National Weather Service United States Department of Commerce

Mid South Severe Weather Outbreak of April 4, 2011

Event Overview

A powerful mid and upper level trough of low pressure originating over the Intermountain West on the evening of Sunday, April 3, 2011, quickly ejected east across the Great Plains and into the Lower and Middle Mississippi Valley on the morning of Monday, April 4, 2011.  A very warm and moist airmass was in place across the Mid South prior to the arrival of the upper level trough and associated strong surface cold front.  This allowed the atmosphere to become moderately unstable and wind fields to strengthen significantly.

Below is a water vapor satellite image from late Sunday night.

Water vapor image

Numerous thunderstorms developed across western portions of the Mid South early Monday morning, with line segments and embedded supercell thunderstorms the predominant storm mode.  These storms quickly become severe in the presence of the moderate instability and strong wind shear in place.  As stronger forcing aloft associated with the upper trough moved into the region by late morning, storms organized into a powerful squall line along the Mississippi river.  Many thunderstorms within the squall line were severe and producing damaging winds and isolated large hail.  Isolated supercell thunderstorms continued to develop in advance of the squall line to the east of the river as the atmosphere further destabilized associated with day time heating.  These isolated supercell thunderstorms were severe as well and produced areas of damaging winds and large hail between quarter and golfball size.


As the intensifying squall line raced east of the Mississippi river, severe winds became widespread with areas of intense winds exceeding 80 mph in some locations where bowing segments and low level mesocyclones developed.  The squall line continued across the remainder of west Tennessee and northeast Mississippi through the early afternoon hours before exiting the forecast area by mid afternoon.   

Below is the radar reflectivity loop.

Radar reflectivity loop

Here is the base velocity radar loop.

Velocity radar loop


Widespread wind damage reports, some significant, were left in the wake of the intense squall line - derecho event across the Mid South.  Numerous reports of trees and power lines were reported along with structural damage in areas impacted by intense straight line winds.  There were also more isolated reports of penny to quarter size hail from storms within the squall line.  All together there were 88 reports of wind damage or large hail in the Mid South and over 1300 across the country.  There was one fatality reported in Shelby County, Tennessee, as a result of fallen power lines.  This event marks the largest severe weather outbreak to date during the 2011 spring severe weather season across the Mid South and one of the largest overall severe weather outbreaks to impact the southern United States.

Storm Reports (click to view)

To view the preliminary storm reports for the Mid South, click here.

For the rest of the country, click here.

Pictures will be included in future updates.  Additional information will be provided when available.  Check back for updates!