How many Tornados, to consider be called an "Outbreak" ?

[Chad Berryhill]

Sorry all, but I have to ask. How many tornados do you need to have an Outbreak? 5...10...15.

I did a quick search for a post about this, if I missed it sorry.

I guess that's like asking how many cows do you need to have a stampede...lol

Thanks to anyone that can answer this post

 

[Chad Berryhill]

My apologies for the misspelling on the title.

 

[Tom Dulong]

Here is the definition from the American Meteorological Society (AMS)Glossary:

<TABLE><TBODY><TR><TD vAlign=top>
</TD><TD>tornado outbreak—Multiple tornado occurrences associated with a particular synoptic-scale system. <DD>In recent years, Galway (1977) has defined ten or more tornadoes as constituting an outbreak.
Galway, J. G., 1977: Some climatological aspects of tornado outbreaks. Mon. Wea. Rev., 105, 477–484. </DD></TD></TR></TBODY></TABLE>

Wikipedia provides the definition of 6 or more tornadoes on one day in a certain geographical region.

 

[Glenn Dixon]

10 or more if you missed them all

2 or more if you bagged 'em

 

[Mike Johnston]

Here is the definition from the American Meteorological Society (AMS)Glossary:

<TABLE><TBODY><TR><TD vAlign=top>

</TD><TD>tornado outbreak—Multiple tornado occurrences associated with a particular synoptic-scale system. <DD>In recent years, Galway (1977) has defined ten or more tornadoes as constituting an outbreak.
Galway, J. G., 1977: Some climatological aspects of tornado outbreaks. Mon. Wea. Rev., 105, 477–484.
</DD></TD></TR></TBODY></TABLE>

Wikipedia provides the definition of 6 or more tornadoes on one day in a certain geographical region.

Another source might be from the SPC's "Probabilistic to Categorical Outlook Conversion Table", which states, in part:

"A HIGH risk area suggests a major severe weather outbreak is expected, with a high concentration of severe weather reports and an enhanced likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events occurring across a large area). In a high risk, the potential exists for 20 or more tornadoes, some possibly F2 or stronger,...."

See http://www.spc.noaa.gov/misc/SPC_Prob_Conv_Otlk_Change_20060214.html

I suppose, then, the difference between an outbreak and a "major" outbreak would be from 10 to 20.

 

[Jeff Snyder]

It's very difficult to define a tornado outbreak explicity since one can define "outbreak" in terms of several quantities such as intensity, number of strong/violent tornadoes, number of tornadoes, areal concentration of tornadoes, etc. For example, one may say that 15 tornadoes is an "outbreak", but what if those 20 tornadoes occur in several different areas of the U.S. - a handful in FL, a few in TX, and a handful in Wisconsin. Is that an outbreak? What if we see 8 tornadoes, with a couple >F2, in southwestern OK over a 4- hour period? So, you probably want to look at the number of tornadoes and the concentration of those tornadoes.

I suggest the following if you are interested in this:
Edwards, R., R.L. Thompson, K.C. Crosbie, J.A. Hart, and C. A. Doswell III, 2004: Proposals for Modernizing the Definitions of Tornado and Severe Thunderstorm Outbreaks. Preprints, 22nd Conf. Severe Local Storms, Hyannis MA.

Tornado outbreak days: an updated and expanded climatology (1875-2003) - Schneider, Schaefer, and Brooks.

 

[Ed Schoenborn]

My understanding is that if the multiple tornadoes are in the same storm that is a FAMILY. If they are in different storms that is an outbreak.

 

[STurner]

I myself personally like to see at least 3 to 4 dozen twisters over a period of less than 24 hours before with several rated EF3+ before calling it a major tornado outbreak. Even though I feel this way some small tornado outbreaks have been just as violent as large tornado outbreaks. The recent EF4 tornado that hit Lone Grove was only one of the dozen tornadoes confirmed that day. I dont know a lot but that day around the Ardmore area seemed very dynamic for violent tornadoes. A small tornado outbreak to me would be at least 6-12 twisters confirmed and at least on significant EF2+ tornado. These are all just my opinions though.