Convective Outlooks Research Articles

Climatology and Analysis of High-Impact, Low Predictive Skill Severe Weather Events in the Northeast United States

Abstract This study identifies high-impact severe weather events with poor predictive skill over the northeast United States using Storm Prediction Center (SPC) convective outlooks. The objectives are to build a climatology of high-impact, low predictive skill events between 1980 and 2013 and investigate the differences in the synoptic-scale environment and severe weather parameters between severe weather events with low predictive skill and high predictive skill. Event-centered composite analyses, performed using the National Centers for Environmental Prediction Climate Forecast System Reanalysis and the North American Regional Reanalysis, suggest low predictive skill events occur significantly more often in low-shear environments. Additionally, a plurality of low probability of detection (POD), high-impact events occurred in low-shear, high-CAPE environments. Statistical analysis of low-shear, high-CAPE environments suggests high downdraft CAPE (DCAPE) and relatively dry lower levels of the atmosphere are associated with widespread severe weather events. DCAPE and dry boundary layer air may contribute to severe wind gusts through strong negative buoyancy and enhanced evaporative cooling of descending saturated parcels.

Determining Criteria for Missed Events to Evaluate Significant Severe Convective Outlooks

Abstract Among the Storm Prediction Center’s (SPC) probabilistic convective outlook products are forecasts specifically targeted at significant severe weather: tornadoes that produce EF2 or greater damage, wind gusts of at least 75 mi h−1, and hail with diameters of 2 in. or greater. During the period of 2005–15, for outlooks issued beginning on day 3 and through the final update to the day 1 forecast, the accuracy and skill of these significant severe outlooks are evaluated. To achieve this, criteria for the identification of significant severe weather events were developed, with a focus on determining days for which outlooks were not issued, but should have been based on the goals of the product. Results show that significant tornadoes and hail are generally well identified by outlooks, but significant wind events are underforecast. There exist differences between verification measures when calculating them based on 1) only those days for which outlooks were issued and 2) days with outlooks or missed events; specifically, there were improvements in the frequency of daily skillful forecasts when disregarding missed events. With the greatest number of missed events associated with significant wind events, forecasts for this hazard are identified as an area of future focus for the SPC.

Standardizing the Definition of a “Pulse” Thunderstorm

AbstractIsolated, short-lived thunderstorms forming in weakly forced environments are referenced through a surplus of terminology. Further, the language used to describe the strongest, severe-weather-producing subset of these storms is applied inconsistently, posing a communication hurdle for the effective dissemination of hazardous weather risks. The term “pulse thunderstorm” was originally coined to describe an anomalously strong airmass thunderstorm often associated with a larger convective complex. However, recent applications of “pulse” have evolved to also describe nonsevere, single-cell storms, and both uses can currently be observed within research, operational, and educational texts. This paper reviews the history of the term “pulse,” performs a content analysis on nearly 1,500 pulse-referencing Storm Prediction Center (SPC) convective outlooks (CO) and mesoscale discussions (MD), and summarizes the deficiencies with the contemporary disorganized convection nomenclature. The larger CO sample (n = 997) establishes that temporal trends in “pulse” references model traditional expectations whereas the detailed MDs (n = 458) showcase examples of pulse-related terminology. The MD content analysis reveals that 1) the term “pulse” frequently appears in conjunction with severe-weather-related language and 2) that pulse-related words (e.g., brief, isolated) are equally represented in multicell-referencing MDs. In the interest of effective communication and reproducible research, the definition of “pulse” is proposed to be standardized according to the term’s original (i.e., severe, multicellular) meaning. Further, thunderstorms forming within synoptically homogeneous air masses in the absence of large-scale dynamical lift are suggested to be termed “weakly forced thunderstorms.” By corollary, pulse storms represent the subset of weakly forced thunderstorms associated with severe weather.

