Extratropical
Highlights –June 2021
1.
Northern Hemisphere
The 500-hPa circulation during June featured above-average
heights across the western United States, the North Atlantic Ocean, and
Scandinavia, and below-average heights extending eastward from northeastern
Canada, across Greenland to the Arctic Sea (Fig. E9). Over the Atlantic basin, the
circulation reflected a positive phase (+0.77 std. dev.) of the North Atlantic
Oscillation (NAO) (Fig. E7, Table E1).
The main land-surface temperature signals during June were
predominantly well above-average temperatures across most of the U.S., Canada,
Europe, and Siberia (Fig. E1). The main precipitation signals included
above-average totals in the southeastern U.S., southern and eastern Asia, the
Mediterranean, and eastern/western Europe, and below-average totals in central
North America and central Europe (Fig. E3).
a. North America
The 500-hPa circulation during June featured above-average
heights across the western U.S. and Canada (Fig. E9). This pattern was associated
with an amplified ridge, with areas under this ridge recording departures in
the upper 90th percentile of occurrences, and contributed to record
breaking surface temperatures across the western and central U.S. (Fig. E1). Multiple states experienced all-time high
temperatures that exceeded 100 degrees Fahrenheit for 6 consecutive days. This heatwave led to a Class-1 degradation to
the already Extreme- to Exceptional- intensity rating
for drought conditions in the region according to the U.S. Drought
Monitor.
The above conditions also contributed to below-average
precipitation in the north-central U.S., with many areas recording departures
in the lower 10th percentile of occurrences (Fig. E3).
The southeastern U.S. saw above-average precipitation and early tropical
cyclone activity due to enhanced divergence and trade winds in the Atlantic (Figs. E3, T20, T23).
b.
North Atlantic/ Scandinavia
The 500-hPa height pattern featured above-average heights
across the North Atlantic Ocean and Scandinavia, and below-average heights over
the Hudson Bay, Greenland and the Arctic Ocean (Fig. E9). This pattern reflected a
positive phase (+0.77 std. dev.) of the NAO (Fig. E7, Table E1). This pattern contributed to
exceptionally warm surface temperatures across Scandinavia and Europe (Fig. E1),
with many areas recording departures in the upper 90th percentile of
occurrences. It also contributed to an east-to-west tripole
pattern of precipitation in Europe, with above-average totals in east and west
Europe and below-average totals in central Europe (Fig. E3).
2.
Southern Hemisphere
The mean 500-hPa circulation during June featured
above-average heights over New Zealand, the Indian Ocean, and the South
Atlantic Ocean, and below-average heights over the Southern Ocean near the
Drake Passage (Fig. E15). In the
Sahel and Southern Africa, precipitation totals were above-average while
precipitation was near normal in northeastern Australia, and below-average in
northeast Brazil (Fig. E3).
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