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<br />12
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<br />THE ARKANSAS RIVER FLOOD OF JUNE 3-:-5, 1921.
<br />
<br />province of Albertn on June 1. 'By the 2d it hnd displnced n low-
<br />pressure area over the upper Missouri Valley, and on the 3d it was
<br />oyer Manitoba and the northern p'art .of North Dakota. Between
<br />the 3d and 4th the movement was very slow, and it advanced only as
<br />far as northeastern Minnesota. By June 5 the area had reached II.
<br />point over the Great Lakes. During this period an area of low pres-
<br />sure (29.7 inches) had developed over western Arizona and remained
<br />over the southern Rocky Mountain plateau for several days. The
<br />difference in pressure between the "high" and" low" . areas' caused
<br />air currents to travel from the" high" to the" low" in an endeavor
<br />tb equalize the pressure. The movement from the" high" area .was .; .
<br />o~tward and downward in a general clockwise direction toward the
<br />"ilow " area. '
<br />The relative positions of the areas of high and low pressure caused
<br />the storm to come primarily from the east or northeast. All along
<br />the front range in Colorado and New Mexico helLYY mins occurred
<br />when the clouds were forced upward by reaching the mountains, but
<br />nowhere else was the rainfall as heavy as in the upper Arkansas'
<br />Valley.
<br />The general line of the front range and foothills bears to the south-
<br />east from the Wyoming boundary to Pikes Peak, a semidetached
<br />mountnin'mass rising abntptl:r from tile plains. South of Pikes Peak
<br />the front range and line of foothills turn sharply westward for 20
<br />miles and then near Canon City swing quickly to the south and south-
<br />east and keep that course for 20 miles. This r:eentrnnt angle in the
<br />general line <.>f the front range had a marked influence on the storm,
<br />causinO' it to concentrate at that point. From the statements of ob-
<br />1._.1:. b .
<br />.' . '>aerrers it is evident that the storm was deflected by Pikes Peak and
<br />t::~.".':" ,~~,a.~e into the ~maller area of. heavy precipitation (pp. ~5-16) from
<br />">>~.;.the north. Owmg to the funnel shape of the valley openmg upon the
<br />{r::-\:ilM~ins, air rushed into it both from. the nort:heast and fro~ the south-
<br />..,.-Y",.... . east around the end of Greenhorn Mountam.' The rushmg together
<br />ga~e the combined air ,currents a more or less circular movement that .'
<br />eallsed them to swing around to the north and northwest, where they
<br />met the storm coming down from the north. This circular motion
<br />caused the clouds to impinge against the side of t.he mountains so
<br />..", . : violently that they rose quickly and precipitated their moisture with
<br />'g~at rapidity. Thunderstorms and hail, which occurred at the be-
<br />'ginning of the min, indicated the strong convective character of the
<br />storm.
<br />." The Weather Bnreau received reports of precipitation at a n1iinb~~'
<br />of points in this general region, which are summarized in the follo,,'-
<br />ing table:' .'0 .
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