Mark L. Kramer
NYC/LI Chapter of the AMS
April 6, 2011
Posted: 06 Apr 2011 11:52 AM PDT
A storm system that stretched from the Mississippi River to the mid-Atlantic states on Monday brought more than 1,300 reports of severe weather in a 24-hour period, including 43 tornado events.
The extraordinary number of reports testifies to the intensity of the storm, which has been blamed for at least nine deaths, but it also reflects changes in the way severe weather is reported to–and recorded by–the Storm Prediction Center. As pointed out by Accuweather’s Brian Edwards, the SPC recently removed filters that previously prevented multiple reports of the same event that occurred within 15 miles of each other. Essentially, all reports are now accepted, regardless of their proximity to other reports. Additionally, with ever-improving technology, there are almost certainly more people sending reports from more locations than ever before (and this is especially true in highly populated areas such as much of the area covered by this storm).
So while the intensity of the storm may not set any records, the reporting of it is one for the books. According to the SPC’s Greg Carbin, Monday’s event was one of the three most reported storms on record, rivaled only by events on May 30, 2004 and April 2, 2006.
Monday’s severe weather map
Accuweather’s comment and SPC investigation
Monday’s Severe Weather: The Largest Outbreak Ever?
By Brian Edwards, Meteorologist
Apr 6, 2011; 12:00 PM ET
Destructive thunderstorms associated with a potent storm system rumbled across the eastern third of the country Monday and Monday night, producing more than 1,300 reports of damage from Texas through Pennsylvania.
Damage reports continue to flurry into the Storm Prediction Center, but the latest numbers show at least 1,377 reports of severe weather between 8 a.m. EST Monday and 8 a.m. EST Tuesday.
With such a large and unprecedented amount of severe weather reports, the question has been raised: Is this the largest single day severe weather outbreak in history?
AccuWeather.com Expert Senior Meteorologist Henry Margusity said that this was the largest number of severe weather reports in a 24-hour time period that he had ever seen.
Looking a little closer at the data, there are several factors that need to be taken into consideration.
First off, in this day and age with the technology that is available, severe weather reports are much easier to receive and report.
In addition, there is the fact that these thunderstorms affected a region of the country with a rather large population. If the same severe weather outbreak occurred in the Plains where the population density is lower, there likely would have been fewer reports overall.
Another factor to consider is that the Storm Prediction Center, which takes all of the storm reports from local offices and combines it into one report, used to use certain filters on the data.
Late last year, the center removed their filters, which did not plot multiple reports of the same event if they were within 15 miles of each other. Therefore, the data this severe weather season is truly raw with every single report being plotted on the map.
In speaking with Meteorologist Greg Carbin of the Storm Prediction Center, he said that “if you add in the filters from a year ago on the preliminary data from Monday, you end up with approximately 850 severe weather reports.”
Only two other events have been observed with more than 850 severe weather reports. Those events occurred on May 30, 2004 and April 2, 2006.
Carbin said, “If you take the numbers literally in terms of overall severe weather reports with the time/space filtering, April 4, 2011 would fall into third place.”
He also stated, “As far as wind reports, it appears that, even with filtering, this most recent event
may have the greatest number of severe wind reports in a single 24-hour period on record.”
However, Carbin mentioned that the final numbers would not be complete for several days, as reports continue to come into the Storm Prediction Center from local offices. Therefore, it is possible that this event ends up being the largest severe weather outbreak ever in terms of the number of reports.
Mark L. Kramer
NYC/LI Chapter of the AMS