Extratropical Highlights –July
2018
1. Northern Hemisphere
The 500-hPa circulation during July
featured above-average heights across the high latitudes of the North Pacific,
Canada, the western United States, and Europe, and below-average heights in the
polar region (Fig. E9).
This overall pattern projected strongly onto the positive phase of several
teleconnection patterns, including North Atlantic Oscillation (NAO, +1.4), the
East Atlantic pattern (EA, +2.4), and the Scandinavia pattern (+2.3) (Fig. E7, Table E1). The
circulation anomalies also projected onto the strong negative phase of the East
Atlantic/ West Russia pattern (-2.2)—(see http://www.cpc.ncep.noaa.gov/data/teledoc/telecontents.shtml)
The main land-surface temperature signals
during July included well above-average temperatures in the western United
States, eastern Canada, throughout Europe, and southwestern Russia, and below-average
temperatures in central Russia (Fig. E1). The main precipitation signals included
above-average totals in the south-central and southeastern U.S., and also southeastern
Europe, and below-average totals in western Canada, Mexico, and northern Europe/
Scandinavia (Fig. E3).
a. United States
The 500-hPa circulation during July
featured a persistent pattern of above-average heights over the southwestern
U.S. (Figs. E9,
E11). This pattern was associated with an
amplified ridge over the western U.S., and with a mean
trough over the eastern U.S. These conditions contributed to well above-average
surface temperatures in the western U.S., where many locations recorded
departures in the upper 90th percentile of occurrences (Fig. E1).
The ongoing anomalously warm conditions in the southwestern U.S. has led to a
continuation of extreme or exceptional drought from Arizona and Utah to
northern Texas and southern Colorado.
b. North Atlantic/ Europe
The 500-hPa height pattern featured
above-average heights over Scandinavia, below-average heights over the polar
region, and a broad trough across southern Europe (Fig. E9). This pattern reflected a continued
strong positive phase (+1.4 std. dev.) of the NAO (Fig. E7, Table E1). A positive NAO pattern has
generally prevailed since January 2018. The pattern also projected on the
strong positive phases of the East Atlantic and Scandinavia teleconnection
patterns, and onto the negative phase of the East Atlantic/ West Russia
pattern.
The overall circulation contributed
to a continuation of 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 (Figs. E3). Southern Europe has recorded above-average
precipitation for the past three months, while northern Europe has recorded
below-average precipitation during that period (Fig. E4).
2. Southern Hemisphere
The mean 500-hPa circulation during
July featured above-average heights in the area south of Africa and over
southern South America, and below-average heights south of Australia (Fig. E15). At
200-hPa, the circulation featured an amplified ridge over western
Australia. This pattern contributed to a continuation of well below-average
precipitation in southeastern Australia, where totals were generally in the
lowest 10th- 30th percentile of occurrences (Fig. E3).