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Center's Two-Day Outlooks Now Rate Tornado, Hail and Wind Risk
in Severe Thunderstorms

March 2, 2001 — NOAA's Storm Prediction Center has changed the way it issues daily national severe weather outlooks to provide more specific information about the type of severe weather threat expected, which will help emergency managers and the general public better prepare for the impending severe weather.

The new, more detailed forecasts began Jan. 31 and include probability forecasts for three separate types of threats associated with severe thunderstorms: tornadoes, damaging winds and large hail. They are included in the daily convective outlooks, which are forecasts identifying where severe storms are likely to occur today and tomorrow. The probabilistic outlooks complement the SPC's categorical outlooks indicating where there is a "slight," "moderate" or "high" risk of severe thunderstorms.

"The new probability outlooks will help emergency managers, the media and the general public make better decisions regarding severe weather safety," said SPC Director Joseph Schaefer. "We've added clarity to our severe thunderstorm forecasts. The outlooks are very useful in judging one's chances of experiencing a severe thunderstorm and what the specific threat is—tornadoes, hail or damaging winds. People can know that today is a day to pay close attention to weather forecasts when there is a risk of severe weather in their local area, and exercise an appropriate personal safety plan."

The outlooks indicate geographic areas where severe weather hazards are expected, as well as the perceived levels of threat for each hazard. The probability values represent the chance of a severe weather event within 25 miles of any point. The higher the probabilities are, the increased threat of that hazard occurring. When a significant or extreme event is predicted, it will also be indicated in the outlook. A significant event is defined as a tornado causing F2 - F5 damage, hail two inches in diameter or larger or winds of 73 mph or greater.

The new probabilistic forecasts represent an important step forward for the SPC in directly conveying the uncertainty inherent in severe storm forecasting. Forecasters' expectations of large hail, damaging winds and tornadoes are explicitly conveyed through individual forecasts. Users who are sensitive to one particular threat, such as emergency managers sending out storm spotters to watch for tornadoes, and positioning recovery equipment, can make more informed decisions.

The SPC's severe weather outlooks are keyed to the well-organized severe weather events capable of causing the greatest damage and injury. A narrative discussion accompanies the outlook to specifically describe and provide rationale for what kind of severe weather is expected as well as where and when it is most likely within a risk area. The probability outlook graphics and discussion are available on the SPC home page.

The Storm Prediction Center is one of nine National Centers for Environmental Prediction, an arm of NOAA's National Weather Service. Their mission is to provide timely and accurate forecasts and watches for severe thunderstorms and tornadoes over the contiguous United States. The SPC also monitors heavy rain, heavy snow, and fire weather events across the U.S. and issues specific products for those hazards.

Relevant Web Sites
Probability Outlook Graphics and Discussion

Media Contact:
Keli Tarp, NOAA's Storm Prediction Center, Norman, Okla., (405) 366-0451