An automated “3-Element” algorithm for forecasting severe weather using AFWA MM5 model output data

The “3-Element Severe” forecast algorithm was developed as a post-processor product for the AFWA MM5 model. According to the algorithm, the following three elements are necessary conditions for severe weather: instability, weak cap, and forcing. The instability element can be triggered by several different stability indices. Likewise, there are different forcing mechanisms, any of which are sufficient by themselves to trigger the ‘forcing’ element. There are two cap indices, either of which can “veto” severe weather by being capped. An important aspect of the 3 Element algorithm is that the instability and forcing thresholds are not fixed, but are variable. The algorithm will forecast severe weather if instability is strong and forcings are weak, if forcings are strong and instability is weak, or if instability and forcing both have moderate values. The 3-Element Severe algorithm provides outlooks of severe weather similar in nature to the Storm Prediction Center’s (SPC) outlooks. The 3-Element forecasts propagate smoothly and predictably across the country, with some diurnal tendency. The forecasts compare favorably to the SPC outlooks, although the 3-Element tends to over-forecast in the Rockies and in the non-peak severe hours. Also, the 3-Element often correctly locates isolated severe weather.