NOAA’s Storm Prediction Center issues outlooks for severe weather every day. High risk outlooks are rare and indicate either a tornado outbreak or widespread damaging winds are expected.
Only a Few Each Year
From 2000 through May 2017, an average of three to four days are covered by an SPC high risk each year.
Both 2010 and 2003 had the most in any year – six – while a few recent years didn’t have a single high risk. From June 2014 until late January 2017, no high risks were issued, the longest stretch this century.
Definition of Severe Storm
The NWS defines a severe thunderstorm as any storm that produces one or more of the following elements:
- A tornado.
- Damaging winds or speeds of 58 mph (50 knots) or greater.
- Hail 1 inch in diameter or larger.
The SPC further defines significant severe thunderstorms as any storm that produce one or more of the following elements:
- A tornado that produces EF2 or greater damage.
- Wind speeds of 75 mph (65 knots) or greater.
- Hail 2 inch in diameter or larger.
The level of categorical risk in the Day 1-3 Convective Outlooks is derived from probability forecasts of tornadoes, damaging winds, and large hail on Day 1, and a combined severe weather risk on Days 2 and 3.
- TSTM (light green) – General or non-severe thunderstorms – Delineates, to the right of a line, where a 10% or greater probability of thunderstorms is forecast during the valid period.
- 1-MRGL (dark green) – Marginal risk – An area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity.
- 2-SLGT (yellow) – Slight risk – An area of organized severe storms, which is not widespread in coverage with varying levels of intensity.
- 3-ENH (orange) – Enhanced risk – An area of greater (relative to Slight risk) severe storm coverage with varying levels of intensity.
- 4-MDT (red) – Moderate risk – An area where widespread severe weather with several tornadoes and/or numerous severe thunderstorms is likely, some of which should be intense. This risk is usually reserved for days with several supercells producing intense tornadoes and/or very large hail, or an intense squall line with widespread damaging winds.
- 5-HIGH (magenta) – High risk – An area where a severe weather outbreak is expected from either numerous intense and long-tracked tornadoes or a long-lived derecho-producing thunderstorm complex that produces hurricane-force wind gusts and widespread damage. This risk is reserved for when high confidence exists in widespread coverage of severe weather with embedded instances of extreme severe (i.e., violent tornadoes or very damaging convective wind events).
Reference: NOAA National Weather Service – Storm Prediction Center: http://www.spc.noaa.gov/misc/about.html#Severe