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Introduction Table 1.2-1 Significant Storms of the 1997 Flood Season (greater than 4 inches total accumulation) Maximum Point Climatic Location County Date Precipitation Re~on Cola-Neb Border Weld May 24 7.0+ inches Great Plains South of Haxtun Phillips June 2 4.5 inches Great Plains Bamesville Weld June 2 4.0+ inches Great Plains Greeley Weld June 13 5.0+ inches Great Plains Holyoke Phillips June 14-15 6.0 inches Great Plains Fort Collins Larimer July 27-28 14.5 inches Front Range Hudson Weld July 28 7.0+ inches Great Plains Northeast Bent Bent July 28 4.0+ inches Great Plains County (unofficial reports) Ramah Elbert July 28-30 17.0+ inches (2- Great Plains day total) Punkin Center LincoIn July 29 4.0+ inches Great Plains Sterling Weld/Logan July 29-30 15.0 inches Great Plains WeIdona Morgan J u1v 29-30 10.5 inches Great Plains Limon-Hugo LincoIn July 29-30 4.0+ inches Great Plains Fort Collins Larimer August 4-5 4.0 inches Front Range 1.5 General Observations A number of kno.wn hydrological concepts were clearly in effect during the circumstances described in this report. These are discussed below in an effort to further examine the nature of the flood-inducing events and the man-made conditions that played roles along the Front Range and elsewhere in eastern Colorado during the summer of 1997. 1.5.1 Meteorology During the summer months, the entire state is subject to convective thunderstorms that are capable of producing large amounts of rainfall over a very short time if sufficient humidity is present in the atmosphere. In eastern Colorado, these storms typically are supplied with moisture drifting in from the Gulf of Mexico and the central plains states. In midsummer and later, monsoonal wind patterns also bring moisture from either the Baja California area, the Gulf of Mexico, or both. This moisture moves into Colorado after coming northward across Mexico and the southwestern U.s., and it is the source of many late summer storms in the state (State of Colorado and FEMA 1997). In late July 1997, this tropical moisture was entering the state from the south. At the same time, a large high-pressure system had stalled over the central plains, and its clockwise rotation supplied moisture to Colorado from the east. A cold front associated with the high pressure d479jchapl.doc 08/13/98 1-4 I