Extratropical Highlights
AUGUST 2008
Forecast Forum
1.
Northern Hemisphere
The
500-hPa height pattern during August 2008 featured positive anomalies over
northeastern
Canada
and the polar region, and negative anomalies in the
Gulf of Alaska
, the eastern
U.S.
, and across northern
Europe
and northwestern
Russia
(Fig. E9). Surface temperatures were above
average in central and eastern
Canada
, and throughout the vicinity of the
Caspian Sea
and
Black Sea
, and below-average in eastern
Alaska
(Fig. E1).
Many
areas recorded significantly anomalous precipitation during the August. Rainfall
was above average in the
Pacific Northwest
,
Gulf
Coast
, and Southeast regions of the
U.S.
(Fig. E5), as well as portions of northern
Europe, southern Scandinavia, and eastern China, and below-average in Alaska,
the Great Lakes region of the U.S., and southeastern Europe (Fig. E3).
a. North Pacific/
North America
The
500-hPa height pattern during August featured amplified troughs over the
Gulf of Alaska
and the eastern
U.S.
, and an amplified ridge over the central North Pacific. The circulation also
featured positive height anomalies over north-central
Canada
. This overall pattern was
associated with enhanced jet stream winds (Fig. T21)
and increased storminess extending from the central North Pacific to
southwestern Canada, which contributed to above-average precipitation in the
Pacific Northwestern U.S. (Fig. E3) for the
first time since January (Fig. E5). It was also
associated with below-average temperatures over eastern
Alaska
and above-average temperatures across central and eastern
Canada
.
For
the first time since December 2007, precipitation was below average in the
Great Lakes
region of the
U.S.
, which was situated in the area of anomalous upper-level convergence and
sinking motion upstream of the mean trough axis (Fig. T23).
In contrast, rainfall was above-average across the
Gulf
Coast
and Southeast regions of the
U.S.
for the second straight month, following eight consecutive months of
below-average precipitation.
Two
landfalling named storms struck the
Gulf
Coast
and southeastern
U.S.
during August. The first was Tropical Storm Edouard, which made landfall in
northeastern
Texas
on August 5th. A very slow-moving Tropical Storm Fay then made
landfall in
Florida
on August 19th, bringing extremely heavy rainfall (150 mm to 350 mm)
to much of the state.
b.
Europe
The
500-hPa circulation featured negative height anomalies across northern
Europe
and northwestern
Russia
, with the mean trough axis situated just west of
Europe
(Fig. E9). This pattern was associated with an
enhanced storm track and above-average precipitation across northern
Europe
, and with below-average precipitation over southern
Europe
.
2. Southern Hemisphere
The
500-hPa height field during August featured an anomalous zonal wave-3 pattern,
with large areas of above-average heights centered south of
Australia
and over the high latitudes of the central
South Pacific Ocean
, and below-average heights located over
New Zealand
, southern
South America
, and southern
South Africa
(Fig. E15). In
Australia
, an extensive area of anomalous northerly flow downstream of the mean ridge
axis extended from the
Great Australian Bight
to northern
Australia
(Fig. T20). This circulation
brought below-average surface temperatures to most of the continent, with many
areas recording departures in the lowest 10th percentile of
occurrences (Fig. E1). In contrast, surface
temperatures were again above average over central
South America
, and below average farther south. These conditions were associated with a deep
upper-level trough and an associated strengthening of the mean cold frontal
boundary (Fig. T20).
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