Extratropical Highlights - May 2021

 

1.  Northern Hemisphere

The 500-hPa circulation during May featured an anomalous wave pattern across the Arctic Ocean, above-average heights over the North Pacific Ocean, the Mediterranean, and eastern Europe, and below-average heights near the British Isles and southern Russia (Fig. E9). Aspects of this pattern reflect a moderate negative phase (approx. -1.0 std. dev.) of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7).

The main land-surface temperature signals included above-average temperatures n western Russia, the Middle East, and northern Russia, with moderate above-average temperatures in Alaska, the western U.S., and Africa and below-average temperatures in central Canada, with moderate below-average signals in Europe and India (Fig. E1).

The main precipitation signals included above-average rainfall in the southeastern U.S., eastern Europe, central Russia, and central Africa.  Northwest and northeast U.S. saw moderate below-average rainfall anomalies while the region near the Gulf of Guinea, Africa had strong below-average rainfall (Fig. E3).

 

a. North America

The majority of North America had near-normal heights (Fig. E9).  The pattern contributed to moderate above-average temperature anomalies across the western U.S. and Alaska, and Quebec and below-average temperatures in central Canada, especially near Alberta and the Northwest Territories.  The precipitation pattern was also near-normal for much of North America with notable exceptions in the southeast U.S. and Victoria, Canada.  Southeastern U.S., experienced record rainfall totals and flooding for much of the month of May.  Over a 5 day period, 21 inches of rain was reported south of Lake Charles and 15 inches fell in Baton Rouge over the same period.  An enhanced southerly jet and above-average sea-level-pressure anomaly over the eastern half of the U.S. helped usher in anomalous moisture from the Gulf of Mexico (Figs. E10, E8).  Departures from normal were well over 150mm for southern Louisiana and southeastern Texas (Fig. E6) and in the 90th percentile (Figs. E6, E5) for the Gulf Coast region.  The U.S. Drought Monitor indicated a Class-1 improvement for parts of the region.

 

b. North Pacific, Eurasia, and Africa

An anomalous high pressure anomaly dominated the North Pacific Ocean which likely contributed to the enhanced southern jet stream and wet conditions in the southeast U.S.  Above-average pressure over western Russia contributed to above-average temperatures and the below-average rainfall.  Precipitation in the area was in the 10th percentile (Figs. E1, E3).  Across Europe temperatures slightly favored below-normal and above-average rainfall with precipitation in the 70th to 90th percentiles for the region (Figs. E1, E3).

Above-average temperatures were observed for most of Africa with regions in western Africa in the 90th percentile.  Niger experienced above-average rainfall while below-average rainfall was observed in the nearby adjacent areas along the Gulf of Guinea (Figs. E1, E3, E4).

 

2.  Southern Hemisphere

The 500-hPa circulation during May featured an anomalous concentric wave pattern (Fig. E15).  Eastern Australia, southern portions of South America, and Madagascar had below-average rainfall (Figs. E15, E4).