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                   Monday, April 25, 2016

A derived parameter for storm chaser traffic impacts: the "Chaser Traffic Index" or CTI

By DAN ROBINSON
Storm Chaser/Photographer
25 Years of Storm Chasing
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The subject of storm chaser traffic problems is a perceived "menace" popular in news articles - always made to sound worse than it actually is, and cited to happen many times more than it actually does. Even so, it's true that chaser traffic can, on rare occasion, create a problem. So, I thought it might be helpful to approach this in an objective manner. In this article, I'll attempt to develop a method to both quantify and predict the impacts from chaser traffic. With this in mind, I propose the Chaser Traffic Index or CTI.

The CTI (Chaser Traffic Index) Scale

    Value     Number of Chasers; ImpactIncidence
CTI-0No other chasers encountered. No impacts.Routine
CTI-1Less than 20 chasers encountered. No impacts.
CTI-2Less than 40 chasers encountered. Short waits to re-enter traffic. Some pull-offs taken. No impact to EMS/LEO.
CTI-3Less than 60 chasers encountered. Waits of 15 to 30 seconds to re-enter traffic. Most pull-offs taken. No impact to EMS/LEO.
CTI-4More than 60 chasers encountered. Lines of vehicles at least 1/2 mile long. Waits of more than 30 seconds to re-enter traffic. All pull-offs taken. Minor jams on secondary dirt roads from parked vehicles. Minor impact to EMS/LEO.2 to 4 events/year
CTI-5More than 100 chasers encountered. Lines of vehicles at least 1 mile long. Waits of more than 1 minute to re-enter traffic. All pull-offs taken. EMS/LEO slowed by having to pass multiple vehicles. A few moderate jams on secondary dirt roads from parked vehicles.Once every 1 to 2 years
CTI-6More than 150 chasers encountered. Lines of vehicles at least 2 miles long. Waits of more than 2 minutes to re-enter traffic. All pull-offs taken. A few secondary dirt roads blocked by parked vehicles. Delays of more than 2 minutes at stop signs, towns or traffic lights. Ability to keep up with storm slightly impacted. EMS/LEO response time slowed by more than 5 minutes.Once every 2 to 3 years
CTI-7More than 200 chasers encountered. Endless lines of chasers. Waits of more than 4 minutes to re-enter traffic. All pull-offs taken. Delays of more than 5 minutes at stop signs, towns or traffic lights. Many secondary dirt roads blocked by parked or stuck vehicles. Ability to keep up with storm moderately impacted. EMS/LEO response time doubled.Once every 5 years or more
CTI-8More than 300 chasers encountered. Endless lines of chasers. Unable to re-enter traffic without someone slowing to form a gap out of courtesy. All pull-offs taken. Delays of more than 10 minutes at stop signs, towns or traffic lights. Ability to keep up with storm severely impacted. Most secondary dirt roads blocked by parked or stuck vehicles. EMS/LEO response time tripled.Once every 10 years or more
CTI-9Countless chaser vehicles. Endless lines of chasers. Gridlock starting at first tornadogenesis. Chasing impossible less than 30 minutes after storm initiation. All secondary dirt roads blocked by parked or stuck vehicles. EMS/LEO cannot reach storm victims.Theoretical, no known cases
CTI-10Endless lines of chasers. Gridlock starting at storm initiation. Chasing impossible. All secondary dirt roads blocked. EMS/LEO cannot reach storm victims.

Forecast CTI formula and calculation

We know the factors that are directly related to chaser traffic: time of year, day of the week, level of tornado risk and proximity to a large population center. Higher chaser/"curious local resident" numbers result from one or more of the following:

  • Event occurring on a weekend or holiday
  • Event occuring close to/during peak tornado season
  • Close proximity to a major city*
  • In the Southern Plains region
  • Less than 3 dominant supercell thunderstorms
  • High-end SPC categorical risk
  • Event forecast days in advance
*In a major metro area, local residents can make up more than 75% of chaser traffic

Taking all of these factors into account, a proposed formula for calculating expected CTI would be:

(M+C+R+S+D+L)/T = CTI

  • Where (M) is the Month value:
    • July-February = -1
    • March = 0
    • April = 1
    • May = 2
    • June = 1
       
  • Where (C) is proximity to a city:
    • Within 50 miles of major city = 2
    • Within 100 miles of a major city = 1
    • More than 100 miles from a major city = 0
       
  • Where (R) is the geographic region:
    • Southern Plains (northern Texas, Oklahoma, southern Kansas) = 2
    • Central Plains (northern Kansas, Nebraska) = 1
    • Northern Plains (Dakotas) = 0
    • Midwest, South, Atlantic Coast = -1
       
