Severe Convective Weather Defined and Forecast - Local news, weather, sports Savannah | WSAV On Your Side

Severe Convective Weather Defined and Forecast

NOAA Storm Prediction Center NOAA Storm Prediction Center

We have all seen the severe weather outbreaks this spring in the plains... with dangerous thunderstorms and even some powerful tornadoes.

We are squarely in thunderstorm season... and this morning I wanted to discuss some terms you may hear us use... but not necessarily understand what they mean exactly.

Lets begin with severe weather. The National Weather Service definition states that a "severe" thunderstorm is any storm that produces one or more of the following elements:

  1. A tornado.
  2. Damaging winds, or winds measured 50 knots (approx. 58 MPH) or more.
  3. Hail 1 inch in diameter or larger.

The government agency required to maintain vigil across the United States for severe thunderstorms is the "Storm Prediction Center" in Norman, OK.

They issue watches for severe thunderstorms and tornadoes... and also issue daily thunderstorm outlooks. In those outlooks... if a severe weather threat exists.. you will see a slight... moderate or high risk. Here are the SPC definitions for each...

A Slight risk implies that well-organized severe thunderstorms are expected but in relatively small numbers/coverage, or a small chance of a more significant severe event. Not all severe storm events will be covered with a SLGT risk, especially during the summer when short-lived, "pulse-type" severe storms are relatively common during the afternoon. I like to think of the Slight risk equaling a lit match next to gasoline.

A moderate risk implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather and greater forecaster confidence compared to a SLGT risk. A MDT risk is usually reserved for days with substantial severe storm coverage, or an enhanced chance for a significant severe storm outbreak. Typical MDT risk days include multiple tornadic supercells with very large hail, or intense squall lines with widespread damaging winds. I like to think of the moderate risk equaling a lit lighter next to gas.

The high risk implies that a major severe weather outbreak is expected, with large coverage of severe weather and the likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events). The HIGH risk category is reserved for the most extreme events with the least forecast uncertainty, and is only used a few times each year. I like to think of the high risk equaling a blowtorch next to gas.

Hope these explainers help in the understanding of words and forecasts related to severe weather.

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