This has been a very bad year for storm chasers. In fact, since I first started chasing in 1997, this will be first year I didn’t chase a single day in the month of May. So, how bad has it been? Here’s a few interesting statistics…
So far, there have been no tornadoes in Oklahoma in the month of May. Nada. The previous lowest May tornado count for Oklahoma occurred in 1988, with two reported tornadoes. 1988 is not a year storm chasers like to talk about.
On the bright side, there have been no tornado deaths in the entire continental United States in the months of April and May. This is extremely unusual.
The May tornado count for the entire country thus far is 114, the eighth lowest number since records have been kept (starting in 1953). With a severe weather event expected in the Texas Panhandle tomorrow, this number could increase a little bit. However, it would take at least 19 tornadoes to pull May 2005 out of the bottom ten.
Furthermore, of those 114 tornadoes, I can only think of two events that would be considered “significant” - that is, having produced F1 or greater damage on the Fujita Tornado Intensity Scale - one of which was perhaps the only true “photogenic” tornado in the month of May, as well. It’s very likely that many of the total 114 tornadoes were nothing more than “dust whirls” and false reports.
So, what happened? A combination of factors are responsible for the slow severe weather season, but an atmospheric blocking pattern that favors entrenched upper-level low pressure over northeast Canada is the primary culprit. Cyclonic flow around the back side of this low has continuously brought dry and cool Canadian airmasses into the central United States - scouring low level moisture and upsetting return flow from the Gulf of Mexico. Sea surface temperatures in the Gulf are actually running a few degrees below normal, courtesy of these persistent invasions of continental air. So, when return flow does set up over the Great Plains, moisture quality remains poor, resulting in high-based thunderstorms that have little chance of producing tornadoes, no matter how much shear, instability, and lift are otherwise present. Of course, low sea surface temperatures in the Gulf suggest a slow start to the Atlantic hurricane season. Following an active 2004 hurricane season, I guess that’s something to be potentially thankful for.
Further complicating matters in the latter half of May has been moderate upper level ridging and persistent northwest flow over the Plains. Whereas this ridging has led to subsidence (sinking air) and warm, dry weather - especially over Texas and Oklahoma - northwest flow makes for poor shear profiles when storms manage to develop. Meanwhile, low pressure over the Gulf of Mexico means that already poor return flow is routed over the southeastern United States before reaching the Plains. So, no juice, no shear, and little focus for lift makes for nothing much worth chasing.
Will June provide redemption? Obviously, it’s too early to say. Blocking patterns have plagued North America for many months now (remember California’s wet Winter?), and as Summer approaches, all it would take is one good ridge to finally shut things down for the Southern and Central Plains. The climatological peak of severe weather season for the Northern Plains has yet to arrive, and with a continued active northern branch of the jet stream, it’s possible the season could still pay dividends in Nebraska, South Dakota, North Dakota, Minnesota, and the Canadian Prairie Provinces in late June and July. A pattern shift is ongoing at the moment, expected to bring southwest flow and better quality moisture to the Southern and Central Plains for the first week of June; however, this new pattern is unusually progressive in nature for this time of year, so there’s a high likelihood it won’t last long. And, even though the quality of moisture is improving, it’s still not all that great. Still, a decent event or two is possible from Texas to Nebraska in the next week or so.
A slow May often gives way to an active June - but not always. The atmospheric patterns that have made for a slow May this year have proved particularly difficult to budge, so guarded optimism is the most I can muster for the time being.
