April 27th 2011 Severe Weather Setup

First Round:  Early Morning Quasi-Linear Convective System (QLCS) with Damaging Winds and Isolated Tornadoes

On April 27th, 2011 a strong cold front with several areas of embedded low pressure extended from the Texas Hill Country northeastward towards the Arklatex and the Ozarks and then into the lower Ohio Valley.  Warm moist air was in place due to strong southerly flow ahead of this front over Mississippi, Alabama, and Tennessee.  This can be seen on the surface map below, with 62 to 66 degree dewpoint temperatures over these areas, and even 70 degree dewpoints further south along and near the Gulf Coast.  An upper level disturbance sparked a broad area of showers and thunderstorms as it moved across the frontal boundary on the previous evening.  The eastern edge of this line of showers and storms continued to move eastward, in concert with the upper disturbance, reaching the northwest Alabama border around 2 AM CDT on the 27th.  Meanwhile, surface winds backed to the south-southeast as the disturbance moved into the area, while winds at the 850 mb level (around 5,000 feet) increased to between 50 and 55 knots and became more southerly.  The combination of high low-level moisture and increasing shear (winds changing direction and speed with height) provided the setup for damaging winds, large hail and brief tornadoes.  This line experienced further intensification as it moved into northwest Alabama, especially after 4:00 AM CDT.

04/27/2011 4 AM Surface Analysis over the Continental United States.

This line of severe storms pushed into northwestern Alabama prompting a tornado watch for all of northern Alabama and portions of southern middle Tennessee until 8 AM. Several tornado warnings and severe thunderstorm warnings were issued for this line as it traversed northern Alabama. One cell which began in Cullman County and moved northeast into extreme northwestern portions of Georgia had particularly strong rotation.  This cell produced a tornado which damaged locations from Cold Springs to Hanceville to south of Holly Pond.  This supercell is circled in red in the loop below.  Several reports of straight line wind damage were also reported along this line.

Hytop, AL Radar-KHTX 0.5 degree Reflectivity Loop - 04/27/2011 (204 AM to 759 AM CDT)

Second Round:  Late Morning QLCS and Discrete Cells.

Relatively high shear and moist surface conditions continued through the morning and into the early afternoon as another line of showers and thunderstorms developed ahead of the approaching cold front.  However, discrete cells became more of a feature as deep layer shear increased through the morning.  The radar loop below shows the evolution of showers and storms during the late morning and early afternoon period, from 7:59 am to 1:55 pm CDT.   

Hytop, AL Radar-KHTX 0.5 degree Reflectivity Loop - 04/27/2011 (759 AM to 155 PM CDT)

This second round of thunderstorms produced wind damage and hail, including ping pong ball size hail in the Phil Campbell community of Franklin County, Alabama.  Wind damage was also reported at many locations in the area.  A gust to 69 mph was recorded by measuring equipment at Joe Wheeler State Park in the community of Rogersville in Lauderdale County, Alabama.  This quasi-line with embedded discrete cells also produced a string of tornadoes, especially across portions of Limestone and Madison Counties between 11:15 AM to 12:05 PM CDT. 

Third Round:  The March of the Supercells.

Deep layer shear and moisture increased dramatically later in the afternoon and evening ahead of the strong cold front. This combination of strong instability and high shear continued through the evening hours ahead of the cold front before it pushed east of the area into Georgia. Below is an image of the 0 to 6 km helicity values at 7 PM.

04/27/2011 at 7 PM: 0-6 km Helicity Values

This produced the last and most violent round of severe weather, which began around 2:30 PM CDT for northern Alabama as supercells began to line up to the southwest of the area.  During the early afternoon hours,the potential for destructive tornadoes was highlighted by the Storm Prediction Center's upgrade to a rare High Risk for severe weather around 1 PM.  This prompted a Particularly Dangerous Situation (PDS) tornado watch, which was issued for northern Alabama and portions of southern middle Tennessee at 1:45 PM.  The potential really ramped up from noon through 9 PM.  During this period, much of Alabama experienced numerous supercell thunderstorms producing strong to violent tornadoes, including five EF-4 tornadoes and one EF-5 in the Huntsville Forecast Area. This was, incidentally, the last EF-5 to strike our area since the Super Outbreak on April 4th, 1974.  

Below is a loop of radar reflectivity from the Hytop, Alabama radar during this time, before communications was lost to the radar at 5:16 PM CDT. By the late evening hours, the cold front finally pushed east of northern Alabama and across Georgia.    

Hytop, AL Radar-KHTX 0.5 degree Reflectivity Loop - 04/27/2011 (Noon to 416 PM)

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