Extratropical Highlights
MAY 2009
Forecast Forum
1. Northern Hemisphere
The 500-hPa height
field during May 2009 featured positive anomalies over the high latitudes of the
central North Pacific and from the western
U.S.
to southern
Europe
, and negative anomalies over eastern
Canada
, the high latitudes of the
North Atlantic
, and north-central
Russia
(Fig. E9).
Over the
Pacific Ocean
the circulation continued to reflect a westward retraction of the East Asian
jet stream in association with the lingering La Niņa signal (Fig.
T21).
The main
temperature signals during May included above average temperatures in the
southwestern
U.S.
and southern
Canada
, and below average temperatures across
Canada
(Fig. E1).
The main precipitation signals included above average totals in the southeastern
quadrant of the
U.S.
, and below average totals in southern
Europe
(Fig. E3).
a. North Pacific/
North America
Lingering La Niņa
signals included a stronger than average tough over the central subtropical
North Pacific, and above average heights over the high latitudes of the central
North Pacific (Figs. T22, E9).
Associated with this pattern, the East Asian jet stream was again retracted
westward toward the western Pacific (Fig. T21). Over
North America
, the circulation during May featured above average heights at 500-hPa over the
southwestern and southeastern
U.S.
, and below average heights over central and eastern
Canada
. This pattern was associated with below average surface temperatures across
much of
Canada
, with departures in central
Canada
dropping into the lowest 10th percentile of occurrences (Fig.
E1). It was also associated with above average
temperatures in the southwestern
U.S.
, with many areas recording values in the highest 90th percentile of
occurrences.
These conditions
were accompanied by an enhanced storm track and 500-hPa trough situated over the
central
United States
. The trough axis delineated areas of below average precipitation to the west
and above average precipitation to the south and east (Fig.
E3). Area-averaged totals were
significantly above average in the
Midwest
, Southeast, and
Ohio
Valley
regions, where departures exceeded the 90th percentile of
occurrences.
b.
North Atlantic
and
Eurasia
The
circulation during May featured a dipole pattern of 500-hPa height anomalies
over the
North
Atlantic
,
with below average heights in the north and above average heights in the south (Fig.
E9). This pattern was associated with a focused
storm track across the central
North
Atlantic
and central
Europe
.
These conditions contributed to below average precipitation across southern
Europe
,
where departures were generally in the lowest 30th percentile of
occurrences.
2. Southern Hemisphere
The circulation during May reflected a zonal
wave-2 pattern of 500-hPa height anomalies in the middle and high latitudes,
with above average heights over the central South Pacific and in the area south
of Africa, and below average heights over the high latitudes of the eastern
South Pacific and southern Indian Ocean (Fig.
T22). The anomaly pattern over the
extratropical South Pacific continued to reflect a lingering influence from La
Niņa.
The main temperature departures during May
were seen in central
South America
, where values were in the upper 70th percentile of occurrences (Fig.
E1). The main precipitation anomalies reflected
below average totals in southwestern and southeastern
Australia
. Monthly totals in portions of the southeast were in the lowest 10th
percentile of occurrences.
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