Has the cold suppressed severe storms?
Posted April 1, 2013
The title is a question we've received recently, given that we've gotten through March and into the start of April without much in the way of severe watches and warnings in central NC, which often begin to ramp up a little as we head this far into Spring.
As is often the case with any broad weather question of that sort, the answer involves some subtleties and isn't as straightforward as we might like. Nonetheless, it has been the case that during the later February and March period, that the persistent, or at least unusually frequent, presence of a broad upper trough over the central and eastern United States has both kept stronger storms systems moving along a rather southerly track and helped to keep the air over a good part of the country too stable to support numerous severe weather outbreaks.
Offsetting that effect somewhat, however, is the fact that we began the year on the warm side of normal for January, before running not too far off normal in most spots for February overall, and then turned notably cooler than average for March. The first three images here are national temperature anomaly maps from the Climate Prediction Center, showing the departure from normal for March (lots of blue on that one!). February (more subtle departures from normal) and January (lots of red and orange there, given the mild nature of the month).
I also included a couple of daily severe weather report maps from the Storm Prediction Center, just to emphasize that although severe outbreaks for NC have been minimal so far this year, there have been some notable ones, especially to our south and west. On these maps, the green dots indicate severe hail reports, the blue dots are damaging straight-line winds and the red dots denote tornadoes. Most of the activity of that sort this year has been concentrated toward the Gulf Coast, parts of TX, and just a couple of sporadic outbreaks farther north over the southeast and Tennessee Valley.
The final two images here are focused on tornado activity for the U.S. so far this year, with the first graph showing how 2013 compares to the previous eight years, along with a plot of the average for those years. On this graph, 2008 stands out as a year that got off to an active start right from the beginning of the year, while 2011 ended up producing almost as many tornadoes, but really was pretty quiet up until April, when the numbers shot up from just a couple hundred to over a thousand in the space of three weeks (this of course includes the major outbreak that brought 30 tornadoes to our state on April 16th that year). It is worth noting that this graph is based on "preliminary" tornado reports, which usually represent a notable overestimate of the actual number. However, it still allows a reasonable year-to-year activity comparison. For 2013 so far, you'll notice the big jump in late January and then only small additions to the numbers since then, resulting in a total as of March 30th of 149, which is third least of the eight years shown, and notably below the average of 250 this far into the year.
A different look at the data is shown in the final image. This is a graph from the Storm Prediction Center that looks similar to the previous one, but shows a line for 2013 that is "inflation-adjusted" to attempt to make it closer to what the final count will be. This number (127 tornadoes through March 30th) is plotted against curves showing the statistical spread of tornadoes for the years 1954-2007. The "50th percentile" curve in green indicates the long-term average number of tornadoes, which is 146 through March 30th. This shows that tornado activity this year isn't all that far below the long-term average, thanks to that quick start in late January.
After a mild day to start the week, we'll spend the rest of this work week in yet another cooler-than-normal airmass, but longer-range indications are finally showing a decent trend toward normal or somewhat above normal temperatures next week and farther into April. If this holds true, we may well see the likelihood and frequency of severe weather outbreaks increase accordingly.