PDS watches: how dangerous are these "particularly dangerous situations?"

In cases where the threat for damage caused by severe convection is unusually high, the Storm Prediction Center (SPC) enhances the wording of the convective watch product with the following statement:

THIS IS A PARTICULARLY DANGEROUS SITUATION

Such watches are known as “PDS” watches. PDS tornado watches (PDS TOR) are issued when there is a threat of multiple strong and violent (F2+) tornadoes in the watch area, while PDS severe thunderstorm watches (PDS SEV) are issued when there is a threat for widespread significant wind damage. PDS watches are rare; for the period 1996-2005, only 22 PDS SEV watches have been issued (out of 5827 total SEV watches), while 216 PDS TOR watches (out of 3058 total TOR watches) have been issued. The greatest frequency of PDS TOR watches from 1996-2005 occurred in the deep south, running from southern Arkansas and northern Louisiana eastward to Alabama, generally corresponding with the greatest density of strong and violent tornado reports. Secondary maxima are noted in the central and southern Plains and from northeast Missouri into central Illinois.

In this study, we investigate some verification measures for PDS watches, particularly in terms of areal coverage of reports and whether the threat for significant damage is truly increased in a PDS watch compared to a regular watch. In order to calculate the area “threatened” by an event, the watches and reports are placed on a grid and the fraction of grid points that had a report occur within 25 miles (~40km) is computed.

The severe wind threat is substantially greater when a PDS SEV watch is issued. Data from 1996-2005 show the overall areal coverage of severe wind in PDS SEV watches is greater than 50% of the total watch area, which is over 2.5 times higher than in a regular SEV watch. Significant wind (65+ knots) coverage is nearly four times higher in PDS SEV watches compared to regular SEV watches.

Meanwhile, for the period 1996-2005, areal coverage of strong (F2+) tornado reports was more than three times higher in PDS TOR watches compared to regular TOR watches (and almost 30 times higher than SEV watches). Areal coverage of violent (F4+) tornadoes, while small in all cases, was nearly five times higher in PDS TOR watches compared to regular TOR watches. These results indicate that there is a greatly increased threat to life and property when a PDS TOR watch is in effect.