Geoscience Reference
In-Depth Information
Figure 10.20
North American rainfall regime regions and histograms showing mean monthly precipitations for each region (January,
June and December are indicated). Note that the jet stream is anchored by the Rockies in more or less the same position at all seasons.
Source
: Mostly after Trewartha (1981); additions by Henderson-Sellers and Robinson (1986). Copyright © 1961. Reproduced by permission
of The Wisconsin Press.
in late summer or autumn, when depression tracks
are in higher mid-latitudes. There is a local maximum
in autumn on the eastern shores of Hudson Bay
(e.g. Inukjuak) due to the effect of open water.
rainfall totals exceeding amounts appropriate for
recurrence intervals over 100 years (Figure 10.21).
The three summer months saw excesses of 500 mm
above the average rainfall with totals of 90 cm or
more. Strong, moist southwesterly airflow recurred
throughout the summer with a quasi-stationary cold
front oriented from southwest to northeast across the
region. The flooding resulted in forty-eight deaths,
destroyed 50,000 homes and caused damage losses of
$10 billion. In September, renewed cyclonic activity
associated with the seasonal southward shift of the
polar front, at a time when mT air from the Gulf is still
warm and moist, typically causes a resumption of
rainfall. Later in the year drier westerly airstreams
affect the continental interior as the general airflow
becomes more zonal.
The diurnal occurrence of precipitation in the
central United States is rather unusual for a conti-
nental interior. Sixty per cent or more of the summer
2
Eastward and southward of the first zone there is
a double maximum in May and September. In the
upper Mississippi region (e.g. Columbia), there is
a secondary minimum, paradoxically in July to
August when the air is especially warm and moist,
and a similar profile occurs in northern Texas (e.g.
Abilene). An upper-level ridge of high pressure over
the Mississippi valley seems to be responsible for
reduced thunderstorm rainfall in midsummer, and a
tongue of subsiding dry air extends southward from
this ridge towards Texas. However, during the period
June to August 1993 massive flooding occurred in
the Midwestern parts of the Mississippi and Missouri
rivers as the result of up to twice the January to July
average precipitation being received, with many point
