Extratropical Highlights –May 2015
1. Northern Hemisphere
The mean 500-hPa circulation during
May featured above-average heights over Alaska, the northeastern U.S., southern
Europe, and central Siberia and below-average heights over the southwestern U.S.,
the high latitudes of the North Atlantic, and eastern Siberia (Fig. E9). At
200-hPa, anti-cyclonic streamfunction anomalies in both hemispheres over the
central and eastern subtropical Pacific Ocean reflected eastward extensions of
the mean subtropical ridges (Fig. T22). These conditions have persisted
for two months and are linked to El Niño (Fig.
T25).
The main land-surface temperature signals
during May included above-average temperatures across Alaska, the eastern U.S.,
southwestern Europe, and northwestern Russia extending to central Siberia (Fig. E1). Below-average
temperatures were observed over portions of the western U.S. The main
precipitation signals during May included above-average totals across much of
the western half of the U.S. and Scandinavia, and below-average totals in eastern
Alaska, western Canada, the eastern U.S., and southwestern Europe (Fig. E3).
a. North Pacific/ North America
At 200-hPa, anti-cyclonic
streamfunction anomalies persisted over the central and eastern subtropical
North Pacific in association with El Niño (Fig.
T22). At 500-hPa, the circulation
featured strong ridges centered over western Canada/ eastern Alaska and over
the northeastern U.S., and an amplified trough over the southwestern U.S. (Fig. E9).
This pattern contributed to well above average surface temperature in Alaska,
with much of the state recording departures above the 90th
percentile of occurrences (Fig. E1). Much of eastern Alaska, along with portions of
western Canada, also recorded well below-average precipitation during May, with
many coastal locations recording departures in the lowest 10th
percentile of occurrences (Fig. E3).
Much of the eastern U.S. also
recorded anomalously warm and dry conditions during May in association a strong
upper-level ridge. Portions of the Northeast and Mid-Atlantic regions recorded
temperature departures in the upper 90th percentile of occurrences
and precipitation totals in the lowest 30th percentile of
occurrences. Area-averaged rainfall totals in the Ohio Valley, Mid-Atlantic,
and Northeast regions were all near the lowest 10th percentile of
occurrences (Fig. E5).
In contrast, much of the western
half of the U.S. and the Gulf Coast states recorded well above-average
precipitation during May, in association with anomalous southwesterly flow
downstream of the amplified trough situated over the southwestern U.S. (Fig. E9). Precipitation
totals exceeded the 90th percentile of occurrences throughout the
Plains states and the Inter-Mountain West (Fig.
E3),
with much of the south-central U.S. recording more than 200 mm of rain.
Area-averaged totals in the Inter-Mountain and Great Plains regions were the
largest in the record dating back to 1979, while area-averaged totals in the
Southwest were near the 90th percentile of occurrences (Fig. E5). The
Southwest has recorded above-average precipitation for the last five months. The
Gulf Coast has recorded above-average precipitation for the last two months.
The U.S. Drought Monitor indicated a
continuation of exceptional drought in central/ southern California and western
Nevada, with extreme drought extending northward into central Oregon and severe
drought extending northward into central Washington. In contrast, drought conditions
ended across Texas, Oklahoma, and Kansas.
b. Eurasia
The 500-hPa circulation featured above-average
heights over southwestern Europe, and below-average heights across the North
Atlantic, Greenland, and Scandinavia (Fig.
E9). This pattern led to above-average
precipitation in Scandinavia and to a continuation of below-average
precipitation in southern Europe (Figs. E3, E4). It was also
associated with well above-average surface temperatures in northwestern Russia
and central Siberia in the region downstream of the mean trough axis. Central
Siberia has recorded warmer than average conditions for the past four months.
2. Southern Hemisphere
The mean 500-hPa circulation during
May featured above-average heights over the western Indian Ocean,
and below-average heights across the high latitudes of the South Pacific (Fig. E15).
At 200-hPa, anti-cyclonic streamfunction anomalies over the central and eastern
subtropical Pacific reflected El Niño (Fig.
T22). Anti-cyclonic streamfunction
anomalies were also observed over eastern Australia in association with a broad
upper-level ridge. This region recorded below-average precipitation during the
month (Fig. E3).
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