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 90^th 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 90^th 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 10^th- 30^th percentile of occurrences (Fig. E3).