Widespread Damaging Wind Event of April 5, 2011

A potent squall line of severe thunderstorms raced eastward across the entire southeastern U.S. from the morning of April 4 through the afternoon of April 5, 2011.  In a matter of just 30 hours, this squall line brought widespread damaging winds to the Ozarks, Tennessee and Ohio Valleys, all of the Gulf Coast states including Florida, and the Carolinas and Mid Atlantic States. This system was one of the most prolific severe weather producers to ever impact the nation. Figure 1 shows all of the reports that came into NWS offices across the country.

Figure 1. Storm reports relayed to the NWS from 1200 UTC April 4 through 12Z UTC April 5, 2011.

Locally, the system swept across the WFO Tallahassee forecast area from Coffee County AL in the northwest to Dixie County FL in the southeast in just over 6 hours. The office received 65 individual reports of severe weather from 40 of the 48 counties in our service area. A synopsis of this event, authored by WFO TAE Senior Forecaster Jeff Fournier is now available. You can also view the summary preliminary local storm report issued for the event which contains particulars for each report of severe weather. Below, we highlight some Doppler radar imagery depicting the squall line as it crossed the region. We also include a few photos of actual wind damage. Figure 2 depicts all of the warnings (polygons) issued by WFO Tallahassee during the event, with severe weather reports overlaid. As can be easily seen, the vast majority of the warnings were verified. The average overall event lead time was 30 minutes.

 Warning polygons with severe reports overlaid for the squall line of April 5, 2011.

Figure 2. Warning polygons issued by WFO Tallahassee on April 5, 2011. Severe (yellow), tornado (red) and special marine (purple) warnings are depicted. Locations of reported severe weather are shown by the icons. Click for a larger view.

 

Figure 3 shows the squall line as it approached Albany, GA. Note the 70-kt outbound winds in the base velocity (right panel) over Lee County and the bowing segment in the associated base reflectivity image on the left. The squall line produced a 63-mph wind gust at the Albany Southwest Georgia Regional Airport at 2:04 AM EDT. Figures 5 & 6 show some wind damage that occurred a few miles north of Leesburg, GA as the squall line passed.

Figure 3. Radar imagery from the KTLH Doppler radar at 0549 UTC (1:49 AM EDT) Tuesday, April 5, 2011. Base reflectivity is depicted in the left panel, with base velocity to the right.

 

 

Figure 4. Telephone poles and power lines north of Leesburg GA that were downed by severe thunderstorm wind gusts during the pre-dawn hours of April 5, 2011.

 

Figure 5. Structural damage to a warehouse north of Leesburg, GA from sustained as a squall line of severe storms crossed the area the previous night.

 

Figure 6 shows a map where wind damage occurred in Cook County GA during the event. Most of the damage occurred near Pine Valley southwest of Adel. Arrows show the direction that several very large trees were lying on the ground, indicative of downsburst winds. Gusts were estimated to be 75 to 80 mph based on the 3-ft diameter of the trees that were felled. Figure 7 shows one of these trees. The person standing next to the tree gives some perspective to the size. Figure 8 shows a large tree that fell near a small house. Figure 9 shows a large tree down near a gas station. Figure 10 (courtesy of WALB) shows a mobile home that was completely flipped onto its roof.

Figure 6. A map showing areas within Cook County that sustained damage from the squall line that raced across the county during the pre-dawn hours of April 5, 2011. Arrows indicate the direction of large felled trees. The pattern is indicative of a strong downburst.

Figure 7. A large tree felled by downburst winds in Cook County, GA during the pre-dawn hours of April 5, 2011.

Figure 8. A large tree felled by downburst winds in Cook County, GA during the pre-dawn hours of April 5, 2011.

Figure 9. Another large tree toppled from severe thunderstorms in Cook County GA on April 5, 2011.

Figure 10. A mobile home that was completely flipped onto its roof by downburst winds in Adel County GA on April 5, 2011. Photo provided courtesy of WALB.

Figure 11 shows the squall line as it approached Leon County, FL. Another line of severe storms had recently developed out ahead of the main line. On the left, base reflectivity from the KTLH Doppler radar is depicted. On the right is the base velocity image, which shows 60-kt inbound winds at an elevation of about 900 ft just west ot the Gadsden-Leon County line at the Ochlockonee River, which is just northwest of Ft. Braden, FL. Outbound winds of a similar magnitude are seen in northeast Leon County at an elevation of about 1100 ft. Storm damage surveys indicate damage consistent with 70 mph winds at the surface.

Radar imagery from the KTLH Doppler radar at 0715 UTC (3:15 AM EDT) Tuesday, April 5, 2011. Base reflectivity is depicted in the right panel, with base velocity to the left.

Figure 11. Radar imagery from the KTLH Doppler radar at 0715 UTC (3:15 AM EDT) Tuesday, April 5, 2011. Base reflectivity is depicted in the left panel, with base velocity to the right.

 

Figure 12 shows the squall line at its most potent as it moved across the eastern portion of the Florida Big Bend. Note the outbound base velocities in the right panel of the KTLH radar image below which show magnitudes of 65-70 knots in and around Perry, FL at an elevation of about 3500 ft. The Perry ASOS measured a hurricane-force wind gust of 65 knots (75 mph) within one minute of the time of this image. Figure 13 shows the squall line as it crossed Dixie County, FL. Radar operators suspected a tornado in the vicinity of the nitch in the base reflectivity image on the left which was coincident with a couplet in the storm-relative velocity (SRM) data (right panel). A good indicator of the widespread nature of this event was the number of high wind events that were captured by automated reporting stations. These are listed below.

Time (EDT) Location Wind Gust (mph)
2:04 AM Albany ASOS 63
2:40 AM Clarkesville SKYWARN 63
3:39 AM Tallahassee ASOS 52
4:16 AM Moody ASOS 59
4:22 AM Valdosta ASOS 45
4:59 AM Perry ASOS 75
5:48 AM Cross City ASOS 40

Figure 12. Radar imagery from the KTLH Doppler radar at 0858 UTC (4:58 AM EDT) Tuesday, April 5, 2011. Base reflectivity is depicted in the left panel, with base velocity to the right.


Figure 13. Radar imagery from the KTLH Doppler radar at 0949 UTC (5:49 AM EDT) Tuesday, April 5, 2011. Base reflectivity is depicted in the left panel, with SRM to the right.

 

Page created by Mark Wool. Thanks go to Don Van Dyke for the radar and warning verification information and to Kelly Godsey for collecting the damage photos from various sources.


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