Robert A. Maddox

Flash flood forecasting : An ingredients-based methodology

Abstract An approach to forecasting the potential for flash flood-producing storms is developed, using the notion of basic ingredients. Heavy precipitation is the result of sustained high rainfall rates. In turn, high rainfall rates involve the rapid ascent of air containing substantial water vapor and also depend on the precipitation efficiency. The duration of an event is associated with its speed of movement and the size of the system causing the event along the direction of system movement. This leads naturally to a consideration of the meteorological processes by which these basic ingredients are brought together. A description of those processes and of the types of heavy precipitation-producing storms suggests some of the variety of ways in which heavy precipitation occurs. Since the right mixture of these ingredients can be found in a wide variety of synoptic and mesoscale situations, it is necessary to know which of the ingredients is critical in any given case. By knowing which of the ingredients...

The Mexican Monsoon

Abstract The pronounced maximum in rainfall during the warm season over southwestern North America has been noted by various investigators. In the United States this is most pronounced over New Mexico and southern Arizona; however, it is but an extension of a much larger-scale phenomenon that appears to be centered over northwestern Mexico. This phenomenon, herein termed the “Mexican monsoon,” is described from analyses of monthly mean rainfall, geostationary satellite imagery, and rawinsonde data. In particular, the authors note the geographical extent and magnitude of the summer rains, the rapidity of their onset, and the timing of the month of maximum rainfall. Finally, the difficulty in explaining the observed precipitation distribution and its timing from monthly mean upper-air wind and moisture patterns is discussed.

Meoscale Convective Complexes

Abstract A particular class of weather system, the Mesoscale Convective Complex (MCC) is identified, defined, and contrasted with other types of convective weather systems. It is found that MCC systems frequently occur over the central United States, grow to tremendous areal extent, and often persist for periods exceeding 12 h. In addition to widespread beneficial rains, a wide variety of severe convective weather phenomena attends these systems. The development and evolution of MCC systems is not explicitly predicted by operational numerical models even though they are shown to be organized in a distinctly non-random mode on scales that cannot be considered subgrid. The MCC is a convectively driven weather system whose physics are not yet understood, much less incorporated into operational parameterization schemes. A preliminary conceptual model of the life cycle of these systems is presented using enhanced, infrared satellite imagery in conjunction with conventional surface and radar data. The outlook f...

Large-Scale Meteorological Conditions Associated with Midlatitude, Mesoscale Convective Complexes

Abstract Objective analyses of composited meteorological conditions attending ten Mesoscale Couvective Complexes (MCC) reveal a number of distinctive characteristics and important interactions with the large-scale environment. The systems appear to be linked to eastward progression of a weak, middle-tropospheric, short-wave trough. Initial thunderstorms develop within a region of mesoscale convergence and lifting that is apparently forced primarily by low-level warm advection. The MCC system acquires mesoscale organization while it moves eastward ahead of the short-wave trough. Diabatic heating eventually produces a system that is warm core in the middle troposphere and cold core in upper levels. The mature MCC exhibits many similarities to tropical convective systems, although it occurs within a considerably different large-scale setting. Inflow within the lower half of the troposphere feeds convection within a region characterized by significant net upward mass flux and widespread precipitation. Thickne...

Synoptic and Meso-α Scale Aspects of Flash Flood Events1

Abstract Meteorological conditions associated with more than 150 intense convective precipitation events have been examined. These heavy rainfalls caused flash floods and affected most geographic regions of the conterminous United States. Heavy rains associated with weather systems of tropical origin were not considered. Analyses of surface and standard level upper-air data were undertaken to identify and define important synoptic and mesoscale mechanisms that act to intensify and focus precipitation events over specific regions. These analyses indicated that three basic meteorological patterns were associated with flash flooding in the central and eastern United States. Heavy convective precipitation episodes that occurred in the West were considered as a separate category event. Climatological characteristics, composite analyses, and upper-air data are presented for these four classifications of events. The large variability of associated meteorological patterns and parameters (especially winds aloft) m...

Weather Radar Coverage over the Contiguous United States

Abstract Terrain and radar beam-elevation data are used to examine the spatial coverage provided by the national operational network of Doppler weather radars. This information is of importance to a wide variety of users, and potential users, of radar data from the national network. Charts generated for radar coverage at 3 and 5 km above mean sea level show that radar surveillance near 700 and 500 hPa is very limited for some portions of the contiguous United States. Radar coverage charts at heights of 1, 2, and 3 km above ground level illustrate the extent of low-level radar data gathered above the actual land surface. These maps indicate how restricted the national radar network coverage is at low levels, which limits the usefulness of the radar data, especially for quantitative precipitation estimation. The analyses also identify several regions of the contiguous United States in which weather phenomena are sampled by many adjacent radars. Thus, these regions are characterized by very comprehensive rad...

Midlevel Cyclonic Vortices Generated by Mesoseale Convective Systems

Abstract Despite the large number of convective systems that occur over the central United States every year, there are typically only a few well-defined, midlevel vortices apparent in satellite imagery after the overlying anvil debris from some convective complexes has dissipated or advected away. A climatology of mesoscale convectively generated vortex (MCV) events for 1981-1988 is presented and the synoptic setting in which the circulation becomes apparent is discussed. Proximity sounding data from numerous cases are used to examine features of the kinematic and thermodynamic setting of MCVs at various lifelycle stages defined by satellite imagery. Features ofthe large-scale environment that appear conducive to the formation and longevity of MCVS include weak flow, weak vertical shear, weak background relative vorticity, and intense horizontal and vertical moisture gradients. The rapid mesovortex generation observed can be explained by the stretching term of the vorticity equation. Most MCVs emerge fro...

Mesoanalysis of the Big Thompson Storm

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 proces...

Comparison of Meteorological Aspects of the Big Thompson and Rapid City Flash Floods

Abstract Analyses and descriptions of the meteorological conditions that produced devastating flash floods in the Big Thompson Canyon on 31 July 1976 and in the Black Hills on 9 June 1972 are presented. The storms developed when strong low-level easterly winds pushed moist, conditionally unstable air masses upslope into elevated, mountainous terrain. Orographic uplift released the convective instability and light winds aloft allowed the storm complexes to remain nearly stationary. Meteorological conditions that produced these flash floods were found to have been very similar. A set of meteorological features is defined for the purpose of identifying the potential for this type flash flood along the eastern slopes of the Rocky Mountains.

A Study of Tornadic Thunderstorm Interactions with Thermal Boundaries

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...