  • Where (S) is the SPC categorical risk:
    • General, 2% tornado = -1
    • Slight, <=5% tornado = 0
    • Slight, 10% tornado = 1
    • Enhanced, 10% tornado = 1
    • Moderate, 15% tornado = 2
    • Moderate, <=10% tornado = 2
    • High, <=15% tornado = 2 (wind event)
    • High, >=30% tornado = 3
       
  • Where (D) is the day of the week:
    • Weekday = 0
    • Weekend or Holiday = 1
       
  • Where (L) is the forecast event lead time:
    • Evident 2 or more days out = 1
    • Evident 1 day out = 0
    • Not evident until day of event = -1
       
  • Where (T) is the number of dominant, mature tornadic supercells within the risk area.
The maximum theoretical CTI would then be 11, which would occur:
  • on a weekend or holiday
  • in May
  • inside of a SPC high risk
  • close to a major city
  • in the southern Plains
  • one dominant supercell
  • Event forecast 2 or more days out
The minimum would be zero (even if the calculated CTI was below zero).

Method for dynamically calculating CTI

Some form of CTI could be dynamically derived using STP (significant tornado parameter), SCP (supercell composite parameter), simulated reflectivity and proximity to major metro areas. This needs some refining, but a possible formula might be:

((STP+SCP)/T) * 50/DIST = CTI

  • STP = significant tornado parameter for the given point
  • SCP = supercell composite parameter for the given point
  • DIST = distance of the given point from the geographic center of Oklahoma City, Kansas City, Dallas/Fort Worth or Wichita
  • T = total number of dominant supercell storms region-wide

It might be possible to incorporate the other factors in the original formula into the derived one to improve accuracy, but I haven't made it that far.

Observed CTI of past storm chasing events

Now that we have a way to quantify chaser traffic, we can classify past events and see what the true impacts of chaser traffic are in the Great Plains. That is, that the more serious instances are rare:

Observed Storm Chaser Traffic
    Event     CTI
May 19, 2010 - C OklahomaCTI-7
April 14, 2012 - C KansasCTI-6
May 21, 2014 - E ColoradoCTI-5
May 16, 2015 - S OklahomaCTI-5
May 10, 2010 - OklahomaCTI-4
April 23, 2007 - KansasCTI-4
May 5, 2007 - KansasCTI-3
June 12, 2005 - TexaCTI-3
June 12, 2004 - KansasCTI-3
May 4, 2007 - KansasCTI-3
April 14, 2012 - NW OklahomaCTI-3
May 31, 2013 - C OklahomaCTI-3
May 12, 2004 - KansasCTI-2
June 9, 2005 - KansasCTI-2
May 18, 2013 - C KansasCTI-2
May 28, 2013 - C KansasCTI-2
April 13, 2012 - SW OklahomaCTI-2
April 12, 2012 - NW KansasCTI-1
April 22, 2011 - MissouriCTI-1
May 29, 2004 - OklahomaCTI-1
June 11, 2004 - IowaCTI-1
April 19, 2011 - IllinoisCTI-0

The average CTI for a typical Great Plains event during peak tornado season ranges from 2 to 4. A CTI-5 event occurs on average once a year, with a CTI-6 happening roughly once every 3 years. In the history of storm chasing, to my knowledge there has only been one event that would classify as a CTI-7 - the May 19, 2010 event northwest of Oklahoma City.

Visual examples

Storm Chasers in Bennington, KS
CTI-2: Bennington, Kansas - May 28, 2013 [ Video clip ]

Storm Chasers near Vici, OK
CTI-4: Vici, Oklahoma - May 11, 2010 [ Larger Image ]

Storm Chasers in Deer Trail, Colorado
CTI-5: Deer Trail, Colorado - May 21, 2015 [ Video clip ]

CTI-7 to CTI-8: Hennessey, OK - May 19, 2010: This event was the worst case of chaser traffic ever documented, and the only incidence of a CTI-7 or CTI-8 in storm chasing history. A combination of VORTEX2 research vehicles, the active Discovery Storm Chasers TV show, a High Risk tornado event near Oklahoma City in mid-May, and a single tornadic supercell combined to concentrate high chaser numbers in a very small area. Link 1, Link 2, Link 3

I'd be interested to hear how you would rank chaser traffic for specific events using this scale, and if you have any suggestions on improving the formula. I'd also appreciate links to more visual examples (photos and videos) that you may have. Please feel free to post in the comments.

Related Posts:
Video evidence refutes claims of storm chaser traffic problems >
Call-out: stop exaggerating storm chaser traffic with Spotter Network screen grabs >

The following comments were posted before this site switched to a new comment system on August 27, 2016:

Love it! Well done Dan!I hope to see this system adopted by traffic flow researchers worldwide! :-) A thought: Perhaps similar methodology could be used to create a "DSPPGI" (Drive South Public Panic Gridlock Index)? :-)
- Posted by Derek

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