What Constitutes as “Severe Weather”

The biggest thing that most under-educated or short-sighted individuals think of when it comes to weather is the qualifications of the word “severe.”  For most of these people, the word “severe” is strictly reserved for thunderstorms that occur in the Spring and Summer in the Midwest, and in some cases into Autumn (year round if you’re in the south United States).  The problem with this mindset is that it completely ignores the definition of the word severe in the dictionary, which Merriam-Webster defines as “very bad, serious, or unpleasant – causing a lot of pain or suffering – very harsh.”

Those of us in the Midwest know that “very harsh” weather can happen any time of year.  That’s especially true for anyone who has lived through the Winter of 2013-14 where we saw record cold temperatures just about everywhere.  -50 F wind chills as far south as southern Iowa and northern Missouri is pretty harsh.  By that definition severe weather takes on many different forms as described by the previous example.

So what are all the characteristics of weather that is severe?

  1. Thunderstorms that produce over one inch of rain per hour.
  2. Thunderstorms that produce hail over one inch in diameter.
  3. Thunderstorms that produce winds in excess of 58 MPH.
  4. Tornadoes – this does not include landspouts, dust devils, gustnadoes, or dirt devils.
  5. Hurricanes, tropical storms and tropical depressions.
  6. Winter storms, or snow systems that produce greater than three inches of snow.
  7. Blizzards – storms that produce winds in excess of 45 mph and freshly fallen snow.
  8. Extreme cold or heat.
  9. Gusty winds in excess of 50 MPH.
  10. Low relative humidity.

Lets take these a part and examine the consequences, and the impacts.  We’ll even take a look at some definitions that might be useful too.

First, lets examine our first three points regarding thunderstorms.  Thunderstorms are the result of atmospheric convection, it is very rare to have a thunderstorm with “light” rain as the atmosphere has to have a certain degree of instability in order for them to form.  The same principle applies in the Winter with thundersnow – enough convection has to be present in the atmosphere for it to occur.  That’s why most thundersnow events are associated with extremely intense snow-squalls.

Lets first do away with the term thunderstorm and replace it with thundershower – as thunderstorm implies that it is a storm not producing rain.  Thundershowers are associated with most severe weather.

Areal Flooding

Thundershowers that produce moderate to heavy rainfall and persist for any amount of time can lead to swelling of lakes, streams, creeks, and ditches.  In some cases they can even impact rivers and other low lying areas like sewers, creating a potentially life threatening situation for people living or travelling in these locations.

Damage to businesses and homes as well as agriculture can occur as well as disruption of transportation and temporal imbalance of local ecosystems.

While areal flooding may not necessarily occur immediately, but over time – the damage it can do is just as harsh as a sudden flooding impact – and in some cases even more so.  The Mississippi River is one of these cases.  Where in the last month excessive rainfall in the north has already caused the river to swell locally and downstream.  Thundershowers producing intense rainfall passed over downstream locations causing the swelling to intensify.  Multiple complexes moving over the same area also led to flash flooding.  Even though these storms did not produce destructive winds, they were still considered severe in the locations they passed over downstream where the river was already in flood stages.

Flash Flooding

Flash flooding is one of the most deadly and catastrophic forms of severe weather to date.  Aside from lightning, flash flooding claims the lives of more people each year than tornadoes, hail damage, or windstorms combined.

Thundershowers that produce torrential rainfall in excess of one inch per hour are to blame for most flash floods, but levee breaks, ice jams, and storm surges can also produce rapid increases in water levels near or away from water.  Logically speaking, earthquakes that produce tsunami‘s are also responsible for a form of flash flooding – but the correct term should always be associated with a tsunami.

Flash flooding is a by-product of convective activity that may not be at severe levels, but could quickly become severe as well.  In the United States, the National Weather Service is allowed to activate the Emergency Alarm System for Flash Flood Warnings for the very reason that posses immediate danger to life and property.  But why?

