Abstract
Convection initiation (CI) remains a formidable forecasting challenge, particularly along the coast. Examining Houston, Texas, radar observations of isolated cells’ CI spatiotemporal patterns for Junes from 2017 to 2022 revealed three patterns: cells forming exclusively over land (LAND), over coastal waters (GULF), or domainwide, initiating first over land (DW-L) or the water (DW-G). CI and dissipation times varied by regime. LAND events tended to typify the diurnal cycle, whereas GULF events tended to initiate overnight; both had durations < 10 h. In contrast, DW events began overnight and lasted until evening, with durations often exceeding 10–15 h. Synoptic-scale composites for each regime revealed only minimal forcing for ascent, suggesting the local environment’s importance for CI. Composite vertical profiles for CI locations revealed surface-based CAPE > 1500 J kg−1 and CIN > −40 J kg−1 for each regime. LAND had the hottest and driest lowest 1 km AGL, but was moistest between 1 and 2 km, suggesting LAND parcels originating below 1 km may be susceptible to entrainment and require moister midlevels for successful CI. We also found conditional instability below 1 km AGL for all regimes but a stable layer for GULF and neutral layers for LAND and DW-G between 1 and 2 km. This indicates saturation of air parcels within this layer is insufficient for CI, and mechanical lifting (e.g., sea breeze) would be necessary for CI. Indeed, all regimes featured potential instability throughout the lowest 4 km. However, only the LAND regime had a coastal density gradient conducive to sea-breeze formation; this indicates other lifting mechanisms may be important in the other regimes.
Significance Statement
Forecasting the timing and location of storm formation is a major challenge, particularly in coastal areas. We endeavor to understand storm formation patterns in the Houston, Texas, area, with the main goal of better understanding how precursor atmospheric conditions may favor or disfavor such storm formation. We find four spatial patterns of storm formation: only over the land, only over coastal gulf waters, or over both land and gulf (but starting over land or the gulf). Average large-scale and local conditions were similar for each regime, with only subtle differences in their low-level temperature and humidity profiles. Results suggest that small-scale features like sea breezes thus are required for initiation, but only the LAND regime has sea-breeze-favorable conditions.
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