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This study uses four-year radar-based precipitation organization and reanalysis datasets to study the mechanisms that lead to the abrupt springtime onset of precipitation associated with isolated storms in the Southeast United States (SE US). Although the SE US receives relatively constant precipitation year-round, previous work demonstrated a “hidden” summertime maximum in isolated precipitation features (IPF) whose annual cycle resembles that of monsoon climates in the subtropics. In the SE US, IPF rain abruptly ramps up in May and lasts until sometime between late August and early October. This study suggests that the onset of the IPF season in the SE US is brought about by a combination of slow thermodynamic processes and fast dynamic triggers, as follows. First, in the weeks prior to IPF onset, a gradual seasonal build-up of convective available potential energy (CAPE) occurs in the Gulf of Mexico. Then, in one-to-two pentads prior to onset, the upper-tropospheric jet stream shifts northward, favoring the presence of slow-moving frontal systems in the SE US. This poleward shift in the jet stream location in turn allows the establishment of the North Atlantic subtropical high western ridge over the SE US which, with associated poleward transport of high CAPE air from the Gulf of Mexico, leads to the establishment of the warm-season regime of IPF precipitation in the SE US. 

This study examines the geographic and temporal characteristics of the springtime transition to the summer precipitation regime of isolated convection in the southeastern (SE) United States during 2009–12, using a high-resolution surface radar-based precipitation dataset. Isolated convection refers herein to isolated elements or small clusters of precipitation in radar imagery less than 100 km in horizontal dimension. Though the SE United States does not have a monsoon climate, it is useful to apply the established framework of monsoon onset to study the timing and regional variation of the onset of the summer isolated convection regime. Overall, isolated convection rain onset in the SE U.S. domain occurs in late May. Onset begins in south Florida in mid-April, continuing nearly simultaneously across the southeastern coastal plain in early to mid-May. In the northern domain, from Virginia to the Ohio Valley, onset generally occurs much later (mid-June to early July) with more variable onset timing. The sharpness of onset timing is most evident in the coastal plain and Florida. Results suggest the hypothesis, to be examined in a forthcoming study, that the timing of isolated convection onset in the spring may be triggered by specific synoptic-scale events within gradual seasonal changes in atmospheric conditions including extratropical cyclone tracks, convective instability, and the westward migration of the North Atlantic subtropical high. This approach may offer a useful framework for evaluating long-term changes in precipitation for subtropical regimes in an observational and modeling context.