An Analysis of the Predictability of Warm-Season Progressive Derechos Using Synoptic-Scale Composites and Convection-Allowing Ensemble Forecasts

Progressive derechos, which are widespread, long-lived convectively-produced wind storms, continue to be a forecast challenge for operational meteorologists despite significant improvements in numerical weather prediction models and severe weather forecasting techniques in the last decade. This study investigates the operational predictability of warm-season (May-August) progressive derechos using two approaches to compare events with lower and higher predictability: 1) composites of derecho environments using the ERA5 reanalysis dataset and 2) convection-allowing ensemble (CAE) forecasts from the NCAR Model for Prediction Across Scales (MPAS). The main goal is to understand what synoptic-scale and mesoscale processes are associated with varying degrees of derecho predictability and evaluate if the use of CAE forecasts can increase forecaster awareness of the potential for a progressive derecho in the medium (3-5 days before the event) and short (1-2 days before) range. The Storm Prediction Center (SPC) severe storm reports and radar imagery were used to identify and select progressive derechos that occurred between 2010 and 2022. The operational predictability of severe weather in general and derechos in particular is evaluated using the SPC Convective Outlooks from five days before the event (D5) to the day of the event (D1). Cases were stratified into low, moderate, and high predictability for severe weather and for derechos.

The ERA5 reanalysis was used to build composite maps of atmospheric variables associated with high/moderate predictability and low predictability derechos. In particular, derechos with higher predictability occur in environments with stronger synoptic-scale forcing for ascent. In these events, there is a rapid increase in both MUCAPE and deep-layer vertical shear ahead of the short-wave trough that initiates the derecho on the day of the event. Conversely, in low predictability events, weak synoptic-scale forcing for ascent is present on the day of the event, with zonal flow, northwest flow, or a low-amplitude ridge aloft over the initiation region. The days before low predictability derechos are characterized by a midlevel ridge west of the derecho location and the presence of moderate or greater instability (MUCAPE > 2000 J/kg) and strong convective inhibition (MUCIN < -100 J/kg) associated with the elevated mixed layer. The presence of substantial conditional instability but weak forcing for ascent appear to be associated with greater practical uncertainty in convective initiation and subsequent upscale growth into a derecho-producing mesoscale convective system (MCS) in low predictability events.

Convection-allowing ensemble forecasts were run for 20 derecho events (10 low and 10 high/moderate derecho predictability) using MPAS with initial conditions derived from the NCEP Global Ensemble Forecast System (GEFS). The MPAS mesh has 60 km horizontal spacing over the globe decreasing to 3 km in an area centered in the derecho wind reports swath. Ten ensemble members were initialized at 0000 UTC at 24h intervals from D5 to D1 for each event. Four aspects of each ensemble member forecast were subjectively evaluated: convective mode, location of the derecho axis, timing of derecho initiation, and the duration/coverage of the wind threat. Composite maps of ensemble members with more successful forecasts (those that produced a bow-echo MCS at the approximate right location and time) were compared with composites of members with less successful forecasts. More successful members predicted an environment with higher CAPE, weaker convective inhibition and stronger low-level warm-air advection compared to members that do not produce a bow echo. Additionally, more successful members are associated with greater precipitation and a larger 2-m temperature decrease in the three hours before the first wind report associated with the derecho, indicating more widespread convection and faster cold pool intensification during the upscale growth stage of the derecho-producing MCS.