THE BIG PICTURE


Let's start by examining the surface map below from the afternoon of April 4.  A strong cold front was moving through the Ohio and lower Mississippi river valleys, with a surface low in southeast Arkansas, just behind a squall line moving through the area. This squall line produced an outflow boundary which served as a focus for new convection, as the surface map shows in Mississippi and  Louisiana.  This dew point map from CoolWx shows better than the surface map a gradient of about 10 degrees across the area.  In addition, the temperatures along with the dew points made for an area with high theta-e.  All these conditions helped make this region ripe for the convection to continue across Alabama, by providing the heat and humidity necessary for thunderstorms.


 This is a surface map at 1800Z on April 4, 2008 that shows a cold front and an outflow boundary moving into the area covered by a tornado
watch, courtesy of the HPC.

The upper-air features played an important role, as well.  The image below of 500-mb heights shows a shortwave trough over the southern Plains which helped produce the area of low pressure I mentioned earlier.  Also, here's a map from 300-mb from
SPC's archives, showing a jet streak moving through the southern Great Lakes.  While it was a little far away, it was still strong enough to produce some divergence in its right entrance region over the lower Mississippi valley, where the tornado watch was located. This divergence, along with the 500-mb shortwave trough, are more ingredients that made this outbreak possible.


This is an image of 500-mb heights, temperature and wind from 1800Z on April 4, 2008 which shows a shortwave trough moving into the area with a
tornado watch, courtesy of  SPC's archives.
With all of the upper-air features in place, it's time to look at the moisture source a little closer to the surface.  The lower-left panel on the image below shows southerly winds at 850-mb.  These winds transported high levels of moisture into the storm which is needed for the formation of thunderstorms and ultimately tornadoes.  With all these features working together, the result was a large storm system, as this satellite image shows from UCAR's archive


This forecast image of heights and winds at various levels from 1800Z on April 4, 2008 shows southerly winds moving into the area covered by the tornado watch which provided moisture for the thunderstorms, courtesy of  PSU's E-WALL.


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