NWS Product Definition Document (PDD) for:

Addition of supplemental Hazard Probability information to the SPC Watch product Suite

Part 1 - Mission Connection

The Storm Prediction Center (SPC) is the National Weather Service’s center of expertise for forecasting convection, especially for economically-disruptive weather events such as tornadoes, large hail, and damaging winds. Severe thunderstorm and tornado watches are issued by the SPC to provide advanced notice of organized severe thunderstorms and tornadoes. The watches provide SPC customers and partners (National Weather Service Weather Forecast Offices; national, state, and local emergency managers; commercial media; general public, etc.) with short-term forecast information on potentially hazardous convective weather events. However, information regarding the intensity and number of severe storms is currently not indicated within the individual watch issuances.

This Product Description Document describes the proposed addition of six hazard probabilities to each watch. This supplemental information would provide those responsible for public safety a more detailed forecast on severe weather conditions within each watch area.

This product has been produced and shared with others in NOAA via the SPC web site for the past year. The proposed addition was discussed at a Partners Workshop held on 24 June 2004 with positive response. At a subsequent National Partners Workshop held in Norman, OK on 8 December 2004, representatives of 9 separate organizations expressed interest in nationwide availability of this information and suggested specific operational SPC products that should ultimately contain the probabilities.

This season (beginning 1 March) the SPC proposes to make the probabilistic hazard information open to all customers through its web site. The format for the information will be essentially unchanged from its current NOAA-restricted form. An example can be found (by NOAA users) within each SPC watch (for example see http://www.spc.noaa.gov/products/watch/ww0322_prob.html). Web feedback from this broader community will be sought via an internet link. Pending continued success, the SPC plans to embed this information in human and machine readable form within one or more of its operationally disseminated AWIPS products. An NWS Public Information Statement (PNS) Service Change Notification (SCN) will be issued prior to AWIPS product modification.

Part II - Technical Description

For the past three years, after a watch had been disseminated, SPC forecasters have entered specific hazard probabilities on an experimental basis. The probabilities ranged from 0 to 100 percent and forecast increments were every 10%. The probabilities that were forecast for included the probability of: (1) two or more tornadoes, (2) one or more tornadoes of F2 or greater intensity, (3) 10 or more severe wind events, (4) one or more wind gusts of 65 kt or greater intensity, (4) 10 or more severe hail events, and (6) one or more hail reports of 2 inch diameter or greater. This year the hazard probabilities would be available via internet (www.spc.noaa.gov), however, given a successful internet test the information would be added to select AWIPS transmitted watch products. These hazard probabilities will provide SPC customers and partners with information that will help them provide better public safety for the United States public.

Submitted 18 January 2005