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On Monday, a mini-epidemic of tornadoes developed in the Richmond area as the remains of Florence ascended the Appalachian spine. More than a dozen tornado activity reports have been forwarded to the National Meteorological Service. Here we look at meteorology and compare Monday's event with another tropical cyclone producing a tornado – Hurricane Ivan in 2004.
It was a busy day for the Wakefield National Weather Office. Kathryn Prociv, a contributor to Capital Weather Gang, notes that he issued 28 distinct tornado warnings for southeast Virginia. The meteorological office has confirmed 6 tornadoes in the region:
- An EF-2 tornado (on scale 0 to 5), which was on the ground at 7.5 miles in Chesterfield County, with winds of 115 to 125 mph. He ripped the roof of a building and destroyed a warehouse where a person was killed.
- An EF-1 tornado, which was on the ground for 9 miles in Chesterfield County, with winds of 90 to 100 mph. It caused structural damage in the Hampton Park neighborhood.
- An EF-1, which was ground for 1 mile in Hanover County, with winds of 95 to 105 mph. Many trees were felled or broken.
- An EF-1, which was on the ground for 3.8 miles on the west side of the city of Richmond, with winds of 95 to 100 mph.
- An EF-0 tornado that was on the ground for 2 miles in Powhatan County, with peak winds of 75 to 80 mph.
- An EF-0 was on the ground for 9 miles in Boydton, Virginia, 80 miles southwest of Richmond, with winds of 80 to 85 mph. Many trees were felled.
Reported death is an unusual phenomenon with a tropical system, with the latest fatality of a tropical storm coming to Debby in 2012.
Surface map showing the great circulation of the tropical depression of Florence with convergent winds (purple) and an unstable air zone (blue zone). The storm cells are covered with yellow / red tones.
The figure above highlights the main cause of this small epidemic: the rest of the whirlwind of Hurricane Florence was moving north-east through West Virginia. Its large wind circulation resulted in an unstable air mass over southern Virginia. This circulation of the wind was rich in the type of vorticity we call "vorticity" because of vertical shear winds. The raging storm cells caused some of this spin upward, causing contraction and intensification of the upward circulation. Some of these small circulations, called "mesocyclones", generated tornadoes.
Rotating thunderstorms, or what we tend to call supercells, have developed along a narrow band of activity concentrated on southeastern Virginia. This band and associated thunderstorms were linked to a band of primary rain feeding the greater circulation of Florence. This group settled in a convergence zone of the wind (indicated by the arrows in the graphs) on the east side of the Florence vortex.
Processes in the upper atmosphere, including an invasion of dry air circulating in the southwestern system, probably also fueled the intensity of thunderstorms.
In the satellite image below, the dry air is colored yellow and the deep plume of moisture fueling the storm off the Atlantic is shown in dark gray. The thunderstorm group in southeast Virginia is colored in dark blue and purple. These storms developed at the edge of the dry air. Dry air that slides over low level humidity can destabilize the atmosphere and generate air currents that play a key role in the formation of tornadoes.
A satellite image showing contrasting wet and dry air masses and violent thunderstorms developing at their interface, southeast of Virginia.
The following graph is a radar representation of individual supercells above and around Richmond at 3:44 pm. The left panel shows the radar reflectivity (rain intensity) with tornado warning polygons (red boxes). At least four mesocyclones were active simultaneously; several of them are framed on the right panel, illustrating the Doppler wind speed models.
Mesocyclones are identified by small circular areas of adjacent red and green color – meaning that the wind is moving 180 degrees counterclockwise for a short distance.
Radar image showing several supercell thunderstorms over Richmond. The intensity of the rain is on the left; Doppler winds right.
The tornado season is rarely intense in the Mid-Atlantic, but it tends to be long. Tornadoes are possible throughout the hot season, although the activity tends to peak with the general case of thunderstorms in the middle of summer.
Virginia is kind of remarkable in the BC region in that tropical cyclones are responsible for a significant portion of the activity of tornadoes in the state. Studies have shown that about 20% of all Virginia tornadoes were caused by tropical cyclones. The numbers are falling in Maryland and are moving rapidly north.
The most prolific tornado machines in the region have tended to come from landfills in the Gulf of Mexico. This is partly related to the dry air injection noted above. Outside the main landing zone, this dry air intake tends to influence tornado outbreaks further away from the landing zone in the days following its onset.
This is the same basic process that happened with Hurricane Ivan, who, curiously, crossed the region the same day in 2004.
Comparison of tornadoes of Ivan and Florence in the wider region. (Capital Weather Gang / The Washington Post)
Roanoke Times Editor and long-time storm hunter Kevin Myatt tweeted Monday's epidemic reminded him of "the passage of Hurricane Ivan in Virginia in 2004". . . Floods are worse today in the south-west of the country than with Ivan, and Ivan has spawned many more tornadoes because of the state. But very evocative.
Ivan was indeed a much larger epidemic locally, at least in number. Hurricane Ivan gave birth to 118 tornadoes in total, which is the record for a tropical system in the United States. Thirty-eight landed in Virginia, a state record. This month, 53 twisters clashed across the state as several tropical systems passed.
While most months of September are not so wild here, it is normal for at least a few tornadoes to be spawned by tropical remains in the fall. Whoever kills is another story.
Although we are heading towards the end of the British Columbia region's storm season, other incidents occurred late in the month, such as the 2001 tornado at College Park. A reminder to keep an eye on when it is storming this time of year.
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