Geoscience Reference
In-Depth Information
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.
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 Mid-
western 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 rainfalls exceeding amounts
appropriate for recurrence intervals exceeding
100 years (
Figure 10.21
). The three summer
months saw excesses of 500mm above the
average rainfall with totals of 900mm 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
(A) Mississippi River, Keokuk, Iowa
600
400
100-year flood
200
0
1800
1900
2000
(B) Missouri River, Booneville, Missouri
800
600
100-year flood
400
200
0
mls
100
0km
100
0
1800
1900
2000
Area of flooding
streams 1993
Twice normal Jan-July rainfall received in 1993
Est. R.I. (years) for point rainfall amounts during June-July 1993
50 - 100
Area of inundation
14 July 1993
100 - 500
>500
Figure 10.21
Distribution of flooding streams and inundation in the US Midwest during the period June to August
1993. Peak discharges for the Mississippi River at Keokuk, Iowa, (K) and the Missouri River at Booneville, Missouri
(B) are shown, together with the historic annual peak discharge record. The isopleths indicate the multiples of the
30-year average January to July precipitation that fell in the first seven months of 1993, and the symbols the
estimated recurrence intervals (RI years) for point rainfall amounts received during June to July 1993.
Sources: Parrett et al. (1993) and Lott (1994). Courtesy of the US Geological Survey.
