Monday, November 9, 2015

SPC Convective Outlooks and "Risk"

When posting strong or severe storm updates, we will often note that the Storm Prediction Center has defined a "risk box" for severe weather.  The Storm Prediction Center (SPC) is responsible for providing watches for severe thunderstorms and tornadoes within the United States, and is part of the National Weather Service (NWS) and the National Centers for Environmental Prediction.  Warnings are issued by local NWS Forecast Offices.  A"risk box" is a term we use for an area of forecast by SPC for severe weather, and can be defined in five categories. Typically the language we use is something like "The Storm Prediction Center has the area in a slight risk box...".  These risks are color coded on a map, and are defined as:

Marginal - limited organization and longevity, or very low coverage and marginal intensity

Slight - organized severe thunderstorms are expected, but usually in low coverage with varying levels of intensity

Enhanced - greater concentration of organized severe thunderstorms with varying levels of intensity.

Moderate - indicates potential for widespread severe weather with several tornadoes and/or numerous severe thunderstorms, some of which may be intense. 

High - severe weather outbreak is expected from either numerous intense and long-track tornadoes, or a long-lived derecho system with hurricane-force wind gusts producing widespread damage. 

The SPC creates Convective Outlooks for Day 1 (today) through Day 8.  The accuracy and certainty is greater on Day 1, than on Day 3 for example.  Regardless, these products are great for awareness and preparation.

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