FLOOD03905 - Laserfiche WebLink

Home Browse Search

" ---,-",:,,::~,,,,~,,;,~,,,,--,,,.,_, system detached itself from the Big Thompson basin and moved slowly n0l1hward producing a swath of 4 inches or more of rainfall which stretched from northem Boulder County into southeastem Wyoming. Serious flooding occurred prior to 900 pm on the North St. Vrain, after 1000 pm on the Cache la Poudre, and after 1100 pm on the South Platte. According to Henz (ibid) while tlle flooding on these rivers was overshadowed by that in Big Thompson Canyon, the potential for loss of life, especially along the Cache la Poudre River, was reduced because of fOliuitous actions by the public. The meteorological conditions leading up to the Big Thompson Canyon event are described by McCain et al (ibid). By early evening, a trailing frontal boundary managed to invade well into the foothills north of Boulder and convective clouds developed rapidly. Strong easterly or southeasterly winds, similar to the upslope fetch present during the Arkansas River Flood of 1921, pushed into the foothills. Data from Tab.le Mountain north of Boulder indicated that strong easterly winds approached 48 knots at 2,000 feet above the surface. An interpolated sounding for Loveland, Colorado at 1800 MDT showed great instability and a potential for convectJve cloud tops exceeding 50,000 feet ms!. As the stonn developed the following sequence of events is described by McCain et al (ibid) and Henz (1996b). During the early evening hours of July 31, 1976 a massive and unforgiving thunderstorm system developed over the foothills of the Rocky Mountains of Boulder and Larimer Counties. The thunderstonn complex grew quickly as a surging stream of steamy plains air focused the storm's wrath over the Big Thompson River Basin in the Larimer County Foothills. Strong low-level inflow allowed a large mass of water to be processed by the convective storm. As the low-level flow approached the Front Range of Colorado, first a shallow layer of stratus fonned. When the low-level moist air was forced upward by topography until it reached the level of free convection, explosive convective growth followed and radar tops reached 62,000 feet ms!. The stoml complex transformed from billowing cumulus clouds into a massive rain-making machine in less than 90 minutes between 1800 PM and 1930 PM LDT. A combination of deep easterly or southeasterly flow and weak mid-level winds kept the intense stonn "locked in" for several hourS. Tbe northwest slope of the storm updraft allowed large precipitation droplets to fonn and fall out of the rear of the updraft enabling the system to remain in a near steady state for a few hours. In agreement with statements by Henz (ibid), between 2200 and 2300 MDT, maximum rainfall intensities moved north- northeastward into the foothills west and northwest of Fort Collins, as the storm previously "locked in" to the Big Thompson Canyon experienced a northerly steering current for the first time and broke free from the "lock". Heavy rainfall continued in the Cache la Poudre River basin until about 0100 MDT on August I with light showers continuing throughout the night. Flood runofffi-om the Cache la Poudre originated from an area about 10 miles wide extending from north to south across the basin. The rainfall that produced the flooding on the North Fork tributaries near Virginia Dale began about 16