Flash flooding is usually the result of these storms causing rapid increases on rivers, lakes, streams, creeks, ditches and sewers.  Open fields with super-saturated soils, low lying areas with poor drainage, and flat roadways and intersections are also at risk.

As little as six inches of water is capable of displacing the wheels of a car and forcing it off the road, or worse – downstream into deeper waters where a driver can become stranded.  Water pressure also begins to increase the deeper the water, and an increase in undercurrent, meaning that you can be swept off your feet much easier as well.  Most fatalities in flash flooding occur in this way.

Severe Thunderstorms & Tornadoes

For information on severe thunderstorms, as well as convective forecasting processes see the related article The Stages of a Weather Alert posted earlier this year.

A note regarding tornadoes:

There is a difference between a waterspout and a tornado over water.  Waterspouts are typically weak and do not cause any significant damage except potentially to smaller marine craft. Tornadoes differ from all spouts.  Waterspouts and landspouts form from weak thunderstorm complexes and typically are not substantially strong.  Some of the stronger spouts are capable of ripping shingles of rooftops, capping small marine craft, and cause damage to unsecured property.  Gust front funnels are also a form of spout and typically occur near a cold front in associated shower and thundershower activity.  These funnels rarely reach the ground and when they do are usually short lived.

However, it is always best to play it safe and not to challenge nature.  Trained persons may be able to tell the difference between a spout (such as identifying if there is an associated rotating column airborne, or identifying the thunderstorm’s magnitude), but the safest thing to do is always to seek shelter.

Blizzards

I wanted to get right to the chase with this.  A blizzard is not – I repeat is not – a supermassive ultra-hyper winter storm that dumps tons and tons of snow.  A blizzard is intense winds that are capable of blowing loose falling or fallen snow around causing whiteout conditions and causing cold temperatures.  That means that yes – it is possible, while not frequent – that blizzards do not require any snow to be falling to occur.  That means, zero inches of new accumulating snow can fall and a blizzard still can occur.

Winter Storms, Ice Storms, and Warnings

The term winter storm can be associated with any combination of winter precipitation and any amount.  Warning criteria varies from south to north.  The further north you go, the higher the criteria for a “Winter Storm Warning” to be issued.  For example, in the Quad Cities, it’s six inches and in St. Louis it’s only four inches.  There’s some variable to that as well that can cause it to be lower, but that’s the general rule of thumb.  Ice Storm Warnings are just about the same everywhere, generally a quarter to a third of an inch.

Why is a warning necessary?  It’s just a little snow right?  Well, not so much.  Six inches is nothing really, it’s the time frame that all six inches are expected to fall in.  If it’s six inches of snow that falls in six hours, that’s rather intense as far as snow goes.  It goes back to the variable of visibility being poor, and when you have snow coming down that heavy it can be hard.  As you go south, the next issue is road crews being prepared and ready to clean transportation routes.

Extreme Temperatures and Low Humidity

Extreme heat and extreme cold is never good.  It’s poor for vegetation – recent and ongoing – and it’s poor for the health of the young and old.  It’s hard on cars, it’s hard on heating, gas and electric bills.  It’s harsh in just about every way.  Brutal for many of us that live in the north and those who live in the south.  Imagine a mucky 105 F day with a dew point of 76 F?  How about a balmy -23 F with winds gusting to 20 MPH?  Heat Index of 120 F, Wind Chill of -50 F.  Both of these scenarios have happened.  Heat stroke, hypothermia, exhaustion, dehydration, and frost bite are rampant in these conditions.

Low humidity does not help either, especially when it comes to agriculture.  Lack of moisture means a more brutal soil, and increased risk of drought and dust accumulation.  It also means a significant risk of wild and brush fires.  If your possessions being burned up isn’t harsh – I’m not sure what is.

CONCLUSION

Next time you hear about severe weather, and it comes and goes without you necessarily seeing anything too significant in your eyes.  Stop and think.  What happened around you?  What happened near by?  Remember, severe weather means more than damage to just something that you own, or more than just an inconvenience to you.

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