Revising Hazard Intensity Information in Storm Prediction Center Outlooks: A Hazardous Weather Testbed Experiment

Storm Prediction Center (SPC) Convective Outlooks consist of individual probabilistic forecasts of severe convective hazards: hail of 1 inch diameter or larger, damaging winds or speeds of 58 mph/50 kts or greater, and tornadoes. Forecasters draw probabilities that represent the chance of each hazard occurring within 25 miles of a point. Forecasters can also delineate “hatched” areas, which represent regions where a 10% chance of significant severe weather (tornadoes producing EF-2 or greater damage, winds of 75 mph/65 kts or greater, and hail 2 inches in diameter or larger) exists within 25 miles of a point.

As part of the 2019 Hazardous Weather Testbed (HWT) Spring Forecasting Experiment (SFE), the SPC tested the feasibility of including additional information about the intensity of individual convective hazards in the Day 1 Convective Outlook. For each hazard, we define three conditional intensity probability density functions (not tied to coverage): “normal”, “hatched”, and “double-hatched”. In plain language, “normal” refers to a typical severe weather day, where significant severe weather is unlikely, “hatched” areas indicate where significant severe weather is possible, and “double-hatched” areas indicate where high-impact significant severe weather is expected.

Outlooks were drawn each day of the HWT SFE, consisting of a “coverage” layer and an “intensity” layer, for each of the three hazards (tornadoes, wind, and hail). This method allows forecasters to communicate situations where the unconditional probability of a hazard is small, but the conditional probability of a significant, high-impact event is large, or situations where severe weather is likely, but large impacts would not be expected. We will show results from surveys administered to experiment participants, demonstrate how the experimental forecasts verified, highlight successes and challenges, and explain how these results can inform future changes to SPC Convective Outlooks.