The Utility of Sounding and Mesonet Data to Nowcast Thunderstorm Initiation

Abstract

Previous studies have shown that thunderstorms often form along boundary-layer convergence lines (boundaries) detected by sensitive Doppler radars similar to the WSR-88D. In this paper, high-resolution mesonet observations (10?15-km spacing and 1-min averages) and sounding data (eight stations within 25 000 km2 and 1?6-h frequency) collected in northeast Colorado are examined to determine their utility for forecasting precisely when and where storms initiate along boundaries. Stability indices derived from mesonet and sounding data were useful in identifying stable regions where storm initiation was unlikely. However, in regions where indices indicated a degree of latent instability, storms often did not form and if they did their intensities were not correlated to the magnitude of the instability. Two-dimensional numerical model studies show that in a near-neutral environment (as typical during a Denver, Colorado summer afternoon), surface temperature and/or dewpoint fluctuations of 2?4°C can be significant for storm initiation. Small-scale fluctuations of this magnitude are common. In addition, observations and numerical model results suggest that in the High Plains, profiles of boundary-layer moisture are necessary to identify the precise locations of storm initiation along convergence lines. Mesonets cannot provide this information and it is impractical to obtain it solely from soundings. Cumulus clouds identify regions where moisture is mixed to the cloud condensation level. Therefore, monitoring cloud location and development with visual observations, very sensitive radars, and satellite imagery is a useful indirect means for identifying regions of deep moisture. It is argued that the rules for forecasting short-term, time-specific locations of thunderstorm initiation, presented in a previous paper by the authors are not significantly changed by the addition of high-resolution mesonet and sounding data. Observations and numerical model results reinforce the importance of using the locations of clouds, stationary boundaries, and horizontal rolls as potential indicators of deep moisture and potential locations of thunderstorm initiation. Mesonet and sounding data are primarily useful for identifying the potential within a mesoscale air mass for thunderstorm initiation. Therefore, mesonet spacing of 25 to 50 km and access to a morning sounding are felt to be adequate.