Why you should heed Severe Thunderstorm Warnings

Last Friday morning, a brief tornado touched down in central Tipton County, TN, about 4 miles northwest of Covington.  Two homes sustained significant damage and a shop was destroyed.  In addition, there were downed trees and a grain bin was overturned.  The National Weather Service rated the tornado an EF-1 with approximate wind of 95 mph and a path length of 1.3 miles.   The tornado was on the ground for about 4 minutes from 6:05-6:09am.  There were no injuries.

A grain bin was overturned and 2 homes and other property were damaged from an EF-1 tornado near Covington on Friday, 5-31-13.  Photos from WMC-TV.

The tornado is one of the type that meteorologists refer to as a "spin-up" tornado, which typically occur along a line of thunderstorms, not from a supercell.  Lines of storms, which can be called squall lines or quasi-linear convective systems (QLCS), can occasionally produce these brief, weak tornadoes and this was a classic example.  A bow echo in the line (named because it arches forward like an archer's bow) pushed out as it crossed Tipton County and on the north end of the bow (a typical place for some rotation), the tornado formed, touched down, produced minor damage, and quickly disappeared.

Radar loop of squall line (QLCS) as it marched across Tipton Co around 6am on Friday, 5-31-13.  The bow echo is visible moving across northern Tipton Co. (center of image).

The NWS NEXRAD Doppler Radar system nominally takes about 5 minutes to produce each set of images  for the forecaster to analyze.  In cases of QLCS tornadoes, most of the time the tornado may show up on one set of imagery, then disappear in the next set.  This tornado lasted 4 minutes, so the forecaster would have had no more than one set of data to look at to spot the tornado.  With QLCS tornadoes, literally by the time the warning meteorologist sees the radar data indicating a possible weak tornado and issues a warning, the tornado is gone.

(Top) Reflectivity (or precipitation) data as the tornado touched down - 6:04am.  The twister occurred in the favored area for QLCS tornadoes, at the northern edge of the bow echo.
(Bottom) Velocity (or wind) at the same time. Rotation is observed in the wind data.

 This can create a dilemma for the NWS on how to warn on these scenarios. Issue a tornado warning after seeing the spin-up on radar for a storm that likely won't produce another tornado after the warning is sent out, hence warning needlessly and after the fact? Warn on every QLCS that hints at possible rotation, likely with the result of a high number of false alarms? Don't issue a Tornado Warning, but allow a Severe Thunderstorm Warning to cover the possibility of brief spin-up tornadoes that produce minor damage in a short path?

(Top) Reflectivity data at 6:09am, about when the tornado lifted.  This panel look less ominous.
(Bottom) Velocity data at 6:09am. The rotation has weakened considerably.

In the case of the Covington tornado, a Tornado Warning was not in effect, but a Severe Thunderstorm Warning was issued well before the line of storms reached the affected area, due to the likelihood of damaging wind. In addition, there was not a Tornado Watch because the threat of tornadoes over a large area was very low (and since this was the only report of a tornado, it makes sense that a Tornado Watch was not in effect).

This event does raise some points worth considering by the public:

• Because of their transient nature, QLCS, or squall line, tornadoes (which are almost always EF-0 or EF-1 in strength and produce minor damage and typically no injuries) are extremely hard to produce timely Tornado Warnings for.
• Whether a Tornado Watch is in existence or not, and no matter how small the tornado threat might be, severe thunderstorms can, and occasionally do, produce tornadoes. If a squall line is heading for you, you must be mindful that a brief tornado is possible.  Not the big monster EF-4/5's that come from supercells, but a 80-100 mph wind producer.
• Severe Thunderstorm Warnings indicate the likelihood of imminent severe weather, which can and often does include damaging wind of at least 60 mph.  Whether it originates from a tornado or straight-line wind, 60-100 mph wind ins a squall line will cause damage.
• Tornado Warnings are NOT always issued for every remote tornado possibility.  Meteorologists have a hard enough time getting people to heed Tornado Warnings with false alarm rates hovering near 75%.  If every possible spin-up in a squall line got a Tornado Warning, the public would become even more siren-weary and nobody would pay attention when there was a serious threat.

The point of this post is to remind you that all severe weather can be dangerous and damaging - not just supercells with big tornadoes.  If a Severe Thunderstorm Warning is issued, go inside, stay away from windows, and if it gets real ugly out, get in your safe place.  Don't wait for a Tornado Warning.  Your time to take action may be limited, especially with fast-moving squall lines that can hit and be gone within a matter of minutes.

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