Mesoanalysis of the Big Thompson Storm

Fernando Caracena Atmospheric Physics and Chemistry Laboratory, NOAA, Boulder, CO 80302

Search for other papers by Fernando Caracena in
Current site
Google Scholar
PubMed
Close
,
Robert A. Maddox Atmospheric Physics and Chemistry Laboratory, NOAA, Boulder, CO 80302

Search for other papers by Robert A. Maddox in
Current site
Google Scholar
PubMed
Close
,
L. Ray Hoxit Atmospheric Physics and Chemistry Laboratory, NOAA, Boulder, CO 80302

Search for other papers by L. Ray Hoxit in
Current site
Google Scholar
PubMed
Close
, and
Charles F. Chappell Atmospheric Physics and Chemistry Laboratory, NOAA, Boulder, CO 80302

Search for other papers by Charles F. Chappell in
Current site
Google Scholar
PubMed
Close
Full access

Abstract

Mesoscale analyses and descriptions of meteorological conditions that produced the devastating dash flood in the Big Thompson Canyon on 31 July 1976 are presented. The storm developed when strong low-level easterly winds to the rear of a polar front pushed a moist, conditionally unstable air mass upslope into the Front Range of the Rocky Mountains. The main thrust of the moisture flux focused initially into the Big Thompson area. Orographic uplift released the convective instability, and light south-southeasterly winds at steering levels allowed the storm complex to remain nearly stationary over the foothills. Heavy rains fell within the storm along a narrow corridor only 5 km wide oriented north-northeast by south-southwest. Minimal entrainment of relatively moist air at middle and upper levels, very low cloud bases and a slightly tilted, updraft structure contributed to a high precipitation efficiency. A deep warm layer of convective cloud fostered precipitation growth through warm cloud processes. The greatest concentrations of precipitation size particles remained at low elevations and as a result of poor vertical beam resolution returned anomalously weak radar echoes to a WSR-57 located 110 n mi away.

Abstract

Mesoscale analyses and descriptions of meteorological conditions that produced the devastating dash flood in the Big Thompson Canyon on 31 July 1976 are presented. The storm developed when strong low-level easterly winds to the rear of a polar front pushed a moist, conditionally unstable air mass upslope into the Front Range of the Rocky Mountains. The main thrust of the moisture flux focused initially into the Big Thompson area. Orographic uplift released the convective instability, and light south-southeasterly winds at steering levels allowed the storm complex to remain nearly stationary over the foothills. Heavy rains fell within the storm along a narrow corridor only 5 km wide oriented north-northeast by south-southwest. Minimal entrainment of relatively moist air at middle and upper levels, very low cloud bases and a slightly tilted, updraft structure contributed to a high precipitation efficiency. A deep warm layer of convective cloud fostered precipitation growth through warm cloud processes. The greatest concentrations of precipitation size particles remained at low elevations and as a result of poor vertical beam resolution returned anomalously weak radar echoes to a WSR-57 located 110 n mi away.

Save