Tornadoes Are Not All Alike

Not all tornadoes are the same.  There can be very weak tornadoes which produce winds of only around 60 mph, not much stronger than one might fine in a large dust devil.  The strongest tornadoes can produce winds of around 300 mph which are capable of destroying all but specially designed, tornado-proof structures.

The most common tornadoes are nearer to the lower end of this spectrum, with wind speeds ranging between 80 and 120 mph. Such storms are, obviously, capable of doing major damage, but the really violent, top-end tornadoes are truly monstrous by comparison.

Severe thunderstorm winds also vary greatly.  A typical, non-severe thunderstorm might produce brief 20-40 mph wind gusts along its leading gust front.  On occasion, straight line winds from a powerful thunderstorm can reach speeds of more than 120 mph.  Such straight line winds are capable of doing as much damage as the most common tornadoes.

Meteorologist John Wheeler

 

Northern Plains Biggest Outbreak of Tornadoes

Twelve people died from the Fargo tornado on June 20, 1957.  In Fergus Falls, 57 people were killed by twin tornadoes on June 22, 1919.  Thirteen died during the Fridley (Twin Cities) tornado outbreak of May 6, 1965.  But the biggest outbreak of tornadoes on record in this region happened just five years ago on June 17, 2010.  There were 76 tornadoes that day; 22 in North Dakota, 48 in Minnesota, and three each in Iowa and Wisconsin.  The 48 in Minnesota is a record number of tornadoes in a single day.  Wadena, MN, took a direct hit and was heavily damaged. Two were killed and nine injured across the region but the casualty list would have been much higher had the tornadoes formed over more populated territory.  This remains the biggest outbreak of tornadoes on record for the summer months.  Most high-count tornado outbreaks happen in spring.

Meteorologist John Wheeler

 

Peak Tornado Month

June is typically the peak month for tornadoes across our region.  What is it about June weather that causes tornadoes to form?  The classic answer, “Cold air from the north clashes with warm and humid air from the Gulf of Mexico,” is an almost a silly oversimplification.  That happens all the time.  Although some weak tornadoes can form from rather mundane thundershowers, the truly life-threatening tornadoes form as a part of rotating thunderstorms with powerful updrafts.  These supercell thunderstorms are also capable of producing large hail and damaging winds and most supercells do not actually produce a tornado.  The specific mechanism for tornado development is still not well understood, but it occurs in the updraft region and may me triggered by various crosswind interactions.  Supercell storms are usually found in an environment of relatively strong upper level wind blowing stronger than, and at an angle to, surface winds and also where surface air is especially buoyant (unstable).

Meteorologist John Wheeler

Tornadoes and Climate Change

Is our warming climate effect the threat of tornadoes and severe thunderstorms?  From the political sides of this debate, this question can be quickly and easily answered either “Yes!” or “No!” depending on the politics.  In the real world, the answer is a lot trickier.  The trend across the United States in recent decades is a decrease in the overall number of tornado days along with an overall decrease in the quantity of tornadoes.  But coupled with this decrease in bad storms is an increase in those few terrible tornado outbreak days, those days when dozens or even hundreds of tornadoes causing staggering property damage and loss of life.  So although severe storms are not exactly increasing, their variability is.  Some of this statistical clustering is skewed by the year, 2011, when several huge tornado outbreaks swept the Midwest and South.  Ongoing research offers no consensus yet of how tornadoes will be affected by climate change in the future.

 

Meteorologist John Wheeler

Are You Tornado Prone?

