The skill of forecasting relatively isolated severe thunderstorm events

The mission of the NOAA/Storm Prediction Center includes forecasting organized severe convection over the contiguous United States. According to NWS instructions, a severe thunderstorm watch should be issued when ”six or more hail events of ¾ inch (penny) diameter or greater or damaging winds of 50 knots (58 mph) or greater” are forecast over an area of 8,000 mi2 during a two hour period. Similarly, when “2 or more tornadoes” are forecast for such an area, a tornado watch is to be issued.

In this paper, a cluster of events is defined as the number of reports recorded during a 2 hour window over an area of 8.000 mi2. A relatively simple technique is used in conjunction with the historical severe thunderstorm database to determine severe thunderstorm cluster sizes. It is shown that because of changes in reporting efficiency, the size of event clusters has increased over the past 50 years. The state of the science for forecasting severe thunderstorms can be estimated by examining the proportion of events that events were in watches (i.e., POD) as the progressively larger clusters are precluded from consideration. When the historical database is sorted by event intensity (tornado Fujita-scale, hail size, gust speed), it can be seen that predictability of a severe thunderstorm event increases as both the intensity of the event and the cluster size that contains it increases.