Sub-Daily Severe Weather Climatology and its Implications on Forecasting

While there has been an abundance of research dedicated to the seasonal climatology of severe weather, very little has been done to study hazardous weather probabilities on smaller scales. As an extension to the local daily climatological tornado estimates described in Brooks et al. (2003), a local hourly climatology of tornado probabilities was developed following the same method. These estimates begin the process of analyzing tornado frequencies on a sub-24 hour scale. Further work will investigate the dependencies of the local climatology, including how the diurnal cycle varies in space and time.

Reports occurring within the 24-hour convective outlook day (12 UTC – 12 UTC) were then analyzed temporally to understand the time scales that severe weather events are contained within. At any given point on a given day, more than 95% of reports within 40 km of the point were captured in a 4 hour period across the United States. Therefore, the SPC’s 24 hour convective outlook probabilities can be interpreted as 4 hour convective outlook probabilities. This research begins to analyze this relationship and the implications of forecasting hazardous weather events on dynamic combinations of temporal and spatial scales.

What we present is part of a larger effort to provide background information for probabilistic forecasts of hazardous weather that are meaningful over broad time and space scales, with a focus on scales broader than the typical time and space scales of the events of interest (including current products on the “watch” scale). A large challenge remains to continue the description as the time and space scales of the forecast become comparable to the scale of the event.