Know Your Severe Weather Risk

spc scale

Dark skies, the bright dazzling flash of lightning and the ominous roar of thunder.  It can terrify some, while fascinating others.  What causes violent severe storms and how can we know in advance that they may strike?  First, let’s define some terms.  The National Weather Service classifies “severe” storms as those containing winds in excess of 58 mph, hail larger than 1” in diameter or producing tornadoes.  A 58 mph wind is strong enough to knock a tree branch down or topple a small, weak tree.   One-inch diameter hail is roughly the size of a quarter.  So even if you see dark clouds and lightning and hear a lot of thunder, the storm may not be considered “severe”, it may just scare you out of your wits.

Meteorologists study current weather patterns and the output from various computer models to determine if the “right” conditions exist for severe weather to occur.  This analysis is based on whether proper amounts of wind shear, temperature, moisture differences and atmospheric energy are present in certain a orientation and strength.  However, it’s not an exact science; even the most seasoned forecaster will admit that.  In many ways, its pattern recognition – if enough ingredients have combined in the past to produce severe weather, odds are they will likely do so again.

It’s the duty of the meteorologists at the Storm Prediction Center (SPC) in Norman, Oklahoma to assess storm potential and identify parts of the U.S. that may be impacted by severe thunderstorms and tornadoes.  They prepare an outlook for risk areas up to a week in advance. For the first three days of that projection, they categorize risk areas in order of ascending danger as follows: general, marginal, slight, enhanced, moderate and high. As of 2014, marginal and enhanced were added in an effort to further provide a distinction. The graphic above shows the changes to scale.

General means that thunderstorms are expected in this area, but they may not become severe.  Of course all thunderstorms are inherently dangerous because they contain lightning, but they may not be classified as “severe” according to the definition above. Marginal risk means the storms are expected to have limited organization, longevity or very low coverage. Slight risk implies organized severe thunderstorms are expected, but with low coverage and varying levels of intensity. There have been occasions when storms producing wind damage, hail and even isolated tornadoes have occurred in areas which were forecast to have slight risk. Enhanced risk indicates a greater concentration of organized severe thunderstorms with varying levels of intensity. Moderate risk means there is the potential for widespread severe weather with several tornadoes and/or numerous severe thunderstorms, some of which may be intense with widespread wind damage and very large and destructive hail. High risk indicates the likelihood of a major severe weather outbreak consisting of numerous intense, violent and long-track tornadoes, damaging large hail or widespread straight-line winds. It could also mean a long-lived derecho system with hurricane-force wind gusts producing widespread damage is expected.

Typically, these risk areas are drawn broadly, covering several states because it is just not possible to forecast with precision the exact location of expected severe weather.  I liken it to baking a cake – if all the ingredients are mixed properly and the pan is set in the oven at the right temperature for the right duration, you will likely get a cake. If any of those ingredients are missing or not in the right proportions, you’ll get something, but it might not be an edible cake.

When severe weather is expected, SPC forecasters usually post a watch anywhere from six to eight hours before the onset of any storm activity.  This is a “heads-up” to people in the affected areas where either severe thunderstorms or tornadoes are expected.  These watches may cover several counties in one state or multiple counties across several states.  A watch is your notice that it is time to plan and prepare.  It also signals local forecast offices in the watch area to be ready to monitor developing storms.

Once a storm reaches severe limits, forecasters at  local National Weather Service offices issue a warning.  This means that one or more of the severe criteria mentioned above has been reached and a real danger exists to the public.  This is the time to take protective action.  Warnings typically can last anywhere from 15 to 30 minutes.  The warning area shape approximates where the storm is expected to move during the warning period.  It may cover a part of a whole county, or segments of several counties, depending on their orientation.  If the storm continues moving, a new warning may be issued covering the new target locations.

Depending on the movement of the storm threat, SPC may decide to remove some counties from the watch or add new ones.  So even if you are not under a watch now, it is important to stay aware of changing weather conditions in the event that the threat moves toward you.  Severe weather is most likely in the spring and fall, but is possible any time of the year, depending on weather conditions.

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