accumulation for April are on the order of 50-150mm below normal for the combined areas of western Washington, Oregon and Northern California (Fig. E6).  Meanwhile, the Mississippi Valley had much above-average rainfall with departures from normal at 150mm or more for the Mississippi Delta area (Figs. E3 and E6). According to the U.S. Drought Monitor for May, the continued restoration of soil moisture levels in the southeastern U.S. has led to Class 2 and 3 Improvements for the region.

 

b. Greenland, Europe and Scandinavia

The 500-hPa height pattern during April featured a high-amplitude ridge over the British Isles that stretched across the Norweigian Sea to Greenland (Fig. E9). Temperature anomalies were only slightly above-normal for April (Fig. E1), however total rainfall was below-average with totals less than 100mm resulting in a low-percentile (10th) for April (Figs. E3 and E4).

 

2. Southern Hemisphere

The 500-hPa height field during April featured above-average heights in the central South Pacific and Indian Oceans, with highest departures from normal over the South Atlantic Ocean.  Below-average heights are primarily in sections of the Southern Ocean and an adjacent area in the eastern portions of the South Pacific Ocean (Fig. E15).  Southeastern portions of South America saw below-average precipitation with totals in the 10th percentile while northeastern Australia had above-average precipitation in the 80th percentile or better (Fig. E4).  In southern Africa, the monsoon season runs from October to April. During April, rainfall was near normal for most of the area with some areas below-average for the 2020-2021 season (Fig. E3).