New Advances in Lightning Forecasting from WFO Grand Junction

The leading weather-related killer in Colorado is lightning. Nationwide, lightning is the number two weather-related killer, exceeding the number of fatalities from hurricanes and tornadoes combined. From a public safety standpoint, skilled lightning forecasts are desired, but lightning prediction remains a challenge due to its high spatial and temporal variability. The ingredients necessary for lightning to occur are fairly well understood (e.g., moisture, lift and instability); however, the mechanisms that create lightning are not. Studies have examined stability indices and their usefulness as lightning predictors, such as lifted index and convective available potential energy (CAPE). Recent research suggests that high relative humidity and temperatures in the -12° C to -18° C range promote stronger negative charging.

Using a combination of moisture, stability indices, and bulk vertical shear, the Grand Junction, Colorado (GJT) Weather Forecast Office (WFO) has developed an algorithm that improves lightning forecast skill spatially and temporally, called the lightning potential index (LPI). Qualitative observations have indicated that moisture is the most important meteorological parameter when forecasting lightning. In the LPI algorithm, moisture refers to the combination of precipitable water, relative humidity from 0 to 3 km above ground level (AGL), and minimum and maximum relative humidity in the 3 to 6 km AGL layer. The 3 to 6 km AGL relative humidity captures the moisture (or lack of moisture) in the critical -12° C to -18° C temperature range. Instability is a required component in the algorithm, but CAPE values and other stability indices make for poor spatial and temporal lightning predictors. Instead, instability defines the area where lightning may occur. The bulk vertical wind shear, when combined with the moisture component, refines the area where lightning becomes favorable. The algorithm was developed using model data, such as the Global Forecast System (GFS), available in the Advanced Weather Interactive Processing System (AWIPS) Two Dimensional Display (D2D). No observations are used to derive the algorithm except that lightning data are needed for overall performance and qualitative verification.

The significant advantage of the LPI, whether in the short term or long term, is that it alerts forecasters on where lightning is more likely to occur, rather than inferring from the meteorological parameters and crudely diagnosing the potential for thunderstorms. Using the Graphical Forecast Editor (GFE), Python scripts run the LPI algorithm and generate output that is available to the public. Grand Junction's LPI is available online at http://www.crh.noaa.gov/gjt/?n=lightningpotentialindex. The latest advances in WFO Grand Junction's methodology in lightning forecasting will be presented, including observational examples.