April 20th, 2011 Widespread Damaging Wind and Isolated Tornado Event

Initial Line in Tennessee Was an Important Player in Later Development over Northern and Central Alabama.

A line of strong thunderstorms pushed east across Tennessee and Kentucky between 11:30 pm on April 19th and 2 am on April 20th.  This produced mainly heavy rainfall, frequent lightning, and some gusty winds as it passed through these areas.  Below is a radar mosaic of this line as it push eastward and fell apart over over eastern Tennessee and northern Alabama.

Although the initial line did not produce severe weather in our area, it was important because it created a mesocscale feature called an outflow boundary extending from northeastern TN southwest into central Alabama.  This boundary is shown in images below as a brown dashed line and is circled by a purple line. 


By about 2:30 am CDT, a  second line of severe thunderstorms developed in central and eastern Mississippi.  This line extended from just south of the Tupelo area southwestward into west central Missippi.  Along this line, several reports of large hail and damaging winds were received in Mississippi.  A few reports of large hail were also reported in southwestern Tennessee, and tornado damage was reported in Carroll county, Mississippi. Below is an image of storm reports received on 04/20/2011.  The reports in Alabama occurred as this line pushed further to the east during the pre-dawn hours.

Main Line of Severe Storms Producing Damaging Winds and An Isolated Tornado moves into Northwestern Alabama.

Around 4 pm CDT this line pushed into northwestern Alabama and then across the remainder of northern Alabama by 7:30 pm CDT.  Click here to see a radar loop during this time.  As this line pushed from north central Alabama into northeastern Alabama it became less organized.  This is because it moved into a more stable atmosphere created by the outflow boundary mentioned earlier with dewpoints in the lower 60s.  Luckily, this lower instability and relatively high wet-bulb heights kept this line of severe storms from producing large hail in northern Alabama.

However, south of the river and near the interface between the boundary and the large scale flow, dewpoints surged just ahead of the approaching cold front.  This caused dewpoints in the mid 60s to move into portions of Franklin, Lawrence, Morgan, and Cullman counties as this line pushed to the east along the cold front.  The afore-mentioned outflow boundary also helped to enhance low level wind shear values in these areas at the same time.  Below is an image of the Storm Relative Helicity from the surface to 6 km.

This increased shear and higher instability combined to produce a tornado touchdown in the Bankhead National Forest in Lawrence county as the cold front pushed through northwestern Alabama.  At this time, the exact track is still being determined, but this tornado uprooted and snapped numerous trees along it's path in the Sipsey wilderness area of Bankhead National Forest along Forest Service roads 210, 208, and 244.  The inaccessibility of densely wooded areas of the Sipsey wildernesss is leading to the uncertainty in track width, length, etc.  However, an aerial survey is attempting to be organized to better determine the details concerning this tornado's path.  Click here to look at the initial damage survey conducted for this tornado by the National Weather Service.

As this line pushed further east, the low level shear shifted southward as the cold front pushed eastward across northern Alabama.  However, widespread damaging winds still continued as this line slowly decreased in intensity.  Again most damaging wind reports were south of the Tennessee river, due to slightly better instability.  These severe thunderstorms with damaging winds were fueled by strong winds aloft.  As seen in the image from 850 mb, winds at this level (about 5,000 feet) were between 40 and 50 KTS.

Winds at 500 mb were also strong, between 55 and 70 KTS.  Dynamic forcing, which likely helped to keep the damaging wind threat going despite more marginal instability, was enhanced by the departing jet streak ortientation relative to northern and central Alabama and fairly strong divergent flow at 300 mb and 200 mb at the top of these storms.  This helped to evacuate air out of the tops of thunderstorms while keeping them fueled by warm, moist air at lower levels.  

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