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 90th
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
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 10th percentile of