Tornadoes in Europe: An Underestimated Threat

Abstract The social and economic impact of tornadoes in Europe is analyzed using tornado reports from the European Severe Weather Database between 1950 and 2015. Despite what is often assumed by the general public and even by meteorologists and researchers, tornadoes do occur in Europe and they are associated with injuries, fatalities, and damages, although their reported frequencies and intensities are lower compared with the United States. Currently, the threat of tornadoes to Europe is underestimated. Few European meteorological services have developed and maintained tornado databases and even fewer have issued tornado warnings. This article summarizes our current understanding of the tornado threat to Europe by showing the changes in tornado injuries and fatalities since the 1950s and by estimating for the first time the damages associated with European tornadoes. To increase awareness of tornadoes and their threat to Europe, we propose a strategy that includes 1) collaboration between meteorological services, researchers, and the general public toward a pan-European database; 2) development of national forecasting and warning systems and of pan-European convective outlooks; and 3) development by decision-makers and emergency managers of policies and strategies that include tornadoes.

Severe Weather Prediction Using Storm Surrogates from an Ensemble Forecasting System

Abstract Probabilistic severe weather forecasts for days 1 and 2 were produced using 30-member convection-allowing ensemble forecasts initialized by an ensemble Kalman filter data assimilation system during a 32-day period coinciding with the Mesoscale Predictability Experiment. The forecasts were generated by smoothing the locations where model output indicated extreme values of updraft helicity, a surrogate for rotating thunderstorms in model output. The day 1 surrogate severe probability forecasts (SSPFs) produced skillful and reliable predictions of severe weather during this period, after an appropriate calibration of the smoothing kernel. The ensemble SSPFs exceeded the skill of SSPFs derived from two benchmark deterministic forecasts, with the largest differences occurring on the mesoscale, while all SSPFs produced similar forecasts on synoptic scales. While the deterministic SSPFs often overforecasted high probabilities, the ensemble improved the reliability of these probabilities, at the expense of producing fewer high-probability values. For the day 2 period, the SSPFs provided competitive guidance compared to the day 1 forecasts, although additional smoothing was needed to produce the same level of skill, reducing the forecast sharpness. Results were similar using 10 ensemble members, suggesting value exists when running a smaller ensemble if computational resources are limited. Finally, the SSPFs were compared to severe weather risk areas identified in Storm Prediction Center (SPC) convective outlooks. The SSPF skill was comparable to the SPC outlook skill in identifying regions where severe weather would occur, although performance varied on a day-to-day basis.

Evaluation of the Storm Prediction Center’s Convective Outlooks from Day 3 through Day 1

Abstract The Storm Prediction Center issues four categorical convective outlooks with lead times as long as 48 h, the so-called day 3 outlook issued at 1200 UTC, and as short as 6 h, the day 1 outlook issued at 0600 UTC. Additionally, there are four outlooks issued during the 24-h target period (which begins at 1200 UTC on day 1) that serve as updates to the last outlook issued prior to the target period. These outlooks, issued daily, are evaluated over a relatively long period of record, 1999–2011, using standard verification measures to assess accuracy; practically perfect forecasts are used to assess skill. Results show a continual increase in the skill of all outlooks during the study period, and increases in the frequency at which these outlooks are skillful on an annual basis.

Objective Limits on Forecasting Skill of Rare Events

Abstract A method for determining baselines of skill for the purpose of the verification of rare-event forecasts is described and examples are presented to illustrate the sensitivity to parameter choices. These “practically perfect” forecasts are designed to resemble a forecast that is consistent with that which a forecaster would make given perfect knowledge of the events beforehand. The Storm Prediction Center’s convective outlook slight risk areas are evaluated over the period from 1973 to 2011 using practically perfect forecasts to define the maximum values of the critical success index that a forecaster could reasonably achieve given the constraints of the forecast, as well as the minimum values of the critical success index that are considered the baseline for skillful forecasts. Based on these upper and lower bounds, the relative skill of convective outlook areas shows little to no skill until the mid-1990s, after which this value increases steadily. The annual frequency of skillful daily forecasts continues to increase from the beginning of the period of study, and the annual cycle shows maxima of the frequency of skillful daily forecasts occurring in May and June.

