Extratropical Highlights –August 2017
1. Northern Hemisphere
The 500-hPa circulation during August
featured above-average heights over the central North Pacific, the western U.S.,
and western Russia, and below-average heights over the Gulf of Alaska, the
central U.S., western Europe, and the polar region (Fig. E9).
The main land-surface temperature signals
during August included above-average temperatures in the northwestern U.S., Europe,
western Russia and the Middle East (Fig.
E1). Surface temperatures were below average
across the central U.S.
The main precipitation signals
included above-average totals in the central and southeastern U.S., Alaska, and
the African Sahel region, and below-average totals in the northwestern U.S., most
of Canada, and the region around the Caspian Sea (Fig. E3).
In the U.S., Major Hurricane (MH)
Harvey made landfall in southeastern Texas with maximum sustained winds of 140
mph. Harvey produced more than 40 inches of rain in the Houston area, causing
widespread and massive flooding. Harvey was the first MH to make landfall in
the continental U.S. since 2005.
a. North America
The 500-hPa circulation during August
featured an amplified wave pattern across North America, with above-average
heights over the western U.S. and an amplified tough over the central U.S. (Fig. E9). This
pattern contributed to anomalously warm (Fig.
E1) and dry (Fig.
E3) conditions in the northwestern U.S., and to below-average precipitation across most of
Canada. It also contributed to cooler and wetter than average conditions in the
central U.S.
Area-averaged rainfall totals in northwestern
U.S. were in the lowest 20th percentile of occurrences (Fig. E5), with many locations recording
totals of less than 25% of normal (Fig. E6). In contrast, area-averaged totals in the U.S. Great Plains
were in the upper 90th percentile of occurrences (Fig. E5). According
to the U.S. Drought Monitor, extreme or exceptional short-term drought expanded
in eastern Montana, and severe short-term drought persisted across the western
half of both North and South Dakota.
b. Europe/ western Russia
The 500-hPa circulation during August
featured above-average heights across southern Europe and extending eastward to
the Caspian Sea, along with below-average heights in western
Europe (Fig. E9).
This pattern was associated above-average surface temperatures throughout
Europe, western Russia and the Middle East (Fig. E1).
c. West African
monsoon
The west
African monsoon extends from June through September, with a peak during July-September.
During August 2017, the west African monsoon system
was enhanced (Fig. E3,
Fig. T24)
with area-average rainfall totals near the 80th percentile of
occurrences (see Sahel region, Fig. E4).
This region has recorded well above-average precipitation during May-August. An
enhanced west African monsoon system, along with
above-average SSTs across the tropical Atlantic and Caribbean Sea (Fig. T18),
typify the warm phase of the Atlantic Multi-Decadal Oscillation (AMO). These
conditions are also typical features of an active Atlantic hurricane season.
2. Southern Hemisphere
The mean 500-hPa circulation during
August featured above-average heights over the three central ocean basins, and below-average heights over southern South
America (Fig. E15).
This pattern contributed exceptionally warm (Fig. E1) and wet conditions (Fig. E3) in southeastern South America,
where surface temperatures and area-averaged rainfall totals were both being
the upper 90th percentile of occurrences (Fig. E5).
The Antarctic ozone hole typically
develops rapidly during August and reaches its peak size in September. The
ozone hole then gradually decreases during October and November, and dissipates
in early December (Fig. S8). During August 2017, the ozone hole intensified
and reached 15 million square kilometers by the end of the month. This size is roughly
the average for the 2007-2016 period.