Extratropical Highlights –April 2018

 

1. Northern Hemisphere

The 500-hPa height pattern during April featured above-average heights over the southwestern U.S., the central North Atlantic and western Russia, and below-average heights over eastern Canada and the high latitudes of the North Atlantic (Fig. E9).

At 200-hPa, the circulation across the subtropical Pacific Ocean in both hemispheres reflected lingering La Niña impacts. The La Niña signal included strong troughs over the eastern Pacific in the subtropics of both hemispheres (Fig. T22), in association with the disappearance of deep tropical convection from the central and eastern equatorial Pacific (Fig. T25). The La Niña signal also included a focusing of the subtropical ridges over Australasia (Fig. T22).

The main land-surface temperature signals during April included above-average temperatures in the southwestern U.S. and Europe, and below-average temperatures in western Canada and the eastern half of the U.S. (Fig. E1). The main precipitation signals included above-average totals in the northwestern U.S. and along the eastern sea-board of the U.S., and below-average totals in the central U.S. and southeastern Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during April featured above-average heights over the southwestern U.S., and below-average heights over both eastern Canada and the eastern U.S. (Fig. E9). This latter feature reflected a highly amplified Hudson Bay trough, and was associated with well below-average surface temperatures across northwestern Canada the eastern half of the U.S. (Fig. E1). Surface temperatures in large portions of these regions were in the lowest 10^th percentile of occurrences.

In the central and south-central U.S., which were located in the area of anomalous sinking motion between the mean ridge and trough axes (Fig. T23), below-average precipitation was recorded during April (Fig. E6). Totals were less than 25% of normal in the north-central U.S., Iowa and Missouri, and from southern California to central Texas.

In the southwestern U.S. an amplified upper-level ridge contributed to above-average surface temperatures, with some areas recording departures in the upper 90^th percentile of occurrences. The ongoing combination of anomalously warm and dry conditions has led to a continuation of extreme or exceptional drought from Arizona and Utah to northern Texas and south-central Kanas.

 

b. Europe

            The 500-hPa height pattern featured an amplified trough-ridge couplet extending from the east-central North Atlantic to western Russia (Fig. E9). This pattern contributed to well above-average surface temperatures in Europe, with many areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). It also contributed to an anomalous precipitation couplet, with above-average totals recorded in southwestern Europe and well below-average totals recorded from southeastern Europe to the Black Sea (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during April featured above-average heights over southeastern Australia, the high latitudes of the central South Pacific, southern South America, and the western South Atlantic. The circulation featured below-average heights over the western South Pacific and Indian Ocean (Fig. E15). At 200-hPa, the subtropical circulation featured an amplified trough over the central and eastern South Pacific Ocean (Fig. T22), which reflects lingering La Niña conditions.

In both southern South America and Australia, anomalous upper-level ridges contributed to exceptionally warm (Fig. E1) and dry (Fig. E3) conditions. Many locations in both of these regions recorded temperatures in the upper 90^th percentile of occurrences and precipitation totals in the lowest 30^th percentile of occurrences.

The South African monsoon season runs from October to April. This area recorded above-average precipitation during April (Fig. E3), with area-averaged totals near the 70^th percentile of occurrences (Fig. E4). For the 2018 rainy season as a whole, precipitation was above average during February-April, and below average during November and January.