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Strong southwesterly flow dominated the upper-level wind pattern over
the southeastern states on the morning of 27 March 1994. At 1200 UTC, a
southerly low-level (850 mb) speed maximum of greater than 25 m/s (approx. 50 kt) was
exiting the region, while a second southerly speed maximum of greater than 25
m/s (approx. 50 kt) was moving over the Louisiana coastline. 500-mb wind speeds of
30 to 40 m/s (approx. 60 to 80 kt) were widespread from Texas to Georgia in the
strong southwesterly flow aloft. A jet streak of 70 m/s (approx. 140 kt) was noted
over Missouri and Illinois at 300 and 200 mb. At the surface, a slow moving
cold front was positioned over the northwestern corner of Alabama, while a
trough extended eastward across northern Alabama and northern Georgia. Early
morning surface dew points were in the upper 60s and lower 70s (F) over
southern Alabama. Quasi-geostrophic forcing was relatively weak over the
southeastern states. Q vectors for both 850-500 mb and 500-300 mb layers at
1200 UTC indicated very little vertical forcing on the synoptic scale.
In spite of the weak quasi-geostrophic forcing, warm and moist low-level
air was being lifted along the surface boundaries. The SHARP Workstation
(Hart and Korotky 1991) presentation of the unmodified
1200 UTC Centreville, Alabama (CKL) sounding indicated an
observed CAPE (convective
available potential energy) of 1,193 J/Kg, with 0 J/Kg of CIN (convective
inhibition). The CAPE increased to 2,665 J/Kg by
1800 UTC.
The mean wind in the lowest 6 km (at CKL) increased from 23 m/s (approx. 43 kt) at
1200 UTC to 28 m/s (approx. 54 kt) at 1800 UTC out of the southwest. This wind speed
increase was associated with increasing temperature gradients (frontogenesis)
throughout the troposphere. The hodographs indicated wind shears of greater
than 50 kt in the lowest 6 km at both 1200 and 1800 UTC. The SHARP
Workstation's computed storm motion was relatively close to the actual storm
motion at 1800 UTC. This storm motion produced a storm-relative helicity (0-3
km) of 510 mē/sē , which resulted in an energy/helicity index (Hart and
Korotky, 1991) of an incredible 8.49. Hart and Korotky (1991) suggested that
1.0 is indicative of a significant tornado potential, while Davies (1993)
suggested that 2.0 or 2.5 may be a better indicator of a significant tornado
event. It was in this moderately buoyant and highly sheared environment that
several supercells developed over northern Alabama on Palm Sunday 1994.
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