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of up-slope flow over the Continental Divide in Montana (from the sur- face to 20,000 feet or more) persisted from before noon on 7 June until late on 8 June, when the surface low had moved far enough to the east across South Dakota to swing the flow to a more northerly direction parallel to thE! mountains in the storm area; also the frontal system had occluded by this time, and a high pressure cell began to move southward out of Canada over Montana. Although the moisture content of the Pacific air was only moderately high (the surface dew point temperatures over the storm area were generally in the mid-50's), the highly efficient dynamic triggering mechanism of the orographic-frontal system complex, coupled with the length of time that this unstable condition persisted, resulted in unusually heavy precipitation over a large portion of Montana. 20. DEPl'H-AREA-DURATI.ON VALuEs The rainfall isohyetal map of the storm, plate 6, and the rainfa II depth';area curves, plate 7, make it apparent that the storm of 7-8 June 1964 ranks with the major storms. of this general area. The isohyetal map and pertinent depth-area values were derived from the regular Weather Bureau precipitation station records for this area, and. from "bucket survey" data obtained shortly after the storm by survey teams from the Corps of Engineers, Bureau of Reclamation, Weather Bureau, and the Forest Service. Plate 7 illustrates the 36-hour storm depth-area amounts, and. also, for convenience, the 36-hour Probable Maximum Storm values and. 36-hour values for several other recorded storms in this genal'lll area. Relative storm magnitudes are aelf-evident. 21. STORM PRECIPITATION The rain occurred generally between the morning of 7 June and the evening of 8 June. The rain fell steadily during this period, with a slightly increased rainfall rate recorded during the morning of 8 June. The heaviest amounts of rainfall were reported along the eastern slope of the Continental Divide:,c from Helena to the Canadian border. Some of the larger "bucket- survey" -rainfall amounts -collected were: - Two Medicine Lake area, 14.5 inches; 27 miles west-northwest of Chouteau, 13.0 iIlches, 13 miles southwest of Augusts, 12.8 inches; and. Heart Butte, 11.0 inches. Plste 8 shows mass rainfall curves from 6 record- ing precipitation gages located within the storm area. - V. FLOOD .DAMAGES PREVENTED 22. DAMAGES PREVENTED IN MON'1'.!\NA a. Existing projects which effected a reduction in flood damages in Montana were: 10