My Thoughts of Dr. Greg Forbes’ Tor:Con Index

06/07/10 1:15 PM

In the severe coverage yesterday I was asked on my thoughts of Dr. Greg Forbes’ Tor:Con Index:

Here is an explanation of this index:

TOR:CON is an attempt, utilizing Dr. Forbes’ extensive forecast experience, to rate, on a scale of 0-10 the likelihood of a tornado within 50 miles of a given location (we’ll call that “area” going forward).  Here are the relative meanings of each number on the scale:

  • 10: Near 100% chance of a tornado within 50 miles (we’ll call that “area”)
  • 8: High probability of a tornado in area
  • 6: Moderate probability of a tornado in area
  • 4: Low probability of a tornado in an area; hail and high winds in t-storms likely
  • 2: Unlikely to have a tornado, but hail and high winds in t-storms possible
  • 0: Near-zero chance of a tornado or severe t-storms in area

TOR:CON is not a forecast of how strong the tornadoes will be, nor a prediction of the number of tornadoes or overall geographic scope of a given severe weather episode, however.

There are three main weather factors Dr. Forbes examines when assessing the TOR:CON index:

  1. Instability:  Warm, humid air near the ground overlaid by cool, dry air aloft maximizes instability in the atmosphere.
  2. Wind shear:  Sharp changes in wind direction and speed with height favor rotating thunderstorms (supercells).  Current research suggests the greater this shear is in the lowest few thousand feet of the atmosphere, the greater the risk for tornadoes.
  3. A “trigger” or source of lift:  This can be a dryline, frontal boundary, outflow boundary from previous thunderstorms, or even a boundary between clear and cloudy skies, across which a temperature contrast develops.

Okay, basically this index is designed to make the forecast as easy to understand to the general public.  The three factors in Dr. Forbes’ forecast are basically the very factors every meteorologist uses when forecasting for severe weather.  So basically what this index represents is the value of other indices like the Showalter index, Bulk Richardson Numbers, Craven/Brooks Significant Severe index, and so on.  As a forecaster, it is important to remember that the Tor: Con index is an index that is two times removed from the actual observational data and thus could have double the amount of error.

As for accuracy, I’m hard press to see a justification of a 100% or even a 80% chance of a tornado for your location.  If anything taught us yesterday, based on this index the New York City and Philadelphia metropolitan area would have been in the 8 to 10 range.  However, I went back and looked at the forecast and instead the values listed were 3 and 4, which by the way was dead on.  Great job Dr. Forbes!

So what does this tell me?  This tells me the scale is only as good as the forecaster that uses it.  Since there is no hard calculation of the index, like the others listed above, the index therefore is an interpretation of the data which is presented in such a way for the layman viewer to understand.  The 50 mile spread gives him some leeway and space to cover himself if your specific location does not get a tornado.  However, lets be honest.  If a forecaster can nail down the exact location of a tornado several hours out, they would be a super genius.

So to recap!  The Tor: Con index is basically a way for Dr. Forbes to as easily as possible to rate the potential for a tornado without getting into the details of lifting parameters, wind shear, and instability.  The index makes the threat easy to understand and straight forward.  However, the index it self does not define the atmospheric potential and the accuracy is dependent more on the forecaster’s understanding of the three main factors.