Extratropical Highlights –May 2018

 

1. Northern Hemisphere

The 500-hPa circulation during May featured above-average heights across the United States, the central North Atlantic Ocean, and Europe, and below-average heights over Alaska, the high latitudes of the North Atlantic, and central Russia (Fig. E9). Over the Atlantic basin, the circulation reflected a strong positive phase (+2.0 std. dev.) of the North Atlantic Oscillation (NAO) (Fig. E7, Table E1). A positive NAO pattern has generally prevailed since January 2018. Over Scandinavia, the circulation projected strongly onto the positive phase (+1.7 std. dev.) of the Scandinavia pattern.

The main land-surface temperature signals during May included above-average temperatures across most of North America, eastern Europe, Scandinavia, and eastern China, and below-average temperatures in northeastern Canada and central Siberia (Fig. E1). The main precipitation signals included above-average totals in the eastern U.S. and eastern China, and below-average totals in the south-central U.S. and Scandinavia (Fig. E3).

 

a. North America

The 500-hPa circulation during May featured above-average heights across the United States (Fig. E9). This pattern was associated with well above-average surface temperatures across the U.S. and southern Canada, with most locations recording departures in the upper 90^th percentile of occurrences (Fig. E1).

Precipitation was above average in the eastern U.S. and below average in the south-central U.S. (Fig. E3). 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 central North Atlantic Ocean and Scandinavia, and below-average heights across the high latitudes of the North Atlantic (Fig. E9). Over the Atlantic basin, the circulation reflected the continuation of a strong positive phase (+2.0 std. dev.) of the NAO (Fig. E7, Table E1). Over Scandinavia, the circulation projected strongly onto the positive phase (+1.7 std. dev.) of the Scandinavia pattern.

The persistent positive NAO pattern has been associated with an anomalously strong and surface pressure ridge over the central North Atlantic (Fig. E8), and with exceptionally strong northeasterly trade winds extending across the tropical Atlantic Ocean (Fig. T20). These conditions have contributed to anomalously cool SSTs across the tropical Atlantic (Fig. T18), with May departures in the eastern Atlantic being cooler than -0.5ºC.

The positive phase of the Scandinavia pattern reflected a blocking ridge over Scandinavia and western Russia. This pattern contributed to exceptionally warm surface temperatures across eastern Europe and Scandinavia (Fig. E1), and to anomalously dry conditions in Scandinavia (Fig. E3).

In late May, NOAA issued its 2018 Atlantic hurricane season outlook. The outlook calls for a 40% chance of a near-normal season, a 35% chance of an above-normal season, and a 25% chance of a below-normal season. The outlook indicates a 70% chance for each of the following ranges of activity: 10-16 named storms, of which 5-9 are expected to become hurricanes, and 1-4 of those are predicted to become major hurricanes (Cat. 3-4-5 storms with maximum sustained surface wind speeds of 111 mph or greater).

           

 

2. Southern Hemisphere

The mean 500-hPa circulation during May featured above-average heights over southwestern Australia and the high latitudes of the eastern South Pacific, and below-average heights over the central Indian Ocean (Fig. E15). In Australia, precipitation was generally below average during May (Fig. E3). The most significant deficits were observed in the southwest and southeast, where monthly rainfall totals were in the lowest 10^th percentile of occurrences.