Published: 2013-05-15 07:56:00
Updated: 2013-10-27 12:32:44
Posted May 15, 2013
Updated October 27, 2013
By Mike Moss
Back around the beginning of April, we checked in here on what had proven to be an unusually quiet severe weather season to that point.
There were fairly frequent episodes in which an upper trough set in across the eastern United States, keeping temperatures, especially maximum temperatures, on the short side of normal. Along with that, the portions of those troughs that would be most favorable for possible severe storms were often east of the U.S.
The relatively cool temperatures associated with the troughs also held down instability, so that many areas saw little thunderstorm activity, and where it did form it was most often routine in nature.
Taking another look now that we're a month and a half farther along in the year shows that the overall trend has held up for the most part through April and the first part of May. While there has been some variability about the weather over the central and eastern U.S., we've continued to see a frequent lack of heat, humidity and instability, resulting in fairly low numbers of severe storms and especially tornadoes.
The first image you see here shows an estimate of tornadoes so far this year from the Storm Prediction Center denoted by the black line, showing a sizable outbreak back in January, a very quiet period after that, and a stretch of modest activity in early to mid-April, all leading to a total of around 211 twisters as of this past Monday.
The other lines on the graph include the most and least active years on record in red and pink, and 75th, 50th (average) and 25th percentile curves in gold, green and blue. The average number by this time of year would be about 431, over twice as many as we've seen so far this season. Judging roughly by the position of the black line between the minimum and the blue 25th percentile line, this year's number at this point is probably among the lowest 10 percent or so in history.
Here's hoping the pattern remains reasonably tranquil and doesn't make up too much ground as we continue along from now on into June. It does appear that in the short-term some very warm, moist air over the nation's mid-section may be acted on by an upper level trough developing into the Rockies and Plains this weekend and early next week, so we could very well be hearing about some tornadoes in the plains states and western Mississippi Valley Saturday into Monday.
The second image above shows an outlook for possible areas of severe weather, stepping eastward somewhat on each of those days.