Extratropical Highlights –April 2017
1. Northern Hemisphere
The 500-hPa circulation during April
featured above-average heights over the eastern U.S., western
Europe, and portions of eastern Asia, and below-average heights across the
central North Pacific, Greenland, and Scandinavia/ northwestern Russia (Fig. E9). Over the North Pacific, this
anomaly pattern projected onto the positive phase of the East Pacific-North
Pacific (+1.0) teleconnection pattern (Table
E1, Fig. E7). Over the
North Atlantic, the anomaly pattern projected onto the strong positive phase of
the North Atlantic Oscillation (NAO, +1.7) and also onto the negative phase of
the Scandinavia teleconnection pattern (-1.4).
The main land-surface temperature signals
during April included above-average temperatures in western Alaska, the eastern
half of the U.S., western Europe, and the eastern half
of Asia (Fig. E1).
The main precipitation signals included above-average totals in much of the U.S.,
southern Canada, and northwestern Russia (Fig.
E3).
a. North Pacific/ North America
The 500-hPa circulation during April
featured an extensive and amplified trough across the central North Pacific
Ocean, a trough over the central U.S., and an amplified ridge over eastern
North America (Fig. E9).
This pattern reflected an amplification of the Aleutian Low and a marked
weakening of the mean Hudson Bay trough. This marks the third straight month in
which the Hudson Bay trough has been exceptionally weak or non-existent. Overall,
these conditions were associated with an anomalous low-level flow of milder
marine air into the northwestern U.S., along with weaker jet stream winds
across the eastern U.S. (Fig. T20).
These conditions contributed to above-average
surface temperatures over much of the central and eastern U.S., with many areas
recording departures in the upper 90th percentile of occurrences (Fig. E1). They
also contributed to above-average precipitation across much of the continental
U.S. (Fig. E3).
Above-average precipitation was
again recorded in the Pacific Northwest U.S. (Fig. E3), with totals in the upper 90th percentile of occurrences
for the third straight month (Fig. E5). This region has recorded above-average totals in every month
since October 2016 (7-months). Elsewhere, area-averaged totals exceeded the 90th
percentile of occurrences in the Inter-Mountain region, the Great Plains, the
Midwest and the Great Lakes region (Fig.
E5). Totals exceeded the 80th
percentile of occurrences in the Ohio Valley region.
According to the U.S. Drought
Monitor, drought ended in the central and southern Plains states, and weakened
or ended in large portions of the southeast and northeast. In the southeastern
U.S., severe drought was recorded in southern Georgia, while the area of severe
or extreme drought expanded in central Florida.
b. North Atlantic/
Eurasia
The 500-hPa circulation during April
featured above-average heights across the central North Atlantic, western Europe, and below-average heights over Greenland,
Scandinavia, and northwestern Russia (Fig.
E9). This pattern projected onto the strong
positive phase of the North Atlantic Oscillation (NAO, +1.7) and also onto the
negative phase of the Scandinavia teleconnection pattern (-1.4).
These conditions were associated
with well above-average surface temperatures in western
Europe and much of eastern Asia, with large areas recording departures above
the 90th percentile of occurrences (Fig. E1). They were also associated with
well above-average precipitation in northwestern Russia and well below-average
precipitation in western Europe (Fig. E3).
2. Southern Hemisphere
The mean 500-hPa circulation during
April featured above-average heights east of New Zealand, and in the areas
south of both South America and Africa. It featured below-average heights over
the eat-central South Pacific and the eastern half of Antarctica (Fig. E15).
In north-eastern Australia, anomalously
dry conditions were present during April, with precipitation totals in many
regions in the lowest 10th percentile of occurrences (Fig. E3).
The South African monsoon season runs
from October to April. This area recorded above-average precipitation during April,
with the largest surpluses occurring in the western and central portions of the
monsoon region (Fig. E3). Overall for the 2016-17 monsoon season, rainfall was below-average in October and March, near-average
in November-December, and above-average in January, February, and April.
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