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from the
Remnants of Lee
The remnants of Tropical Storm Lee moved into the Tennessee valley from the Gulf coast region, and combined with a stalled frontal boundary in the mountains, created an environment for isolated tornadoes as well as copious amounts of rain.
The official track that Lee took out of the Gulf and into Louisiana while still a tropical storm is shown below, but this does not include the complex transition to a remnant low pressure center, which continued to move very slowly northeastward, but remaining south and west of the Appalachian chain.
By Labor Day evening (Monday September 5th), the surface low pressure was still over Alabama, and had connected with a frontal boundary that had moved down from the northwest, but had then become stationary near the Blue Ridge (see map below)
This pattern allowed for tropical moisture to spread northward and then northwest over the frontal boundary, getting some additional lift from the mountains. Rainfall from early Monday morning September 5th to Wednesday morning September 7th (almost 60 hours) ranged from 3 to 9 inches along the Blue Ridge and the adjacent eastern foothills, with 1.5 to 3 inches over the rest off the area. This actually could have been worse, except that the deep tropical moisture moved fairly quickly off to the northeast of the area. While minor flash flooding was reported in a few locations, the very dry conditions leading up to this event helped prevent those problems from being worse.
The images below show a broader scale of the rainfall over a two week period in early September, most of which was associated with the remnants of Lee, and then a closer view of the Blacksburg County Warning area, for a 48 hour period Sept 5-6 when most of the rain associated with Lee fell over our area.
The heaviest rainfall shown in the image above occurred near the Blue Ridge, with totals of 5 to 9 inches.
As if often the case with remnants of tropical systems moving inland, heavy rainfall was not the only concern. Winds began to increase with height above the ground and turned sharply in direction with height as well with the system approaching. Combined with some daytime heating (mainly along and east of the frontal boundary), the ingredients were coming together for some stronger updrafts to develop within the rain bands, and to acquire rotation as they developed, thus presenting a threat for tornadoes. A Tornado Watch was issued for the area immediately to the south of the Blacksburg County Warning area at 3:45pm, and then another for much of the area near the Blue Ridge and points eastward in NC and VA by 6:30pm. Several tornado warnings were issued throughout the afternoon and evening on Labor Day, and two tornadoes were confirmed, one at about 5:20pm in the foothills of northern Wilkes County NC, and the other during the late evening (about 10pm) at the base of Blue Ridge in Cana (Carroll County) VA.
There were a number of rotational signatures seen in Doppler radar data across the region Monday afternoon and evening, and again early Tuesday morning. One of the strongest signatures was in fact associated with the Cana tornado, and the image below shows this couplet of higher outbound Doppler velocity (brighter red) close to higher inbound velocity (brighter green), just moments before an EF0 tornado very briefly touched down along Highway 52 (Fancy Gap Hwy) in Cana (not labeled on the map but indicated by the yellow arrow).
The maps below show the track (1.3 miles) for the Wilkes County tornado, and brief touchdown location for the Cana tornado, as well as some photos of the damage if you click on the blue pins. At the bottom of this page are a few frequently asked questions regarding tropical remnant tornadoes.
View Wilkes County Tornado September 5th, 2011 in a larger map
View Carroll County Tornado September 5th, 2011 in a larger map
Frequently Asked Questions Regarding Tropical Cyclones (Remnants) and Tornadoes
1) Why do tropical cyclones spawn tornadoes?
2) What parts of a tropical cyclone are most favored for tornado formation?
3) Why are TC tornadoes especially difficult to deal with?