Convective Meteorology (Mesoscale Dynamics)

Forecasting Tornadoes using Convection-Permitting Ensembles

Burkely Gallo

School of Meteorology

12 February 2016, 3:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Hourly maximum fields of simulated storm diagnostics from experimental versions of convection-permitting models (CPMs) provide valuable information regarding severe weather potential. While prior studies have focused on predicting any type of severe weather, this work uses a CPM-based Weather Research and Forecasting (WRF) ensemble initialized daily at the National Severe Storms Laboratory (NSSL) to derive tornado probabilities using a combination of simulated storm diagnostics and environmental parameters. Daily probabilistic tornado forecasts are developed from the NSSL-WRF ensemble using updraft helicity (UH) as a tornado proxy. The UH fields are combined with simulated environmental fields such as lifted condensation level (LCL) height, most-unstable and surface-based CAPE (MUCAPE and SBCAPE, respectively), and multi-field severe weather parameters such as the significant tornado parameter (STP). Varying thresholds of 2-5km updraft helicity were tested with differing values of σ in the Gaussian smoother that was used to derive forecast probabilities, as well as different environmental information, with the aim of maximizing both forecast skill and reliability. Addition of environmental information improved reliability and the critical success index (CSI) while slightly degrading the area under the receiver operating characteristic (ROC) curve across all UH thresholds and σ values. Based on objective tests, four sets of probabilities were chosen to be evaluated in the 2015 Spring Forecasting Experiment at the Hazardous Weather Testbed, and found to be useful to forecasters, despite a perceived overforecasting tendency.

Future work regarding the establishment of a relationship between numerical guidance and observed storm characteristics will also be discussed. The low-level rotational velocity of a storm has recently been shown to relate to the conditional probability of tornado damage rating. However, relationships between model-derived supercellular characteristics, such as updraft helicity or vorticity, and observed storm characteristics, such as low-level rotational velocity, have yet to be explored. Ongoing work with the Storm Prediction Center seeks to characterize those relationships based on the NSSL-WRF ensemble and an extensive severe weather database, with the goal of creating an unconditional model-generated tornado probability with an observational basis.

Convective Meteorology (Mesoscale Dynamics) Seminar Series website