Weather 101: Understanding the new SPC risk categories

Understanding the new SPC outlooks

Within the past year, the Storm Prediction Center, or SPC, has revamped its risk categories used when forecasting severe weather zones. 

The SPC is a specialized arm of NOAA, forecasting severe weather hot spots and issuing, in coordination with the local National Weather Service offices, severe thunderstorm and tornado watch boxes.

The old system was a three-tiered system, using the designations of slight, moderate and high.

In October 2014, SPC implemented the new five-tiered system, adding the categories of "marginal" and "enhanced", in addition to the original three-tiered designations. 

Each tier has criteria that are used when explaining them.

The "marginal risk", or "level 1" risk zone will typically highlight an area that may have one or two strong to severe thunderstorms. The word marginal is just that. The conditions for severe weather are usually very borderline and conditional; any severe weather produced in this area will typically just meet the criteria for a severe storm (wind gusts of at least 58 mph, hail of at least 1" diameter or a tornado). 

The "slight risk", or "level 2" risk zone will highlight an area where the dynamics for severe weather are a little higher. Any severe weather in this area will produced isolated severe weather reports of wind damage, large hail and tornadoes.

The "enhanced risk" or "level 3" risk zone highlights an area where scattered severe weather reports could be produced. Storms are more persistent within this risk zone. Severe weather reports may include several wind damage reports, damaging hail and tornadoes. 

The "moderate risk" or "level 4" risk zone will highlight an area where numerous severe storms will be possible where widespread wind damage, destructive hail may be produced along with strong tornadoes (EF-2 or greater).

The "high risk" or "level 5" risk zone highlights where widespread severe storms are likely. These risk zone very rare and used on days where the potential for severe weather could be very deadly and destructive. Typically, these are during derecho (high wind events) or tornado outbreaks and may be issued once or twice a year. 

The last time a "high risk" area was issued for North Carolina was April 16th, 2011. This was during the worst tornado outbreak in the state's history that produced 10 tornadoes across the counties of Bladen, Columbus, Robeson, Cumberland, Sampson, Duplin and Onslow. In total, 28 tornadoes were confirmed. 

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