Evaluation of the Storm Prediction Center’s Day 1 Convective Outlooks

Abstract The Storm Prediction Center has issued daily convective outlooks since the mid-1950s. This paper represents an initial effort to examine the quality of these forecasts. Convective outlooks are plotted on a latitude–longitude grid with 80-km grid spacing and evaluated using storm reports to calculate verification measures including the probability of detection, frequency of hits, and critical success index. Results show distinct improvements in forecast performance over the duration of the study period, some of which can be attributed to apparent changes in forecasting philosophies.

An Assessment of Areal Coverage of Severe Weather Parameters for Severe Weather Outbreak Diagnosis

Abstract The areal extent of severe weather parameters favorable for significant severe weather is evaluated as a means of identifying major severe weather outbreaks. The first areal coverage method uses kernel density estimation (KDE) to identify severe weather outbreak locations. A selected severe weather parameter value is computed at each grid point within the region identified by KDE. The average, median, or sum value is used to diagnose the event’s severity. The second areal coverage method finds the largest contiguous region where a severe weather parameter exceeds a specified threshold that intersects the KDE region. The severe weather parameter values at grid points within the parameter exceedance region are computed, with the average, median, or sum value used to diagnose the event’s severity. A total of 4057 severe weather outbreaks from 1979 to 2008 are analyzed. An event is considered a major outbreak if it exceeds a selected ranking index score (developed in previous work), and is a minor event otherwise. The areal coverage method is also compared to Storm Prediction Center (SPC) day-1 convective outlooks from 2003 to 2008. Comparisons of the SPC forecasts and areal coverage diagnoses indicate the areal coverage methods have similar skill to SPC convective outlooks in discriminating major and minor severe weather outbreaks. Despite a seemingly large sample size, the rare-events nature of the dataset leads to sample size sensitivities. Nevertheless, the findings of this study suggest that areal coverage should be tested in a forecasting environment as a means of providing guidance in future outbreak scenarios.

Numerical prediction of severe convection: comparison with operational forecasts

AbstractThe prediction of severe convection is a major forecasting problem in Australia during the summer months. In particular, severe convection in the Sydney basin frequently produces heavy rain or hail, flash flooding, and destructive winds. Convective activity is a forecasting challenge for the Sydney basin, mainly from October to April. Currently, there is a need for improved numerical model guidance to supplement the official probabilistic convective outlooks, issued by the operational forecasters. In this study we assess the performance of a very high resolution (2 km) numerical weather prediction (NWP) model in terms of how well it performed in providing guidance on heavy rainfall and hail, as well as other mesoscale features such as low level convergence lines. Two cases are described in which the operational forecasts were incorrect on both occasions. Non‐severe thunderstorms were predicted on 1 December 2000 but severe convection occurred. Severe convection was predicted on 8 December 2000, but no convection was reported. In contrast, the numerical model performed well, accurately predicting severe convection on 1 December and no convection on 8 December. These results have encouraged a program aimed at providing an enhanced numerical modelling capability to the operational forecasters for the Sydney basin. Copyright © 2003 Royal Meteorological Society

Storm Prediction Center Forecasting Issues Related to the 3 May 1999 Tornado Outbreak

Abstract Forecasters at the Storm Prediction Center (SPC) were faced with many challenges during the 3 May 1999 tornado outbreak. Operational numerical forecast models valid during the outbreak gave inaccurate, inconsistent, and/or ambiguous guidance to forecasters, most notably with varying convective precipitation forecasts and underforecast wind speeds in the middle and upper troposphere, which led forecasters (in the early convective outlooks) to expect a substantially reduced tornado threat as compared with what was observed. That, combined with relatively weak forecast and observed low-level convergence along a dryline, contributed to much uncertainty regarding timing and location of convective initiation. As a consequence, as the event approached, observational diagnosis and analysis became more important and were critical in identification of the evolution of the outbreak. Tornadic supercells ultimately developed earlier, were more numerous, and produced more significant tornadoes than anticipated...

