Guest post by Paul Homewood
It does not seem long since the February outbreak of tornadoes had Joe Romm noting :-
The unexpectedly fierce and fast tornado outbreak so early in the season has folks asking again about a possible link to climate change.
And talk in some quarters of another year of extremes.
So how has the tornado season developed thus far? In fact, the data tells us it has been remarkably quiet.
Figure 1, from NOAA’s Storm Prediction Centre,(SPC), shows there have been 871 preliminary tornado reports so far this year, compared to an average of 1118 at this date between 2005-2011.
(It is worth spending a bit of time explaining how the SPC calculate their tornado statistics. In their own words :-
Tornado data usually reaches SPC first from local storm reports (LSRs), warnings or other bulletins sent by local NWS forecast offices. Such reports are usually sent within the first day or two after a severe weather event, before all the information on a tornado is known. In fact, some tornado information might not be known for many weeks or months–for example, if someone who was injured dies from his injuries a long time afterward. That is why we call all tornado data “preliminary” until the National Climatic Data Center publication Storm Data is completed. Storm Data contains the “final” information on all severe weather events.
They go on to explain how they survey damage and thereby determine the Enhanced Fujita Scale.
This varies from place to place; and there are no rigid criteria. The responsibility for damage survey decisions at each NWS office usually falls on the Warning-Coordination Meteorologist (WCM) and/or the Meteorologist in Charge (MIC). Budget constraints keep every tornado path from having a direct ground survey by NWS personnel; so spotter, chaser and news accounts may be used to rate relatively weak, remote or brief tornadoes. Killer tornadoes, those striking densely populated areas, or those generating reports of exceptional damage are given highest priority for ground surveys. Most ground surveys involve the WCM and/or forecasters not having shift responsibility the day of the survey. For outbreaks and unusually destructive events–usually only a few times a year–the NWS may support involvement by highly experienced damage survey experts and wind engineers from elsewhere in the country. Aerial surveys are expensive and usually reserved for tornado events with multiple casualties and/or massive degrees of damage. Sometimes, local NWS offices may have a cooperative agreement with local media or police to use their helicopters during surveys.
In practice, this all means that it takes about three months before tornado data can be finalised and accurate statistics produced.)
So back to Figure 1! To assure a comparison of like with like, the preliminary tornado figure for this year is compared with the preliminary numbers for previous years at the same date, and not actual numbers. (Actual numbers normally come out at between 65% and 85% of preliminary figures – this is partly due to the same tornado being spotted twice, and partly due to storms being wrongly identified as a tornado). It is clear, then, that after the early season outbreaks in February and March, the last three months have been remarkably quiet.
Figure 2 shows the same information as Figure 1, but compares with each year since 2005 separately. Only 2005 has recorded a lower number by the end of June.
Figure 3 is slightly different, as it attempts to compare with a baseline of 1954-2011. Direct comparisons are meaningless, as in the SPC’s own words – “The increase in tornado reports over the last 54 years is almost entirely due to secular trends such as population increase, increased tornado awareness, and more robust and advanced reporting networks.”. To get around this problem, tornado numbers in the earlier part of the record have been “adjusted” upwards. Also this years figure of 740 is adjusted to 85% of the preliminary LSR’s, to allow for double counting etc.
Again it appears that this year’s numbers are low by comparison, close to the 25th percentile.
June Tornado Events
According to data from the Storm Prediction Center, the count of preliminary tornado reports during June — 114 — was much below the 1991-2010 average of 243. June is typically one of the most active tornado months during the year. In terms of the number of tornadoes, this marks the least active June since 2002, when 97 tornadoes were confirmed. Once the final tornado count is confirmed, it is likely the June 2012 count will be revised lower. There were four tornado-related fatalities reported during the month — three due to a single EF-2 tornado in Missouri on June 4th and a single fatality from a tornado spawned by Tropical Storm Debby on June 24th in Florida.
A storm system moving through the Mid-Atlantic on June 1st, spawned numerous severe thunderstorms from western Pennsylvania, through the Washington D.C. metro area, and into southeastern Virginia. There were 28 preliminary tornado reports, with 12 tornadoes confirmed in and around Washington and Baltimore. The tornadoes were weak in nature, rated as EF-0 and EF-1, causing only minimal damage. The largest impact from the storms was flash flooding, with 2-4 inches of rainfall being observed. No fatalities were reported with the severe weather outbreak.
Confirmed Numbers for February
The SPC have now confirmed their figures for January and February. F1+ tornadoes totalled 78, compared to a baseline average from 1970-2011 of 33. Figure 4 shows how this year stacks up against earlier decades for tornadoes during January and February. ( Changes in recording practices, such as Doppler radar, make comparisons of the weaker F0 tornadoes with years before 1998 meaningless – for more information read here.)
While this year had a busier start than usual, this is not uncommon. Since 1970, four years have experienced more early season tornadoes during January and February.
Next month, we will take a closer look at the March, and hopefully April numbers. In the meantime, let us hope the summer remains quiet.
1) Detailed information is available from the Storm Prediction Centre.