The largest tornado outbreaks of 2015

Post last updated on May 27, 2015.

In an effort to better track outbreaks every year, we’re going to create maps of them following each large event. For now, we’ll classify large events as those that reach 25 or more filtered reports in the Storm Prediction Center database. 25 is a sizable number of tornado reports, and it leaves some room for decrease in final tallies, which happens plenty.

For multiple days to be grouped together as an outbreak, they must have no greater than 12 hours between reports/touchdowns, and be associated with the same storm system. With this methodology, it is possible one system could produce more than one outbreak. Something we’ll evaluate going forward.

Though it should, tornado outbreak generally has no fully agreed upon definition. As such, this is an idea in progress, and we’re certainly eager to hear challenges on the “rules” if it can build a better picture. Once official tornado tracks for 2015 are available, at the beginning of next year, we will replace reports with actual tracks in the maps below and to come. It’s possible we’ll redefine or remove events in final processing as well.

Anyway, let’s get to it.

So far in 2015, there have been six larger events fitting the criteria above. We’ve also included a map for one event that fell just short, as well as brief info on a few others in that zone.

Large tornado outbreaks of 2015

Events with at least 25 filtered tornado reports

April 18-19, 2015 | 33 reports, 38 warnings, 7 watches
April 18 SPC event page | April 19 SPC event page

Tornadoes also occurred prior to and after these days, but there were only a handful reported and on either side of long breaks. The tornadoes were the result of a slow moving upper-level low.

. . .

May 6, 2015 | 59 reports, 102 warnings, 5 watches
SPC event page

This was the first big volley amidst an active stretch. About 5-10 tornadoes occurred in the day before and after. And after that. Then the next outbreak came along…

. . .

May 9-10, 2015 | 87 reports, 117 warnings, 10 watches
May 9 SPC event page | May 10 SPC event page

A classic Plains severe weather situation with a big upper-level trough ejection from the Rockies. A lack of cap meant lots of rogue convection, which probably held this one back from top-end stuff. The tornadoes the day prior were caused by the same general system, but there was a 20-hour break between events.

. . .

May 16, 2015 | 50 reports, 68 warnings, 9 watches
SPC event page

Another in a series of big mid-level troughs during peak season. Like the May 9 portion of the prior outbreak, this one had some setbacks. But, also as with recent times, a number of notable tornadoes formed on the edges of areas stabilized by daytime precipitation. A particularly long-lasting tornado passed from near Elmer, Oklahoma to near Tipton, Oklahoma. We’ve included a few reports and warnings after midnight to capture in entirety, but the majority of the event occurred on May 16.

. . .

May 19, 2015 | 29 reports, 42 warnings, 3 watches
SPC event page

Red River, again. This one can easily be considered an overperformer, and it was a very tricky forecast. The case of 2015: morning convection displaced the main tornado zone, but tornadoes still dropped aplenty. This one had a relatively small footprint, though there were confirmed tornadoes over a large area despite the focus on north Texas and south Oklahoma.

. . .

May 25, 2015 | 28 reports, 87 warnings, 7 watches
SPC event page

While this wasn’t a classic tornado outbreak by any means, it certainly fits the bill overall. A large squall line raced across the Southern Plains with embedded tornadic circulations that caused numerous tornadoes. Supercells also formed out ahead of it in spots. Flooding was a big issue once again, as has been the case throughout the past few months in the region.

. . .

June 21-22, 2015 | 30 reports, 43 warnings, 4 watches
May June 21 SPC event page | June 22 SPC event page

This event probably looks like two events, and it’s possible it was. But tornadoes on both days were both spawned by the same system and it came within the 12 hour window for consecutive tornadoes. The focus was across Illinois where nine tornadoes dropped in the Chicago NWS region.

. . .

June 27, 2015 | 28 reports, 24 warnings, 1 watch
SPC event page

Northwest flow doesn’t cause big tornado events in many places, but it does in the northern Plains during mid-summer. At least 19 tornadoes were confirmed mainly across North Dakota. This was mainly two or three supercells that became tornado machines. An event like this may get dropped from the top in final analysis, though in a year like 2015 it certainly deserves mention.

. . .

November 16, 2015 | 47 reports, 62 warnings, 7 watches
SPC event page

Another event that might not make the cut in your average year based on numbers, it was big in 2015 and for the region it impacted considering the time of year. With three EF3s, it ranks up there with the top days of the year for the most significant tornadoes. Final tornado tallies appear set to settle somewhere in the 20-30 range. A separate smaller outbreak occurred in Mississippi overnight November 17-18, from the same system but with enough time between to count them as two events.

Close, and worth mention

Info on events that fell just short

April 9, 2015 | 24 reports, 45 warnings, 6 watches
SPC event page

Tornadoes occurred the day prior from the same system, but it was a relatively modest event by recent standards, and there was a lengthy break. The first violent tornado of the year occurred on this day, an EF4 that passed through Rochelle/Fairdale, Illinois.

Other days

Both April 24 and April 26 fell just shy of the 25 filtered reports mark, with 23 each. March 25 has only 9 filtered reports, but brought the first tornado fatality of the year. All other fatality-causing events are shown above in maps. There have been five killer tornadoes this year.

May 24 reached the 25+ filtered reports on SPC “rule,” but it is a reason to perhaps not always follow the rule hard and fast. About 25 reports came from one storm alone, which as of this time only has two confirmed tornadoes. May 24 and May 25 also had less than a 12 hour break between tornadoes, which we’ve used to classify other multi-day outbreak above, however they were not really from the same impulse and are likely separate events. The messy job of classifying outbreaks…

Outbreaks ranked

The top events from high to low based on filtered reports

#1: May 9-10, 87 tornado reports
#2: May 6, 59 tornado reports
#3: May 16, 50 tornado reports
#4: November 16, 42 tornado reports
#5: April 18-19, 33 tornado reports
#6: May 19, 29 tornado reports
#7: May 25, 31 tornado reports

Katie Wheatley and Ian Livingston are developing this process and will post the updates for 2015 here as they occur. This post was last updated on November 20 based on data as of November 20.