Pds Watches : How Dangerous Are These “ Particularly Dangerous Situations ? ”

Such watches are known as “PDS” watches. PDS tornado (TOR) watches are issued when the forecaster has high confidence that multiple strong (F2-F3 on the Fujita Scale) or violent tornadoes (F4-F5 on the Fujita Scale) will occur in the watch area, while PDS severe thunderstorm (SEV) watches are issued when there is a threat of a high end “derecho” (e.g. Johns and Hirt 1987, Coniglio et al. 2004) with widespread wind damage. In this study, we investigate verification measures for PDS watches for the period 1996-2005 designed to evaluate whether the PDS wording was appropriate in these forecast products. Hales (1998) discussed verification of PDS TOR watches for earlier time periods. Our main focus will also be on PDS TOR watches, since strong tornadoes pose a significant threat to life and property. We will also briefly discuss PDS SEV watches, which are rarely issued (only 22 during the period 1996-2005). All report data in this study is drawn from the National Climatic Data Center’s Storm Data publication.