Tornado watches issued on September 18 (left), tornado reports (none) on September 18 (right) (National Weather Service Storm Prediction Center, adapted by CWG)

SPC issued 7 tornado watches Tuesday affecting tens of millions of people covering the Raleigh, Norfolk, Richmond, Washington, D.C., Baltimore, Philadelphia, and New York City metropolitan areas. Not one tornado touched down according to preliminary reports.

The atmosphere displayed certain signs it could spawn tornadoes. Profiles of the atmosphere showed ample turning of the winds with altitude or wind shear, a necessary condition for the generation of tornadoes. But one ingredient proved elusive: the instability necessary to sustain rising air motions (updrafts) within the thunderstorm.

Instability arises from heat and moisture - serving as essentially atmospheric fuel. It is often measured using a quantity called Convective Available Potential Energy (CAPE). During the June 29 derecho, we had astronomical levels of CAPE as temperatures soared to over 100 degrees. Not so much Tuesday.

Rather, forecasters were dealing with a low CAPE/high shear environment - the opposite of conditions during the derecho (which lacked the shear necessary for tornadoes). Low CAPE/high shear environments are notoriously inconsistent in generating tornadoes. Sometimes they do, sometimes they don’t.

“The spectrum from damaging wind line segment to brief/strong tornado is incredibly narrow in these cases,” said Greg Carbin, warning coordination meteorologist at SPC. “We cannot say how close we are to tornado potential in these types of environments.”

Despite the uncertain circumstances for tornado development, Carbin was emphatic that tornado watches were necessary as opposed to severe thunderstorm watches.

“The only scenario I can envision where SPC might not issue tornado watches in this type of environment is when the atmosphere has already shown its hand and we are dealing with a fast-moving, nearly solid, squall line in a low CAPE regime with little chance for a tornado,” he said.

Carbin added the high population density of the region at risk was a consideration in issuing the watches.

(National Weather Service Storm Prediction Center, adapted by CWG)

Although SPC’s tornado forecast didn’t work out (a 10 percent probability of a tornado within 25 miles of a point), the moderate risk forecast (based on a 45 percent probability of damaging winds within 25 miles of a point) matches up closely with actual reports that occurred within the moderate risk area.

“This outlook verified quite well with respect to severe thunderstorm wind,” Carbin said.