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Abstract This study analyzes the effect of the location of the North Atlantic Subtropical High (NASH) western ridge on the daily variability of precipitation organization in the southeastern United States (SE US). The western side of the NASH, also known as the NASH western ridge, plays an important role in the variability of summertime precipitation in this region. In this study, the mean summertime position of the NASH western ridge was determined and used to classify each summer day during 2009–2012 into one of four quadrants. Composites of synoptic‐scale circulation and precipitation from mesoscale and isolated precipitation features (MPF and IPF) were calculated for each NASH western ridge quadrant. MPF contributed most (about 65%) of the total summertime precipitation and accounted for most of the differences between the four NASH quadrants. Domain‐averaged precipitation was highest (lowest) during NASH‐SW (NASH‐NW) when IPF (MPF) precipitation was strongest (weakest). The regionality of MPF precipitation maxima was generally associated with the location of low‐level jets and upper‐level troughs. For instance, positive MPF anomalies occurred across the SE US during NASH‐SW when the Great Plains low‐level jet turned eastward bringing moisture to fuel convection in the SE US. In contrast, IPF rain was distributed more uniformly across the SE US. Finally, this study revealed a dipole of precipitation that is controlled by the position of the NASH western ridge and its associated low‐level jets. In one extreme of the dipole NASH‐SE, periods are associated with enhanced MPF precipitation along the coast and offshore for days at a time, and suppressed MPF precipitation inland. The opposite pattern occurs during NASH‐NW when MPF precipitation is enhanced inland and suppressed along the coast and offshore. 

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.