TORCON: Are Scientists Becoming Better at Predicting Tornadoes (Photo Credit: NOAA’s Storm Prediction Center, Dr. Greg Forbes)
SPC Day 1 Convective Outlook (left) and Dr. Greg Forbes’ TOR:CON index (right) for the late-winter tornado outbreak of February 23-24, 2016. 61 tornadoes were reported across eleven states and three deaths. Issued at 11 pm CST on the 22nd and 9 am CST, respectively.
Now that spring is upon us, odds are that millions of Americans will find themselves under a TORCON of 6 or above or even a “slight” or “moderate” risk of severe weather. But, what does this mean exactly? TOR:CON, or the Tornado Condition Index, was created by Dr. Greg Forbes of the Weather Channel using a rating system describing the likelihood of a tornado within 50 miles of a location.
The three main parameters used when assigning numbers in the TOR:CON are based off of instability (when warm humid air is overlaid by cold dense air above), wind shear (changes in wind speed and direction with height- higher values close to the ground usually represent a greater risk for tornadoes), and a source of lift such as a dryline or frontal boundary. TOR:CON values range from 0-10 with 0 being near-zero chance of a tornado or severe thunderstorm, 2 low chance of a tornado, 4 moderate chance of a tornado nearby and/or hail and high wind gusts possible, 6 high probability of a tornado, 8+ very high probability of a tornado.
The SPC categorical risk maps, also known as convective outlooks, are used to describe the likelihood of large hail, damaging winds, and/or tornadoes ranging from marginal risk (a stray severe storm is possible), enhanced (slight risk of severe storms but not widespread in coverage), enhanced (greater risk of severe storm coverage and intensity), moderate (widespread severe weather with several tornadoes likely), and high risk (a severe weather outbreak is expected from numerous intense tornadoes). A Day 1 convective outlook would show a 30-60% of significant severe tornadic activity coinciding with a high risk category.
Some caveats, though. High TOR:CON values do not reflect a forecast of how strong tornadoes will be, nor are a predictor of the number of tornadoes within a geographic area. Low TOR:CON values may also mean that the environment is locally favorable for tornadoes. A TOR:CON index of 4 means there’s could be a 40% chance of a tornado within 50 miles of any location in the indicated area, but it also means there is a 60% chance of a tornado will not occur in your area, whereas the analogous SPC tornado probabilities apply within 25 miles. A separate concern with SPC risk categories is the extent of severe storms don’t necessarily correspond with how strong they are. Sometimes, there is a widespread wind or hail threat to merit a moderate risk, but there are few tornadic supercells and/or little damage.
Since tornadoes are such small-scale events, TOR:CON is a fair way of showing the general threat area for tornadic activity. TOR:CON is more of a “heads up” as to what can be expected and if these storms can spawn tornadoes. A high TOR:CON value doesn’t necessarily mean to spend the night in the basement, but it’s important to be weather-aware through different media avenues.
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©2019 Meteorologist Sharon Sullivan