P12.6
Multi-scale Analysis of the 13 October 2001 Central Gulf Coast Tornado Outbreak
Dan Darbe, NOAA/NWS, Mobile, AL; and J. M. Medlin
On 13 October 2001, a mini-tornado outbreak occurred along the central Gulf Coast over a small land area (9 tornadoes per ~ 3200 sq. mi. ). This event was part of a larger scale severe weather outbreak that occurred over the preceding five days and extended from Nebraska to Georgia. The number of tornadoes (five F0s, two F1s, one F2 and an F3) accounts for 12.6% of all Baldwin County (71) tornadoes reported between 1950-2001. The event became the new 4th highest October MTOTAL (3.966) since 1948 (source - http://eyewall.met.psu.edu/ranking/month.html ).
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.
Poster Session 12, Tornado and Severe Storm Environments
Thursday, 15 August 2002, 3:00 PM-4:30 PM
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