The purpose of this paper is to: (1) assess the event synoptic and mesoscale evolutions, and (2) examine particular radar reflectivity and velocity characteristics of six tornado-producing mesocyclones.
Over the region of concern, the event was marked by a notable difluent upper tropospheric wind flow pattern, a broad and strengthening low-level jet (20-30 m/s) and the propagation of an upper level jet streak (40-50 m/s) atop a strengthening surface front during a period of peak solar insolation. It was evident that relatively strong and decelerating boundary layer winds flowing from ocean to land allowed high levels of water vapor (~19 g/kg) to continually pool along coastal areas. In turn, this supported three separate episodes of severe deep convection over 12h. Radar analysis revealed 52 shallow mesocyclones (rotation depths generally < 1.5 km). The latter does not account for those shallow mesocyclones that developed outside of the concerned time range nor those that eluded detection.
Convective Available Potential Energy (CAPE) was computed as ~ 3877 J/kg. The sounding was constructed using a surface parcel defined as [q=300K (~ 81F) and w=19.9 g/kg (~ 76F)]. 0-3 km Storm-relative Helicity (SRH) values were sustained around ~225 m2 s-2 during the event. The greatest contribution to horizontal vorticity production (and corresponding helical area) was found within the lowest 1.5 km. Hourly storm motion vectors veered from 176-198 and strengthened (11-20 m/s) between 12-18 UTC. The presence of increased 0-3 km CAPE (~289 J/kg) at updraft low-levels (0-3 km) along with relatively strong ambient vertical wind shear likely aided the development of additional lifting forces which potentially set this event apart from past similar, but non-tornadic events.
Mean radial velocity trends showed significant change prior to tornado production (T0-4). 0.5° horizontal shear (Vr shear) nearly doubled from 0.020 s-1 (T0-4) to 0.038 s-1 (T0) as mesocyclone core diameter halved to ~0.8 km. Simultaneously, the height of the maximum horizontal shear lowered from 925 m (T0-4) to 631 m (T0). Consistent with similar past findings, results suggest that when Vr-shear persists around ~.015 s-1 and a sudden doubling is observed (when correlated with the descent of the maximum shear below 1 km) a tornado may already exist or soon form. The latter finding could potentially provide tornado lead times of ~10-15 min, which would be of great assistance given the rapid nature of these tornadic evolutions.
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