After last week’s first tornado outbreak of the season produced yet another tornado in the city of Moore, Oklahoma, there has been much speculation as to why Moore seems to have become be a tornado target in recent years.  Some have speculated it could be a subtle feature of geography; a combination of the slope of the land and proximity to urban areas causing natural enhancements to thunderstorm inflow.  However, actual results seem to defy such explanations.  Tornadoes are so isolated and uncommon in any one place that it is very hard to find empirical evidence for such conclusions.  Likely as not, when a particular location seems to be tornado prone, it is just random luck.  The forces within a supercell thunderstorm are strong enough to dwarf any local geographical effects.   Meteorologist John Wheeler

Tornado Absence

 

March is typically one of the peak months for tornadoes in the United States.  Though rare so early in our region, it is common for there to be big outbreaks in March across the Southern Plains or the Southeast.  This year, for just the second time since 1950, there were no tornadoes reported across the country from March 1-15.  The other year was 1969. The cause for this year’s absence of twisters is, of course, the jet stream pattern, which has kept the Great Plains extremely dry this month so far.  And although the Southeast has been rainy, the rains have been general soakers.  The pattern just has not favored the development of tornado-producing supercell thunderstorms.  This should not be taken as a sign that our warm season will be free of tornadoes.  Meteorologist John Wheeler

A good Year for Few Tornadoes

The average annual number of tornadoes reported in the United States is about 1,260.  Fairly good records go back to 1953.  The average has been on the rise in recent years, mostly due to a much more active storm spotting network.  The most number of tornadoes in a year was 1,870 in 2004.  Other very active tornado years were 2008 and 2011 with more than 1,600 each of those years.  But for the last three years, there has been a dearth of tornadoes with fewer than 900 observed tornadoes in 2012, 2013, and so far this year.  In fact, barring an unusual Christmas tornado outbreak, 2014 will go down as having the fewest number of tornadoes on record in the modern era (since 1953).  So far, there have been just 814 tornadoes this year.   Meteorologist John Wheeler

Tornadoes #14 and #15

Friday evening, September 19, a supercell thunderstorm produced a sequence of two tornadoes in northwestern Minnesota.  The first tornado touched down near Northcote and on U.S. 75 and travelled east, crossing U.S. 59 south of Lancaster before lifting and travelling a bit further as a funnel cloud.  There was quite a bit of damage to a number of farm buildings in central Kittson County.  The tornado earned an EF2 rating, indicating peak winds between 111 and 135 mph.  the storm then produced a second tornado which traveled several miles across western Roseau County west of Greenbush.  This storm was earned an EF1 rating, suggesting winds between 86 mph and 110 mph.  Though neither tornado would have been considered a “monster” storm, it was an impressive feat for so late in the storm season.  Despite the late-season tornadoes, 2014 continues to be a relatively benign year for tornadoes in our region.  Those two tornadoes Friday were only the fourteenth and fifteenth tornadoes this year over eastern North Dakota and northwestern Minnesota, and the first since July.    Meteorologist John Wheeler

Light Summer, Tornado-Wise

It has not been a very stormy summer.  Across the whole of the United States, there were just 27 tornadoes during August.  This is the lowest tally since 1963.  This statistic becomes even more impressive when you consider that in the 1960s there was no radar and almost no storm spotting or storm chasing networks to observe tornadoes in rural areas.  Several areas of the United States did experience storms with heavy rainfall during August, but the atmospheric mechanics were just not there for tornadoes.  Year to date, this calendar year has produced the fewest tornadoes nation-wide since 2002.  Over the area of eastern North Dakota and northwestern Minnesota covered by the Grand Forks National Weather Service, there were no tornadoes in August and just 13 so far this year.     Meteorologist John Wheeler

Tornado Capital of the World

Caddo County, just west of Oklahoma City, has been hit by at least 111 tornadoes from 1950 through 2012.  The rest of central Oklahoma is about as unlucky.  Geography is to blame, particularly from mid April through mid June when conditions ideal for tornadoes occur with regular frequency.  The Gulf of Mexico and its warm, humid air is close by to the south.  Just to the west and northwest is Colorado and, in particular, the Colorado Front Range with its peaks to 13,000 feet.  The Front Range acts to strengthen low pressure areas which draw dry air from the west and humidity from the south.  The southerly winds near the ground undercut the dry air blowing in rapidly from the west/northwest which creates rapidly rising air and spin.  The result, more often than any other place in the world, is tornadoes.   Meteorologist John Wheeler