How rare are November tornado outbreaks?
Wednesday, November 20, 2013, 12:10 PM
The clean-up continues across the U.S. Midwest and Ohio Valley in the wake of Sunday’s deadly tornado outbreak. Damage surveys are still ongoing, but as of Tuesday afternoon, around 49 tornadoes have been confirmed.
VIOLENT TORNADOES RIP ACROSS U.S. MIDWEST
Two of Sunday’s tornadoes have been given preliminary ratings of EF-4 on the Enhanced Fujita Scale -- one in Washington, Illinois and the other in New Minden, Illinois. These tornadoes are the strongest November tornadoes on record in the state of Illinois. Prior to Sunday, the strongest November tornado recorded in Illinois was a F-3 (Nov. 15, 1988).
The tornado that ripped across Washington, Illinois had a path length of nearly 75 km, with peak winds reaching 300 km/h. Hundreds of homes were destroyed as a result.
According to the National Weather Service in Chicago, since 1986, 194 tornado warnings have issued in Illinois in the month of November. Of those, 101 (or 52%) were issued during Sunday’s outbreak.
In Indiana, preliminary NWS surveys have confirmed 26 tornadoes from Sunday’s outbreak -- the third-highest number of tornadoes in one day in state history.
RARE HIGH RISK ISSUED BY SPC
Ahead of the outbreak, NOAA’s Storm Prediction Center issued a rare High Risk in their Day 1 severe weather outlook. High Risk outlooks are reserved for when a significant severe weather outbreak is expected, and typically only issued a few days per year. Only two other Day 1 High Risks have been issued in the month of November since 1998. Note how much farther south the previous High Risk areas are compared to Sunday’s.
NOAA’s Storm Prediction Center logged an incredible 85 preliminary tornado reports from Sunday’s outbreak, making it the most active tornado day of 2013. Still, the outbreak was well-forecast by the SPC.
In Chicago, the NFL drew criticism from meteorologists for not postponing the Bears-Ravens football game, despite the high confidence in severe weather. The outdoor game went on as scheduled, even after reports were coming in of severe storms to the southwest. Fans were eventually evacuated from their seats after the NWS issued a severe thunderstorm warning specifically mentioning the threat to the stadium.
The game resumed after a two-hour delay. Several tornadoes have been confirmed just south of the city of Chicago.
HOW RARE ARE TORNADO OUTBREAKS?
In the wake of Sunday’s severe weather, many are asking how rare November tornado outbreaks are. The short answer is - they’re rare, but not unheard of, especially in the southern states.
Dangerous tornadoes can occur during any month of the year in the US. Each region has a peak season, with May and June being the most active across the traditional Tornado Alley areas. The upper Midwest and Ohio Valley can see strong tornadoes into late summer (as can many parts of Canada).
To get tornadoes, you need instability (usually driven by warm, humid air from the south), and strong wind shear (driven by the jet stream). Therefore, peak tornado season shifts along with these two ingredients. As the jet stream rides farther north in the summer, so does the tornado potential. During the winter, the jet stream pushes back south.
While much of the U.S. sees a gradual decline in tornado frequency in the fall and winter, the southern Gulf States actually experience a slight increase in frequency. This is sometimes referred to as the “secondary tornado season”.
So while November outbreaks aren't unheard of in the Gulf States, they’re rare in the upper Midwest. The states of Illinois, Indiana, and Ohio average only one tornado per year in the month of November.
Sunday’s outbreak was driven by a surge of warm humid air, an incredibly strong jet stream aloft, and a rapidly strengthening low pressure system at the surface.
SEE ALSO: Major damage in 12 states following U.S. tornado outbreak
WHAT ABOUT CANADA?
The same system that affected the US Midwest also brought severe weather to southern Ontario Sunday night, although to a much lesser extent. By the time the storms tracked in from Michigan, the biggest impacts were wind and rain. No tornado watches or warnings were issued in Canada. (And no reports have been received either.)
November tornado outbreaks are virtually unheard of in Canada. However, an isolated tornado is possible, as evidenced by the Hamilton tornado of Nov 9, 2005. This tornado was only the third to occur in Canada later than November 9 since record-keeping began in the early 1900s. (The other two Ontario tornadoes occurred in Leamington on Nov. 29, 1919 and Exeter on Dec. 12, 1946.)
US TORNADO SEASON IS WINDING DOWN
Despite Sunday’s outbreak and the devastating tornadoes that occurred this spring in Tornado Alley, the 2013 tornado season is still running below average in the US. The preliminary total number of tornadoes is 903, as of November 18. The average for this time of year is 1424.