Low Tornado Season In 2018 So Far, Could Change

By Earth Networks Greta Easthom

Low Tornado Count So Far in 2018

Steven Weiss has been predicting where tornadoes will spin up since the National Oceanic and Atmospheric Administration’s Storm Prediction Center (SPC) used teletype and carbon paper to send out weather watches.  Whether it was the deadly EF-5 2007 tornado in Greensburg, Kan., or the EF-4 1987 Saragosa, Texas, the climatological warm season for tornadoes has always kept Weiss on his toes. But as he prepared to retire after 44 years at SPC, he found this year to be “the quietest in any during his career.”

Tornadoes usually make their grand entrance in March. By now, these strong storms typically tear up the south-central U.S. but this season never quite touched down. This May was the first since 1970 with fewer than 100 tornadoes observed. As of June 27, only 571 tornadoes have been reported, far below the ten-year average of 906 tornadoes. And at this point, it is unlikely we will add significantly to the total for 2018.

“I’ve learned to never say never, but it is very unlikely. The bulk of a year’s tornadoes tends to occur before mid-June or after mid-October,” said Dr. Patrick Marsh, Warning Coordination Meteorologist for NOAA’s SPC. This is because the convective ingredients, which include:

• Moisture
• Instability
• Lift
• Wind shear

are most favorable for storm formation in “tornado alley” in the south-central U.S. and usually peak between March and June. “Tornado reports are running below normal, with tornado reports about as low as we’ve seen before.”

Storm Chasing Tornadoes: What to Look for

For Earth Networks Meteorologist and storm-chaser Mark Ellinwood, there are a couple of key factors he looks for on a storm-chasing day: “Southerly to southeasterly winds at the surface, winds becoming westerly by the time you climb up to 500 mb, [halfway up in the atmosphere, where fast-moving winds influence surface weather], decent wind speeds in the low levels (10-15 mph minimum generally) and good-low level moisture.” The southerly winds feed in moisture typically from the Gulf of Mexico, while the wind speeds provide enough rotation, or what meteorologists call shear, for the storm system to spin.

In addition, the cloud bases need to be low enough for the storms to touch down. We need two things for this to happen. The first is surfaces temperatures lower than 85 degrees. The second is a capping inversion to prevent clouds from forming before the atmosphere can build up “plenty of instability,” according to Ellinwood. Clouds typically form when warm, less-dense surface air can rise then cool and condense. A capping inversion occurs when warmer-than-the-surface temperatures aloft trap surface air and prevent it from rising. The warmer temperatures are like a lid on a pot: Blocking air from rising and cooking up convection.

Current Conditions Resulting in Low Tornado Count

We have a low tornado count this season because we didn’t get moisture-laden, warm, air masses.“The large-scale pattern across the United States through the end of April favored cold-air intrusions across most areas east of the Rocky Mountains,” said Marsh. “These air-masses, originating in Canada, tend to be very dry in addition to being cold.”

Thus, the cyclonic, clockwise, rotation across the Plains heated the dry air mass quickly, resulting “in conditions more favorable for mega wildfires than severe thunderstorms and tornadoes,” according to Marsh.  To further add fuel to the fire, this April was “below normal in terms of CAPE,” according to Weiss. CAPE stands for convective available potential energy and is a measure of the strength storms can achieve.

Once warm, moist air finally meandered its way into the central U.S. in May, the wind shear was missing. Wind shear is necessary for storms to turn into monster systems capable of producing tornadoes. “Typically wind shear tends to be maximized, or at least sufficient for severe weather, across the Southern Plains during May. However, this year the winds aloft were quite calm,” said Marsh.

La Niña Plays a Part

There are upper-air patterns at play to explain why the winds were so weak. La Niña, can enhance thermally driven westerly winds across the U.S. But La Niña, like the weak one this spring, can be a “bit temperamental,” says Ellinwood.

For instance, “the lingering La Niña effects in April 2011, promoted more Dragon (low-pressure) over the western and central U.S. and ridging (high pressure) along the East Coast,” says Ellinwood. This meant Gulf of Mexico moisture had an accessible channel straight into the heart of the “Dixie Alley” (a region of the southern Plains to lower Mississippi Valley) and combined with strong mid-to-upper level winds, to become the “most active April on record,” according to Ellinwood.

Out of 1,584 total tornadoes for 2011,758 occurred that April and there were 361 reported fatalities in that month alone. One particularly, potent day—the April 27, 2011, ‘Super Outbreak’—spawned four intensely destructive EF-5 and EF-4 tornadoes across Alabama, Georgia, Kentucky, Mississippi and Tennessee.  This April, “the La Niña event was weak to borderline moderate, so the stronger mid-to-upper level winds faded more quickly and did not remain across the southern U.S. long enough to take advantage of the Gulf moisture,” diminishing the tornado chances.

“We Can’t Let Our Guard Down”

As such, teleconnections like La Niña can only give us so much of the picture.  “Climatology provides the background and then we have to assess the threats,” said Weiss.  Although tornadoes didn’t spin up as often this year, “wind reports are running near normal thanks to several big wind events in the East,” said Marsh. The SPC takes every threat seriously whether it’s in a populated area such as eastern Mississippi or in an open area in the Plains. However, we still can’t quite let our guard down for the fall and next couple of years.

“Tornadoes are such rare events to begin with that it is very hard to attribute a climatological reasoning to their fluctuations,” said Marsh, “However, if we continue to have several years below normal activity, then it might become easier to pin down the climatological factors contributing to the reduction in tornadoes.”