Scott M. Rochette

The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States

Twenty-one warm-season heavy-rainfall events in the central United States produced by mesoscale convective systems (MCSs) that developed above and north of a surface boundary are examined to define the environmental conditions and physical processes associated with these phenomena. Storm-relative composites of numerous kinematic and thermodynamic fields are computed by centering on the heavy-rain-producing region of the parent elevated MCS. Results reveal that the heavy-rain region of elevated MCSs is located on average about 160 km north of a quasi-stationary frontal zone, in a region of low-level moisture convergence that is elongated westward on the cool side of the boundary. The MCS is located within the left-exit region of a south-southwesterly lowlevel jet (LLJ) and the right-entrance region of an upper-level jet positioned well north of the MCS site. The LLJ is directed toward a divergence maximum at 250 hPa that is coincident with the MCS site. Near-surface winds are light and from the southeast within a boundary layer that is statically stable and cool. Winds veer considerably with height (about 1408) from 850 to 250 hPa, a layer associated with warm-air advection. The MCS is located in a maximum of positive equivalent potential temperature ue advection, moisture convergence, and positive thermal advection at 850 hPa. Composite fields at 500 hPa show that the MCS forms in a region of weak anticyclonic curvature in the height field with marginal positive vorticity advection. Even though surfacebased stability fields indicate stable low-level air, there is a layer of convectively unstable air with maximumue CAPE values of more than 1000 J kg21 in the vicinity of the MCS site and higher values upstream. Maximumue convective inhibition (CIN) values over the MCS centroid site are small (less than 40 J kg 21) while to the south convection is limited by large values of CIN (greater than 60 J kg 21). Surface-to-500-hPa composite average relative humidity values are about 70%, and composite precipitable water values average about 3.18 cm (1.25 in.). The representativeness of the composite analysis is also examined. Last, a schematic conceptual model based upon the composite fields is presented that depicts the typical environment favorable for the development of elevated thunderstorms that lead to heavy rainfall.

Initiation of an Elevated Mesoscale Convective System Associated with Heavy Rainfall

Abstract A mesoscale convective system (MCS) developed during the morning hours of 6 June 1993 and moved across northern and central Missouri, resulting in a narrow swath of excessive rainfall (>150 mm). The MCS developed well north of a surface warm front above a cool, stable boundary layer and moved east–southeast across the state. Although some features of the synoptic environment agree with the frontal flash flood composite model, predicting the elevated thunderstorms that composed the MCS posed a unique forecasting challenge. This paper first describes the diagnostic parameters of the prestorm environment that would have been helpful to predict the initiation of the MCS and the resultant locally excessive precipitation. Attention is then drawn to the MCS itself via IR satellite and WSR-88D imagery. Finally, the similarities and differences of this episode to previous studies of flash flooding and elevated thunderstorms are noted, and a summary of key parameters useful in the anticipation of this type...

The Role of Sublimational Cooling in a Late-Season Midwestern Snow Event

Abstract Analysis is provided of a surprise late-season snow event over eastern Missouri and western Illinois. While snow totals failed to exceed 15 cm (6 in.) at any single location, the system was noteworthy because of the poor performance of public, private, and media forecasts in anticipating the event. Using observed data and a successful simulation with a mesoscale numerical model, the event is scrutinized to determine the forcing mechanisms for the precipitation over a small area. A region of enhanced frontogenesis is diagnosed over the region both in the observed data as well as the model output. That the precipitation fell as snow is shown to be the result of a dry layer of air between the surface and the cloud base that saturated and cooled due largely to snow sublimation–evaporation in just a few hours to permit the fall of snow uninhibited from the cloud base to the ground.

Composites of Heavy Rain Producing Elevated Thunderstorms in the Central United States

Composite analyses of the atmosphere over the central United States during elevated thunderstorms producing heavy rainfall are presented. Composites were created for five National Weather Service County Warning Areas (CWAs) in the region. Events studied occurred during the warm season (April–September) during 1979–2012. These CWAs encompass the region determined previously to experience the greatest frequency of elevated thunderstorms in the United States. Composited events produced rainfall of >50 mm 24 hr−1 within the selected CWA. Composites were generated for the 0–3 hr period prior to the heaviest rainfall, 6–9 hours prior to it, and 12–15 hours prior to it. This paper focuses on the Pleasant Hill, Missouri (EAX) composites, as all CWA results were similar; also these analyses focus on the period 0–3 hours prior to event occurrence. These findings corroborate the findings of previous authors. What is offered here that is unique is (1) a measure of the interquartile range within the composite mean fields, allowing for discrimination between variable fields that provided a strong reliable signal, from those that may appear strong but possess large variability, and (2) composite soundings of two subclasses of elevated thunderstorms. Also, a null case (one that fits the composite but failed to produce significant rainfall) is also examined for comparison.

A Case Study of Anomalous Snowfall with an Alberta Clipper

An Alberta clipper moved over western New York state on 11-12 January 2004, producing snowfall amounts of up to 27 cm in portions of the region during a roughly 12-h period. In addition, lightning and thunder were reported. Such systems, known primarily for their fast motion and relatively dry nature, are not generally associated with significant snowfalls. A postmortem analysis of this event, following an ingredients-based methodology, revealed that as the weak low approached the lower Great Lakes, it came under the influence of coupled 300-hPa jets that produced enhanced divergence and significant upward vertical motion over western New York, resulting in the enhanced convective snowfall over the region for a limited time. Instability and possible enhancement via the Great Lakes are also investigated, which show that while there was at least modest instability over the region during the time of heavy snowfall, lake enhancement was unlikely.