Alabama was located in the base of a significant upper trough, with westerly upper-level winds. The associated surface low was located more than 1000 km to the northeast of Alabama (in Ohio), and surface winds in Alabama were out of the southwest; this is climatologically atypical for tornadoes in the southeast U.S. However, with the westerly upper-level flow, and a surface thermal boundary positioned over central Alabama, there was sufficient low-level shear for tornadoes. 0-1 km storm-relative helicity (SRH) values were near 350 m2 s-2. However, as is typical in cold season severe weather events in the Southeast, thermodynamic instability was limited across most of the area. What would normally be an excellent proximity sounding in both time and space was released at 2302 UTC at Birmingham, AL (KBMX), only 41 minutes prior to, and 25 km southeast of, the time and location of the tornado. This sounding indicated weak mid-level lapse rates of 6.3 K km-1, and with a surface dewpoint Td = 53°F, the surface-based CAPE was only 11 J kg-1 at KBMX.
But, a sequence of surface analyses shows not only that the tornado was near the leading edge of a thermal boundary, but also that the thermal boundary was frontogenetical. Differential heating due to rain and clouds on the north side of the boundary and sunshine to the south of the boundary, along with modest confluence, produced frontogenesis over north-central Alabama of approximately 0.35 K per 100 km per hour. Several studies (e.g., Markowski et al. 1998; Rasmussen et al. 2000; Thompson et al. 2008) have discussed the interactions of supercells and tornadoes with thermal boundaries. Thermal boundaries generally produce a thermally-direct circulation that may enhance the storm-relative helicity along the boundary and lead to enhanced convergence on the warm side of the boundary. Both of these effects would be enhanced by frontogenetical forcing and associated geostrophic adjustment. Doppler radar techniques are used to estimate ageostrophic circulation near the boundary.
It is likely that the frontogenetical boundary produced enhanced SRH and convergence, and these processes may partially explain why a significant tornado formed in this environment. Moreover, a meso-gamma scale analysis of surface dewpoint, produced using mesonet observations, reveals a narrow (~20 km wide) but significant pool of enhanced moisture along the leading edge of the boundary, near the tornado track. Mesonet stations within 10 km of the tornado path indicated Td ~59°F. The meso-gamma analysis reveals that the lower Td at KBMX was not purely an anomaly. It appears that the thermally-direct circulation along the boundary produced moisture flux convergence and the pool of enhanced moisture. Modifying the KBMX 0000 UTC sounding using surface data from mesonet stations shows that the surface-based CAPE near the tornado track was greater than 500 J kg-1, drastically different than the 11 J kg-1 at KBMX just 25 km away. Time-series of surface data from mesonet stations also indicate that the surface Td increased quickly between 2100 UTC and 2345 UTC, so the variability in Td, and the associated variability in CAPE, was large in both space and time.
Such large-magnitude variability, at a small spatial and temporal scale, in the thermodynamic environment for tornadoes is quite significant. It shows how large the gradients in CAPE can be. It primarily shows the potentially large effects a thermal boundary, especially one that is frontogenetical, on not only the wind shear but also on the instability in the tornado environment.
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