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).