A rarely studied cause of flash flooding-the leading cause of weather-related fatalities in the United States-is from a type of convective cell called a low-echo centroid (LEC), where most of the reflectivity is within the warm portion of the cloud-bearing layer. Since the inception of the LEC classification, the definition has remained ambiguous and difficult to quantify. Operational identification of LEC convection is critical to anticipating flash flooding, because rainfall estimates from the Weather Surveillance Radar-1988 Doppler often are too low. Four LEC convective cases are examined in order to refine the definition and to provide the characteristic features of these storms. From the sample events, two types of LEC storms were identified that shared features that would assist in their operational identification. Analyses indicated that radar reflectivity 60 dBZ should exist within the cell, the cell should possess a long-lived steady-state reflectivity of 45-55 dBZ, and the LEC cell should have increasing reflectivity as height decreases. The development of a system to assist operational forecasters in identification of the atmospheric parameters favoring LEC convection, and the real-time recognition of LEC events to support flash-flood-warning decision making, is an important effort to reducing unwarned and/or unexpected flooding events.
A database of upper-air soundings was collected for weak (EF0/EF1), significant (EF2/EF3), and violent (EF4/ EF5) tornadoes that occurred within 100 km and 6 h of the rawindsonde observation. After case filtering and quality control, a total of 50 proximity soundings for violent tornadoes and randomized samples of 100 proximity soundings for significant tornadoes and 102 for weak tornadoes were obtained. Key convective parameters were analyzed between the tornado datasets. Low-level instability parameters (0-3-km lapse rates and 0-3-km mixed-layer convective available potential energy) were noteworthy predictors of the highest tornado damage rating, whereas mixed-layer lifted condensation level, wind shear, and effective storm relative helicity displayed little predictive skill distinguishing significant and violent tornado environments. The ability of the significant tornado parameter (STP) to discriminate between significant and violent tornadoes also was analyzed. This analysis found that STP does statistically discriminate between violent and significant tornadoes, with mixedlayer convective available potential energy the best discriminator of its variables. Because of the skill in the lowlevel instability parameters, this study also offers a new violent tornado parameter that includes the low-level instability fields in order to better differentiate between significant and violent tornado environments. ABSTRACT (Manuscript
Tornadoes in eastern Texas generally track to the east as predominant westerly upper flow acts on their parent storms. However, an examination of tornadoes from 2000 to 2018 finds that 22% of all tornadoes in the region move in much more northward directions. These tornadoes’ parent storms develop in the open warm sector prior to the arrival of a main linear forcing mechanism (e.g., front, dryline). In fact, some of the more notable tornado outbreaks in recent years across Texas have occurred from northward-moving thunderstorms. This bifurcation of storm/tornado motions is important to understand for forecasting, warning, and messaging of these events. The results show these tornadoes typically occur eastward of large, slow moving, mid to upper-level long-wave troughs and underneath the left quadrant exit-region of an upper-level jet streak. The composite pattern also shows that a low-level jet in eastern Texas, a surface low centered in west-central Texas, and a warm/stationary front extending northeast of the surface low were common for these events. The typical air mass was indicative of weak instability, low convective inhibition, and high shear. Radar analysis of the northerly moving, tornadic storms showed mesocyclonic circulations with smaller diameters and lower rotational shear when compared with tornadic storms that moved in an easterly direction.
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