THE UTILITY OF SOUNDING AND MESONET DATA TO NOWCAST THUNDERSTORM INITIATION

Previous studies have shown that thunderstorms often form along bounda ry-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 nor theast Colorado are examined to determine their utility for forecastin g precisely when and where storms initiate along boundaries. Stability indices derived from mesonet and sounding data were useful in identif ying stable regions where storm initiation was unlikely. However, in r egions where indices indicated a degree of latent instability, storms often did not form and if they did their intensities were not correlat ed to the magnitude of the instability. Two-dimensional numerical mode l studies show that in a near-neutral environment (as typical during a Denver, Colorado summer afternoon), surface temperature and/or dewpoi nt fluctuations of 2-4-degrees-C can be significant for storm initiati on. Small-scale fluctuations of this magnitude are common. In addition , observations and numerical model results suggest that in the High Pl ains, profiles of boundary-layer moisture are necessary to identify th e precise locations of storm initiation along convergence lines. Meson ets 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 rad ars, 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, presen ted in a previous paper by the authors are not significantly changed b y the addition of high-resolution mesonet and sounding data. Observati ons and numerical model results reinforce the importance of using the locations of clouds, stationary boundaries, and horizontal rolls as po tential indicators of deep moisture and potential locations of thunder storm initiation. Mesonet and sounding data are primarily useful for i dentifying 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.