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Robert A. Maddox
,
Faye Canova
, and
L. Ray Hoxit

Abstract

Meteorological characteristics of flash floods and heavy precipitation events over the western United States are often considerably different than those associated with flash floods in the east. Data were compiled and studied for 61 western United States flash flood events and attendant meteorological conditions were documented. The events were classified by similar 500 mb flow regimes and four characteristic patterns were associated with the flash flood events. Synoptic storms occurred in conjunction with both an intense cyclonic surface system and a strong 500 mb trough. The remaining flash floods (49 events) were associated with weak and slow moving 500 mb short-wave troughs. It is shown that the National Meteorological Center's broadscale analyses (surface and standard level charts) are not sufficient for monitoring the subtle features that often interact to produce flash floods. The field forecaster must enhance, or reanalyze, these facsimile charts if he is to identify regions of developing flash flood potential. Distinct geographical. seasonal and diurnal characteristics were noted for the four basic types of western flash floods and mean values of important meteorological parameters were computed.

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Robert A. Maddox
,
L. Ray Hoxit
, and
Charles F. Chappell

Abstract

It has been frequently observed that thunderstorms which interact with a warm front, or an old thunder-storm outflow boundary, are likely to increase in severity and become tornadic. The physical mechanisms responsible for this observed characteristic of severe storm evolution are not well understood. A physical model of subcloud wind profiles near thermal boundaries has been developed and a number of cases have been analyzed. Within a hot, moist and conditionally unstable air mass, warm thermal advection and surface friction cause the winds to veer and increase with height. Whereas within a cool, Moist air mass (such as a thunderstorm outflow region) cool thermal advection and friction combine to produce a wind profile that has maximum speeds near the surface and veers little with height. The spatial distribution of differing vertical wind profiles and moisture contents within the boundary layer may act in concert to maximize mesoscale moisture contents, convergence and cyclonic vorticity within a narrow mixing zone along the thermal boundary. These characteristics may explain, in part, why storms often reach maximum intensity within the environment attending thermal boundaries.

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Helmut K. Weickmann
,
Alexis B. Long
, and
L. Ray Hoxit

Abstract

Infrared satellite photographs of the tropical oceanic regions within and around the GATE A/B array point to the existence of large, rapidly growing, cumulonimbus clouds. The region along 10°N experiences the greatest frequency of these storms. Also a pronounced diurnal variation is found in the times of initial development with maximum frequencies near midnight. In most cases, this anvil cloud grows to an areal extent ∼7000 km2 in about 4 h, then dissipates in another 3 h. The velocity divergence in the anvil is 1−3×10−4s−1 and the mass outflow is 100–200 kton s−1. These storms account for some of the difference in diurnal variation of high cloudiness that is observed between tropical oceanic and continental regions. A large number of rapidly growing cumulonimbus clouds was observed on 10 August 1974 in connection with a disturbance in the tropical easterlies that ultimately developed into Tropical Storm Alma.

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Fernando Caracena
,
Robert A. Maddox
,
L. Ray Hoxit
, and
Charles F. Chappell

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.

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