Extratropical Highlights –June
2018
1. Northern Hemisphere
The 500-hPa circulation during June
featured above-average heights across the southwestern and central United
States, central Canada, the eastern North Atlantic Ocean, and Europe, and
below-average heights extending from eastern Canada across Greenland to central
Russia (Fig. E9).
Over the Atlantic basin, the circulation reflected a strong positive phase
(+1.4 std. dev.) of the North Atlantic Oscillation (NAO) (Fig. E7, Table E1). A positive NAO pattern has
generally prevailed since January 2018.
The main land-surface temperature signals
during June included well above-average temperatures across most of the United
States, central Canada, Europe, and central Siberia, and below-average
temperatures in northeastern Canada (Fig.
E1). The main precipitation signals included
above-average totals in the north-central U.S. and southeastern Europe, and
below-average totals in the south-central U.S. and northern Europe (Fig. E3).
a. North America
The 500-hPa circulation during June
featured above-average heights across the southwestern and central U.S., and
central Canada (Fig. E9). This pattern was associated with an amplified
ridge over central North America, and with an anomalous low-level flow of warm,
moist air extending northward from the Gulf of Mexico to the north-central U.S.
(Fig. T20).
These conditions contributed to a continuation of well above-average surface
temperatures across the U.S. and central Canada, with most locations recording
departures in the upper 90th percentile of occurrences (Fig. E1).
The above conditions also
contributed to above-average precipitation in the north-central U.S., with many
areas recording departures in the upper 70th percentile of
occurrences (Fig.
E3). Farther south, anomalous sinking motion and
upper-level convergence (Fig. T23)
resulted in below-average precipitation across the south-central U.S. (Fig. E3) in the region downstream of
the mean ridge axis. The ongoing
combination of anomalously warm and dry conditions in the southwestern and
south-central U.S. has led to a continuation of extreme or exceptional drought from
Arizona and Utah to northern Texas and central Kanas.
b. North Atlantic/ Europe
The 500-hPa height pattern featured
above-average heights across the eastern North Atlantic Ocean and Europe, and
below-average heights over the northwestern North Atlantic and Greenland (Fig. E9). This
pattern reflected a continued strong positive phase (+1.4 std. dev.) of the NAO
(Fig. E7, Table E1). This
pattern contributed to exceptionally warm surface temperatures across Europe (Fig. E1), with many areas recording
departures in the upper 90th percentile of occurrences. It
also contributed a north-south dipole pattern of precipitation in Europe, with
above-average totals in southeastern Europe and below-average totals across
northern Europe (Fig. E3).
2. Southern Hemisphere
The mean 500-hPa circulation during
June featured above-average heights in the area south of New Zealand, over the
eastern South Pacific, and over much of Antarctica, and below-average heights over
the high latitudes of the central South Pacific and the central South Atlantic
Ocean (Fig. E15).
In eastern Australia, precipitation was below average during June (Fig. E3),
with totals generally in the lowest 30th percentile of occurrences.
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