The prediction of two cases of severe convection: implications for forecast guidance

Severe convection in the Sydney basin regularly produces destructive winds, heavy rain or flash flooding, and damage from large hail. Such convective activity is a major forecasting challenge for the Sydney basin, especially during the October to April warm season. There presently is a need to provide timely, accurate and reliable numerical guidance to supplement the current probabilistic convective outlooks, issued by the operational forecasters. Initial work has been carried out that examines two cases of severe convection in the Sydney basin. The performance of a very high resolution (2 km) numerical weather prediction (NWP) model is assessed in terms of how well it performed in providing guidance on heavy rainfall and hail, as well as other key mesoscale phenomena such as low level convergence lines. The model results discriminated well between severe convection that actually did occur in the first case (1 December, 2000) and the failure of severe convection to develop in the second case (8 December, 2000). The operational forecasters predicted severe convection to occur in both cases. It is now planned to take the next step of augmenting the quasi-subjective approach of the operational forecasters with the NWP guidance to provide an enhanced capacity to forecast severe convection in the Sydney basin.

Operations of the National Severe Storms Forecast Center

The National Severe Storms Forecast Center in Kansas City, Missouri, is composed of several operational forecasting units, all national in scope. It includes the Severe Local Storms Unit (SELS), the National Aviation Weather Advisory Unit (NAWAU), and a Techniques Development Unit (TDU). Within SELS there is a synoptic interpretation message (SIM) section and a mesoscale discussion (MD) section. The SELS Unit has been in Kansas City since 1954 and is responsible for providing a wide array of products related to severe weather: convective outlooks, public weather statements, mesoscale discussions, status reports, and tornado and severe thunderstorm watches for the 48 conterminous states. SELS is also responsible for preparing a national weather summary. NAWAU provides alerts, nationally, of hazardous weather, known as AIRMETs and SIGMETs, and convective SIGMETs to aviation interests. NAWAU also issues aviation area forecasts. The SIM section prepares a diagnostic interpretative message that incorporates satellite and other information and relates them to model trends. The MD section supports the SELS program by providing short-range updates on developing mesoscale systems. The Techniques Development Unit has been responsible for working hand in hand with SELS to reap the benefits of applying interactive computer-processing technology, progressively giving the forecaster more effective means to provide improved services. Illustrating these improvements is the fact that severe weather watches have significantly increased in accuracy, especially in handling violent death-dealing tornado situations. An illustrative example of SELS products is shown for the severe weather outbreak of 15–17 June 1992. The example shows how skillfully this episode, producing 172 tornadoes and 462 other reported severe weather events, was forecast. Emphasis is made of the need for consistent and well-coordinated watches and other advisories when dealing with widespread, rapidly moving, severe weather-producing systems.

Focus on forecasting1

A forecast experiment was conducted from 1 June to 12 August 1983 at the Program for Regional Observing and Forecasting Services (PROFS) in Boulder, Colorado. The exercise included probabilistic forecasts of severe-weather convective outlooks prepared with the use of existing technology and severe-storm warnings made with the use of advanced technology to display a wide variety of data sets on the PROFS workstation. The convective outlooks were evaluated using signal-detection-theory analyses and Brier scores. Considerable skill using signal-detection theory was found for both severe-thunderstorm and tornado outlooks, treated as separate outlooks. On the other hand, Brier skill scores relative to sample climatology show small skill for the thunderstorm outlooks and no skill for the tornado outlooks. The signal-detection analyses provide valuable baseline values for future evaluation studies. Skill scores for the warnings issued at PROFS are compared with a similar set of skill scores for warnings from the...