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Extratropical Highlights
JULY 2007
Forecast Forum
Extratropical
Highlights – July 2007
1. Northern
Hemisphere
The 500-hPa height field during July featured an anomalous wave-like pattern
across most of the middle and high latitudes, with positive anomalies over the
high latitudes of the central North Pacific, western North America, Greenland,
the polar region, and central Russia, and negative anomalies in the Gulf of
Alaska, the eastern U.S., and northern Europe (Figs. E9,
E11).
The main
surface temperature departures during July reflected warmer than average
conditions in Alaska, western North America, eastern Europe, and western Russia,
and cooler than average conditions in the eastern United States (Fig.
E1). The main precipitation anomalies included above average totals in
the south-central U.S., northern Europe, and southern Scandinavia, and below
average totals in the north-central and eastern U.S., southern Europe, and
Mongolia (Figs. E3, E6).
a.
North America
The 500-hPa circulation pattern during July featured an anomalous wave pattern
with deep troughs in the Gulf of Alaska and the eastern U.S., and a very strong
ridge over western North America (Fig. E9). The surface temperature
anomalies were very consistent with this pattern, with above-average
temperatures in the western U.S., Alaska, and western Canada, and below-average
temperatures in the eastern United States. Temperatures across much of the
western U.S. and western Canada were generally 2°-4°C above average, with many
regions recording values in the upper 90th percentile of
occurrences. Similar anomalous warmth was recorded in western Alaska. In
contrast, anomalous northwesterly flow and a deeper-than-average upper-level
trough brought below-average temperatures to the eastern United States.
This
anomalous wave pattern also strongly controlled the precipitation patterns
across North America (Figs. E3, E5).
Above-average precipitation was recorded in western Canada in the area
immediately downstream of the mean trough axis, and below-average precipitation
was recorded over large portions of the north-central and eastern U.S. in the
region between the mean upper-level ridge and trough axes. Long-term
precipitation deficits have led to moderate drought conditions across much of
the eastern U.S. east of the Mississippi River, and to severe drought in
portions of the upper Midwest and Inter-Mountain regions of the western United
States.
b.
Eurasia
The 500-hPa circulation pattern during July featured a strong trough/ jet stream
configuration over the eastern North Atlantic and western Europe. This pattern
led to well above-average precipitation in northern Europe and southern
Scandinavia, with severe flooding recorded in areas of England and northern
Europe during the month. In contrast, a persistent upper-level ridge over the
eastern Mediterranean Sea contributed to below-average precipitation and
above-average temperatures in that region.
2.
Southern Hemisphere
The 500-hPa height anomaly pattern during July featured a
strong zonal symmetry, with above-average heights over Antarctica, and
below-average heights in the middle latitudes (Fig.
E15). This pattern projected strongly onto the negative phase of the
Antarctic Oscillation (AAO). The circulation across Australia featured an
anomalous trough-ridge-trough pattern, with the ridge located over the middle of
the continent and the troughs centered just off the west and east coasts. This
pattern led to a continuation of well below-average precipitation in eastern
Australia, with large portions of the region recording totals in the lowest 10th
percentile of occurrences (Fig. E3).
A strong ridge-trough pattern flanking southern South America was also
evident. This circulation contributed to below-average temperatures across the
southern half of the continent, with departures in many areas in the lowest 30th
percentile of occurrences. A very unusual cold air outbreak brought snow to
Buenos Aires, Argentina for the first time in 89 years.
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